Casimir Annual Report 2010 by Casimir Research School

Casimir Research School Delft â&#x20AC;&#x201C; Leiden Report 2010

Report 2010

Casimir Research School Delft-Leiden Report 2010

Contents Part I Preface: Casimir Research School, from 2010 to 2020 --------------------------

1. About the School-----------------------------------------------------------------1.1. Core business 1.2. Research themes 1.3. Education activities 1.4. Organization

2. People -----------------------------------------------------------------------------2.1. PhD students 2.2. Recruitment 2.3. New PhD cohort 2010 2.4. Staff developments

3. Education -------------------------------------------------------------------------3.1. Casimir PhD Courses 2010 3.2. Casimir Spring School â&#x20AC;&#x201C; Arnemuiden 3.3. Les Houches Summerschool

4. Research --------------------------------------------------------------------------4.1. Timeline of research and grant highlights 2010 4.2. NeCEN/NIMIC 4.3. Theses 4.4 Highlights of research achievements per Casimir theme

5. Outlook to 2011 by new scientific director ------------------------------------ 31

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Preface: Casimir Research School, from 2010 to 2020 In a recent book1, former TU Delft applied physics student Jo Ritzen (graduated in 1970) wrote: “Europe's universities are very well represented among the world's top 200 universities, but almost absent in the top 50. They are economically, culturally and socially underexploited and underperforming.” It is no secret that university rankings are dominated by US universities. The emergence of this US domination is very well documented in another recent book2, written by Jonathan Cole, professor at Columbia University. He concludes that these impressive institutions are “creative machines unlike any other that we have known in our history” . In most cases a typical university scientist will read these stories, consider them the concern of politicians, funding agencies, or university managers and simply request more money for doing research. Indeed, one of Ritzen’s statements is that universities in many European countries are underfinanced. But the analysis of Cole focuses on a different issue: the emergence of US universities as research universities. As developed in Germany and the UK, research universities carry out both research and teaching programmes that interact most intensely at the level of the MSc and PhD training. The research universities receive public funding for research on a competitive basis. Within the Netherlands, we are quite familiar with many of the American top ranked universities. A large number of the Casimir staff has accumulated valuable experience in the US as a PhD student, post-doc, faculty member, or visiting professor. The common experience is that the difference between a typical Dutch university and an American university is not very large. Nevertheless, there are clear differences. American universities work more easily with multiple missions such as fundamental and applied research, a broad programme allowing cross-fertilization, and access to highly qualified students. In American universities the graduate school brings together ground-breaking research and gifted students applying and selected from all over the world. Being able to work with excellent colleagues is wonderful, but equally wonderful is to be able to work with very bright young students. Indeed, when it comes to talented students, professors in the US as well as in Europe tend to feel strongly involved. It has been and continues to be the ambition of the Casimir Research School to improve our own academic landscape in such a way that we can play a similar role as the American graduate schools do. In our view a graduate school brings together active research and the best of young talent, recruited from a large reservoir of potential candidates. Achieving this ambition is also dependent on developments in the world outside Casimir. Luckily, the importance of graduate education and PhD training is getting more widely appreciated at each of our universities. But it should not be overlooked that many countries in Europe have a very different way of organizing research and education than the Netherlands has. A recent European Commission conference was entitled “Conditions for Achieving Excellence in Universities and Other Research 1

Jo Ritzen, A Chance for European Universities, Amsterdam University Press (Amsterdam, 2009) Jonathan R. Cole, The great American university; its rise to pre-eminence, its indispensable national role; why it must be protected. (Public Affairs, 2009). See also http://university-discoveries.com/.

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Organisations.” The wording of the title is a clear reminder that for some countries in Europe the focus for research is on the universities but for many other countries research is carried out largely or entirely at separate research organizations. Furthermore, in many cases the money for individual projects is traditionally not allocated on a competitive basis and the PhD students do not form a mutually interacting club of aspiring newcomers. Inevitably, the European Commission is very much interested in a modernization agenda. In October 2010 the European Union launched its strategy for 2020, called “Innovation Union”, presented by Máire Geoghegan-Quinn, EU-commissioner for Research, Innovation and Science5. It is a very extensive programme based on the awareness that the needed improvement of the European research structure is more far-reaching than only money. One statement is: “Our education systems at all levels need to be modernized. Excellence must even more become the guiding principle. We need more world-class universities, raise skill levels and attract toptalent from abroad.” Commissioner Geoghegan-Quinn publicly stated that Europe needs fresh ideas for organizing research and human capital. As an example she mentioned new structures such as European research schools, as proposed by Delft and Leiden. The Delft-Leiden Casimir Research School should continue to focus on its corebusiness: carrying out world-class research in ‘interdisciplinary physics’ and training young MSc and PhD students for this frontline research. At the same time it should encourage a broad stimulating intellectual climate, where PhD students learn and mature through mutual interaction, as well as facilitate recruitment of the best students (PhD and MSc) from all over Europe (and beyond). In doing so Casimir is well prepared for opportunities to fund its programme of graduate education and recruitment arising through national bodies like the KNAW and NWO, as well as those arising through the new European Marie Curie programme for Innovative Doctoral Programmes. It is fair to expect that new European funding instruments will become available to further encourage successful university research training models. We believe that this yearly report testifies that Casimir is ready to meet that challenge for the next decade.

March 31st 2011 Prof.dr.ir. T.M. Klapwijk, Scientific director Prof.dr.J. Zaanen, Co-director

Prof. Jan Zaanen and Prof. Teun Klapwijk

See http://ec.europa.eu/research/innovation-union 5

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1. About the school 1.1. Core business

Casimir focuses on interdisciplinary physics as performed in the training of Ph.D. students at Leiden and Delft. In using the term ‘interdisciplinary physics’ Casimir tries to capture two aspects. Firstly, the school unites researchers who follow a physics approach. At the same time it wants to emphasize that in all these physics subjects there is strong overlap, interaction with and collaboration with other disciplines. Often it concerns different disciplines for different physics subjects. In addition Casimir has a strong desire to overcome the traditional dichotomy between fundamental and applied research. We consider challenges emerging from fundamental questions of equal importance as challenges emerging from practical questions, encouraging research in both directions at both institutions. In its support of PhD training, a great deal of attention goes into obtaining, exchanging and communicating new research results as well as acquiring the skills to carry out research. For the so-called ‘soft skills’ Casimir relies on the general programmes offered by the Universities of Leiden and Delft as well as those offered by the NWO-funding agency FOM. Casimir builds in its own programme elements, which address directly relevant soft skills as well. Nevertheless, central goal remains the performance of high-level research as well as the training of young people in this profession. Casimir is convinced that the combination of independently thinking young people and research forms the best breeding ground for innovative research. Finally, although Casimir is built upon the groups participating in the Kavli Insititute of Nanoscience at Delft, i.e. the departments of Bionanoscience and Quantum Nanoscience, and the Leiden Institute of Physics, the research school has the freedom to allow others to join. It is our view that the organization should facilitate and not limit the enrichment of PhD training nor stand in the way of new research interactions and endeavors. 1.2. Research themes In the past 6 years it has become clear that the spectrum of research subjects carried out at Leiden and Delft can be well summarized in six themes (see Table):

6) Dynamic complex systems

Informatics Astronomy, instrumenttechnology Chemistry, biology, mechanical and civil engineering

Industry

Chemistry

Application

Activities

Experiment

Interacting disciplines Biology Electronics, chemistry

Theory

Research Themes 1) Molecular biophysics 2) Physics of nanostructures 3) Quantum matter and functional materials 4) Quantum information and quantum optics 5) Universe physics: theory and instrumentation

7) Future themes? For each of the themes the relevant disciplines for interaction are indicated. The columns in the Table indicate that, ideally, we would like to see in each of the 6

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themes research being carried out which is theoretically oriented, is primarily experimental in its nature or originates in applied problems. Finally, each of the themes interacts with different industrial environments or spin-off companies. We do not quantify to what extend each theme focuses on theory or experiment etc. Instead we prefer to use the Table simply as a guideline in the realization of the Casimir goals. The empty 7th slot signifies future themes, indicating that Casimir could allow others to participate within one of the existing themes or by adding an extra theme, which would fit into the overall vision of Casimir. For example, Prof. Ekkes BrĂźck joined in 2008 as part of the existing theme â&#x20AC;&#x153;Quantum matter and functional materialsâ&#x20AC;?. Below we elaborate on the 6 Casimir themes: 1) Molecular Biophysics The focus is on the study of processes of living objects on a molecular-physical level. The possibility to address these questions is due to newly developed experimental techniques such as optical and scanning probe techniques. In addition molecular biology is providing excellent materials control, which allows selecting suitable experimental systems for collaboration between biologists and physicists. The combination with microlithography creates possibilities for unique experimental research as well as for lab-on-a-chip ideas. In addition, the strong Life Sciences activities at Leiden University enable collaborative projects. In Delft a strong investment in bionanotechnology has been implemented recently, led by Spinoza-winner Cees Dekker, while in Leiden the theoretical physics thrust is being expanded with theory in biophysics. 2) Physics of Nanostructures Nanolithography finds its way to numerous old and new devices using a variety of functional materials. The length scale is to be compared to physically relevant length scales such as the wavelength of the electrons or photons or spin waves. There are new devices such as spin-valve devices, magnetic tunnel junctions in which the dynamics of magnetism plays a key role. Additionally nano tunnel devices are used for the coherent detection of very weak signals. Downsizing to nano-dimensions has led to the emergence of a new field called nanophotonics in which sub-wavelength optics plays a prominent role. A fourth field of interest concerns nano-mechanical properties and single-molecule transistors. This field counts several researchers in Delft and Leiden, all strongly driven by the current-day experimental skills in making very small structures leading to unique properties. Among them Michel Orrit, who was awarded an ERC Advanced Researchers Grant, Carlo Beenakker and Yuli Nazarov, who are specialized in theory of mesoscopic systems, and Gerrit Bauer who is a leading expert in the theory of magnetic mesoscopic systems. In Delft a new nanolaboratory (Van Leeuwenhoek Laboratory) has been built supporting a variety of experimental programmes. 3) Quantum Matter and Functional Materials Fascinating new developments are taking place in the field of condensed matter physics and functional materials. Numerous new materials have recently been discovered which cannot be described with the traditional textbook classifications such as phonons, electrons, bandstructures etc. The complexity is largely due to the strong interactions between the relevant particles and in which subtle differences in the interactions may lead to a material being magnetic, insulating, or superconducting. Some of these effects have been studied in model systems such as 7

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Josephson-arrays; in many real and usable materials the search for the relevant concepts is actively being pursued theoretically and experimentally. In addition there are many materials with new functionalities such as the organic semiconductors with unique optical and electrical properties and of course the one-atom thick graphene layers. In both cases a strong interaction with chemists is crucial as well as the control of the thin film technology known from the semiconductor world. One of our leading researchers in this field is Spinoza-winner Jan Zaanen. In addition a large programme Nano-Imaging under Industrial Conditions (NIMIC) has been created by Henny Zandbergen in Delft and Joost Frenken in Leiden. Frenken and Zandbergen both received an ERC advanced researchers grant. Frenken is also a winner of the Jacob Kistemaker prize for applied physics. 4) Quantum Information and Quantum Optics Strongly supported as a FOM Concentration group an integrated effort on quantum information processing is established in Delft and Leiden. The ambition is to develop techniques and theory and to execute experiments on quantum information processing using semiconducting quantum dots, superconducting circuits, and optics. It is a challenging engineering goal with many practical and theoretical questions, all strongly related to quantum informatics. Several approaches are studied simultaneously using single-photon-detection, superconducting devices, GaAs quantumdots, as well as semiconducting nanowires and spin-dependent defects in diamond. In addition apart from quantum-technology experiments fundamental aspects of quantum mechanics are tested. The quantum computation effort is led by Spinoza-winner Leo Kouwenhoven and includes professor-emeritus Hans Mooij. The quantum optics effort involves researchers such as Dirk Bouwmeester, jointly at UCSB and Leiden, who is a recipient of a Marie Curie Excellence Grant. 5) Universe physics: theory and instrumentation The interplay of the major physical forces gravitation, electromagnetism and the nuclear forces manifest themselves dominantly in the evolution of the universe. Thanks to advanced instruments we now know very accurately the fluctuations in the cosmic microwave background leading to immense theoretical challenges (dark matter and dark energy) as well as a desire for innovative new experiments. It also has led to a renewed intensity of collaboration between astronomers, astrophysicists and instrument-developers. Casimir researchers based in Leiden focus on the theory of inflation in order to embed it in the microscopic theories (supergravity and string theory). They are eagerly awaiting results from the Planck satellite, which was launched in May 2009, together with the Herschel Space telescope, which is partly based on device-development executed within Casimir at Delft. The latter instrument will provide insight into the evolution of galaxies and will for the first time be able to detect the availability of water elsewhere in the universe. These results will be complemented with the Atacama Large Millimetre Array (ALMA) in which Delft researchers are strongly involved as well. This theme is evolving towards a stronger presence within Casimir through Ana Acachurro and Koenraad Schalm, whereas the research and development of new sensitive sensors interacts with the Leiden Spinoza-winning astronomer Ewine van Dishoeck, as well as the Netherlands Institute of Space research (SRON).

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6) Dynamic Complex Systems Next to the study and manipulation of individual objects, most prominent in the field of nanotechnology, the collective behaviour of many individual objects is a challenging field of study as well. Examples are the behaviour of granular material, self-assembly, fracture in materials and problems in hydrodynamics. In a combination of numerical methods with the development of new concepts many new experiments have appeared. Many of those problems are directly related to major engineering disciplines and some arise directly in an industrial context, most notably with (bio-)polymers. This theme used to be led by Wim van Saarloos in Leiden, founding Director of the Lorentz centre at Leiden and winner of the 1998 Descartes-Huygens prize. He is currently director of FOM and was succeeded in Leiden by Martin van Hecke. 1.3. Educational activities

Support of MSc students For students with an interest in a research career beyond the MSc phase, Casimir has established a special pre-PhD track within the existing MSc programmes Physics (in Leiden) and Applied Physics (in Delft). The track is funded by the Dutch Research Council (NWO) through a competitive programme for Graduate schools. It consists of a particular set of courses and research experience in more than one department. A selection takes place for entrance into this track. For a limited number of students within this track, a PhD position is guaranteed. This so-called prize-PhD position is co-funded by NWO, TU Delft and Leiden University independent of specific research funding. Students apply to these positions by writing a research proposal themselves. Other students within the Casimir pre-PhD track will have excellent PhD job prospects. More information can be found on the Casimir website.

Support of PhD students The Casimir Research School organizes workshops and offers special graduate and advanced graduate courses. Casimir PhD students are required to acquire 15 ECTS credits in graduate education during their PhD. This number is chosen as to allow PhD students to attend at least two graduate courses and several workshops, concentrated mostly in the first years of their PhD project duration. Casimir uses the following formats for its educational activities: • • • • •

Graduate courses throughout the year Personal development courses One-week Casimir schools The bi-annual Casimir Science days A bi-annual Spring School for PhD students and post-docs only

Each PhD student has its own educational plan, detailing the workshops and courses to be attended. The PhD supervisors coach the students in drawing up and updating this plan, and monitor progress in an informal and formal way. Fully in the spirit of Hendrik Casimir, the research school aims to provide PhD students with more than just training for a specific subfield of physics. Personal development courses are part of the educational programme, too. These courses are offered by the participating universities and the funding agencies FOM and NWO.

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A full list of courses can be found on the Casimir website. They cover topics such as presentation skills, scientific integrity, time management and business orientation. In addition, Casimir provides PhD students with financial support for visiting summerschools or winterschools, which would otherwise not be funded. Participation in these schools is intended to broaden and deepen the student’s background in the science of their research-field. The maximum support is 1000 Euro per PhD student. Finally, Casimir offers PhD students support in printing their thesis. This is done by providing an ISBN code and by sharing experiences in the printing and publication process. Casimir has arranged with both participating institutions a uniform policy of printing costs-reimbursement. 1.4. Organization In 2010, the Casimir organization consisted of the following persons: Scientific director Prof.dr.ir. T.M. Klapwijk Co-director Prof.dr. J. Zaanen Casimir Board Prof.dr. J. van Ruitenbeek Prof.dr.ir. H.S.J. van der Zant Prof.dr. J.W.M. Frenken

Casimir Education Committee Dr. C. Danelon Dr.ir.R. Hanson Dr. T. Oosterkamp Prof. H. Schiessel

Casimir PhD platform Joris Berkhout (Leiden) Floris Braakman (Delft) Marijn van Loenhout (Delft) Jennifer Mathies (Leiden) Jan van Ostaay (Leiden) Jos Seldenthuis (Delft) Casimir scientific advisory • Prof. B. Noordam, Professor of Physics, University of Amsterdam and ASM Lithography (together with P. Gosling he is the author of “Mastering your PhD; Survival Success in the Doctoral years and beyond”, Springer, Berlin/Heidelberg, 2006) • Prof. J.P. Kotthaus, Professor of Physics and former Director of the Center for Nanoscience, München, Germany • Prof. M.R. Beasley, Professor of Applied Physics and Electrical Engineering and former Dean of the School of Arts and Sciences, Stanford University, Stanford, USA • Prof. Zheng-Yu Weng, Professor of Theoretical Physics, Tsing-Hua University, Beijing, China • Prof. P.B. Littlewood, Professor of Physics and Director of the Cavendish Laboratory, Cambridge University, Cambridge, UK (from 1 February 2009) • Prof. Jonathan Howard, Max Planck Institute for Molecular Cell Biology & Genetics and Professor of Biophysics, Dresden University of Technology

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2. People 2.1. PhD students At the end of 2010, 165 PhD students were part of the Casimir Research School. 31 PhD students finished their project in 2010 and published their results in a thesis. Staff* Leiden: Delft: Total:

41 40 83

Postdocs

PhD students

Theses

34 43 77

79 85 165

12 18 31

PhD Dropouts 0 4 4

* Number of Casimir staff members (not FTE) including part-time appointments and retired staff members still active in our research community Year

Theses completed

2005 2006 2007 2008 2009 2010

23 28 22 17 25 31

Average time to thesis approval (years) 4.54 4.26 4.49 4.02 4.20

Graphical representation of the total time-to-thesis (in years) for students receiving their degree in 2010 (in blue) and the duration beyond end-of-contract (purple).

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2.2. Recruitment of PhD students To further enhance the quality of our research it is very important that each vacant PhD position can be filled with a talented candidate. Until recently the recruitment of PhD candidates occurred mostly through advertising for individual positions and selection by the intended supervisor who has acquired the needed budget. This method is time-consuming, leads to considerable delays and in practice to fairly strong internal competition. Ideally one would like to become attractive, as an institution, for the best possible candidates at least out of the population of Europe. But to achieve that goal through the method of individual recruitment is not very efficient. Therefore it becomes important to consider methods to recruit candidate PhD students collectively. Various laboratories in Europe are experimenting with this methodology, which is common in the UK and at US universities. As part of the NWO Graduate school programme Casimir is also experimenting with methods to recruit collectively. In a recent internal evaluation it has become abundantly clear that the various project-leaders are very interested in extra options to get access to qualified candidates. An important aspect is the choice to recruit students with a BSc diploma to enter the MSc studies, which allows a careful evaluation on the basis of performance in the MSc studies, and recruitment of PhD students who already have an MSc diploma from elsewhere. In the latter case there is little time to adjust to a different culture and different living circumstances. The following steps have been taken: 1. From November 2009 to February 2010 a selection committee consisting of Carlo Beenakker, Joost Frenken and Nynke Dekker assessed a total of 179 open applications. 14 candidates were brought to the attention of the group leaders. Four candidates were of sufficient interest to some group leaders and two candidates have obtained a PhD position within Casimir. The overall conclusion of this first round was that it had not yet provided a reservoir of sufficiently good potential candidates in comparison to those who entered through the traditional pathways. 2. Various PhD positions have been filled with students who entered our universities as MSc student from elsewhere and turned out to be suitable candidates for a PhD position within Casimir. The conclusion is that it is very important to continue to get scholarships for gifted students from elsewhere to enroll for the MSc study. 3. The Leiden Faculty of Sciences has recently introduced the Leiden/Huygens fellowships for Astronomy, Physics and Mathematics to attract gifted PhD candidates (http://huygens.researchschool.nl/). In total five excellent candidates were offered a fellowship and have accepted, two of whom are in the physics department. 4. As follow-up of the pilot run in 2009/2010 (see 1) the Casimir office has made available the methodology of uploading applications to a database to support group leaders in bringing suitable candidates to the attention of colleagues. 5. Finally, Casimir also participates in a programme set up jointly by KNAW and NWO with the Chinese Academy of Sciences to identify excellent Chinese candidate PhD students with interest in PhD studies at Casimir. This programme is in an early phase

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but may certainly contribute to the increased awareness of the PhD research opportunities in physics in Delft and Leiden. 2.3. New PhD cohort 2010 New PhD students

Male/female

MSc degree from

Male

Leiden Delft

Female

Nationalities

Other Dutch Abroad Dutch Universities

Other Outside Europe Europe

Delft

Leiden

Total

2.4. Staff developments Vincenzo Vitelli (Soft Condensed Matter Theory, Leiden) I obtained my BSc in theoretical physics from Imperial College London in 2000 with a thesis on the thermodynamics of quantum entanglement, supervised by Martin Plenio and Miles Blencowe. As an undergraduate, I was awarded a Nuffield Foundation Fellowship to spend a semester at MIT doing research on self-organized criticality with Tom Chang and Dimitri Vvedensky. I then moved to Harvard where I completed my doctorate in Physics under the supervision of David Nelson in 2006. I was awarded the Callan Prize for my PhD work on the interplay between geometry and superfluid order. From 2006 to 2009 I was a post-doc in the soft matter theory group at the University of Pennsylvania. There I started work on smectic liquid crystals with Randy Kamien and on non-equilibrium properties of amorphous solids with Andrea Liu and Sid Nagel. In 2009, I was a Professeur Invitè at Paris 7 and a Feinberg Foundation Fellow at the Weizmann Institute of Science. Since January 2010, I have been an Assistant Professor at the Institute Lorentz for Theoretical Physics in Leiden. Erik Bakkers (Quantum Transport, Delft) I obtained my PhD in 2000 at the Utrecht University under supervision of Daniel Vanmaekelbergh working on the charge transfer between colloidal semiconductor quantum dots and metal electrodes. I studied the rate of photoinduced electron tunneling between CdSe nanocrystals and a gold substrate by time-resolved electrochemical techniques as a function of the distance. I spent the final part of my PhD at TUD in the group of Leo Kouwenhoven to study the electronic properties of single quantum dots by using a low-temperature STM set up. Directly after my PhD I started to work at Philips Research Laboratories in Eindhoven on a project called ‘nanoelectronics’. The focus was on semiconducting nanowires for several applications, such as light-emitting diodes, transistors, and chemical sensors. Important aspect has always been to figure out the details of the nanowire growth mechanism. We found a way to grow III-V wires directly on silicon substrates, a 13

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regime in which hollow semiconductor tubes are formed, and we fabricated twinning superlattice structures. From the early start there was a fruitful collaboration with Leo Kouwenhoven who studied electronic transport through the wires. From begin of 2010 I joined both the Eindhoven and Delft University of Technology. In Eindhoven we are setting up equipment for the materials synthesis. One goal is to make efficient, but relatively cheap solar cells. In Delft opto-electronic nanowire devices are fabricated and studied at the level of single electrons and photons. The wires offer unique possiblities to combine different semiconductor materials, such to enhance the functionality. Therefore, these structures are very promising in the fields of quantum information processing. Leonardo DiCarlo (Quantum Transport, Delft) I am an Argentine-born, US-educated assistant professor who joined the Quantum Nanoscience Department at TU Delft last October. My research focuses on superconducting quantum circuits, with applications in quantum information processing. My first steps in research were as an undergraduate in Electrical Engineering and in Physics at Stanford University (1994-2000), where I participated in Gravity Probe B, a NASA-Lockheed project aiming to test Einstein’s general relativity theory through precise measurement of tiny changes in the rotation of superconducting gyroscopes in an earth-orbiting satellite. There I found a passion for feedback control while working with engineers whose goal was to levitate these gyroscopes electrostatically, a daunting task owing to the mission’s specifications. At GP-B I learned that cutting-edge science demands excellent engineering, and the pursuit of both has guided my research career of 15 years. At the start of my PhD in 2001, no field seemed a better match for basic science and engineering than experimental quantum computing. Believing strongly that a quantum version of a computer will ultimately be an integrated electrical circuit, I went to Harvard to learn about quantum circuits. I pursued a PhD with Prof. Charles Marcus, graduating in 2008 with a thesis titled “Mesoscopic electronics beyond DC transport”, which combined shot-noise measurements, electrometry and photovoltaic effects in semiconducting nanostructures (GaAs/AlGaAs and graphene). During this exciting period I learned about many quantum mechanical signatures in electronic transport through devices of mesoscopic scale at mK temperatures, though I wasn’t quantum computing yet. That was left for my post-doc in superconducting quantum circuits at Yale, where guided by Profs. Robert Schoelkopf and Michel Devoret, I fell under what I call “the spell of the Hamiltonian”. The possibility to design superconducting circuits with well known Hamiltonians whose parameters can be tuned and calibrated to better than .01% proved irresistible. Over three years, we developed a programmable two-qubit processor that actually executes rudimentary but truly quantum algorithms, and extended entanglement from two to three qubits, both firsts for circuit-based quantum computing. The “spell of the Hamiltonian” remains the basis for the research I am undertaking with my young and growing group at TU Delft. Looking forward, it is evident that 14

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further progress in quantum computing demands that we go beyond open-loop control. Here, we are developing a toolbox of quantum measurements and control techniques whose integration will allow us to close the feedback loop needed to perform quantum error correction. I look forward to the many exciting spin-offs that will likely result from this pursuit! Liberato Manna (Molecular Electronics and Devices, Delft) I received my master degree thesis in Chemistry in 1996 from the University of Bari (Italy) and my PhD degree in Chemistry from the same University in 2001. As part of my PhD project I worked at UC Berkeley, where I developed the synthesis shape-controlled semiconductor nanocrystals, which relied on specific surfactants to promote or depress the growth rate of selected nanocrystal facets and additionally on manipulating the relative stability of crystallographic phases at the nanoscale during nucleation and growth. After receiving my PhD degree, I continued work in Berkeley with a postdoc position at the Lawrence Berkeley Lab. The project involved the development of nanocrystals with anisotropic shaped and with controlled optical properties, for example enhanced fluorescence quantum yield and optical stability, but also with capability to confine carriers in separate regions of a single nanocrystal. The latter could be achieved for example by synthesizing nanocrystals made of sections of different materials and/or crystal phases. In 2003 I returned to Italy to an independent position as Junior Scientist at the National Nanotechnology Laboratory (NNL) in Lecce. At that time, I was also a visiting scientist at the Center for Nanoscience in Munich, where I also developed methods for stabilizing in aqueous phase various types of colloidal nanocrystals. I later became head of the Nanochemistry Division in Lecce (2006). My work in 20032009 was on various fronts: from the synthesis of nanocrystals of various materials and combination of materials (the so-called â&#x20AC;&#x153;hybrid nanocrystalsâ&#x20AC;?), to the study of their assembly, to their exploitation in optics, photonics, photovoltaics and biology. In 2009 I then moved to the Italian Institute of Technology in Genova (Italy), as director of the Nanochemistry Department. The focus of my research is now on the exploitation of nanostructures, grown by chemical approaches, in energy-related fields and in medicine. Since November 2010 I am part-time professor at the Department of Quantum Nanoscience, of Delft University of Technology, in the Molecular Electronics and Devices group. My broad project at TU Delft is the integration of colloidal nanostructures in nano-electronic devices, as well as the study of the structural/physical properties of individual nanoparticles and their assemblies with advanced techniques. Sander Otte (Molecular Electronics and Devices, Delft) The Leiden-Delft connection has been a recurring theme throughout my (short) scientific career. In 2002, while finishing my Master work in Leiden on the electrical conductance of metal point-contacts, my first encounter with Delft was through a short research internship under Cees Dekker, who was then still quite actively working on carbon nanotubes. It was during this internship that I first learned how to operate a low-temperature scanning tunneling microscope (STM), which would become the main tool for my later experiments. 15

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The following year I returned to Leiden to start my PhD research in Jan van Ruitenbeek's group. I initially continued my earlier work on point-contacts, but gradually developed an increasing interest in magnetism and particularly in the possibility to address the spin state of an individual atom. So with that goal in mind I slightly changed topics and soon I was constructing a sub-Kelvin STM system optimized for single-atom spectroscopy. During these years I followed several courses in both Leiden and Delft through the recently founded Casimir Research School, to further develop my knowledge on spin state control. In 2006 I had the opportunity to join the STM team at the IBM Almaden Research Center in California, led by Andreas Heinrich, as a part of my PhD project. The year before, his team had just managed to observe the spin states of individual magnetic atoms: precisely the experiment I had been dreaming of. For six months I helped them improve the technique with great success. The discoveries we made during that short period form the basis for my current research program. After obtaining my PhD in Leiden in 2008 I moved to Maryland for a Postdoc in Joe Stroscio's group at NIST. Here I was involved in the development of an even more advanced STM system: one that was capable of operating at 10 mK. We managed to demonstrate the spectacular spectroscopic energy resolution enabled by this machine through ultra-sensitive measurements on Landau levels in epitaxial graphene. Finally, in 2010 I joined the Department of Quantum Nanoscience in Delft as an assistant professor, where I intend to expand my single-atom spin research to larger, integrated atomic structures. Again the Leiden-Delft link came into my life as I took on the role of coordinator for the Casimir Pre-PhD Master Program. At the end of the day, I just can't seem to make a final choice for either of the two universities. And why should I? Gary Steele (Quantum Transport, Delft) I was born in Toronto, Canada, and studied honours physics at McGill University in Montreal before moving to MIT for my doctoral studies to study condensed matter physics. At MIT, I worked with Ray Ashoori, where I studied the Quantum Hall effect using a low temperature scanning capacitance microscope. By manipulating the electron gas under the tip by applying DC voltages to the tip, and probing its compressibility by measuring its AC capacitance to the tip, we imaged incompressible strips and localized states in the Quantum Hall liquid. After my PhD, I moved to Leo Kouwenhoven’s group in Delft to work on carbon nanotube quantum dots. During my postdoc, I developed a clean suspended nanotube device that was significantly different from those made before. Essentially we “did the fabrication backwards”, starting with electrodes and gates on the substrate, and growing nanotubes over top in the last step. The crucial development is that the carbon nanotube was not in any way exposed to the fabrication process, preserving the characteristics intrinsic to their defect-free molecular structure. This technique led to a breakthrough in both the electrical and mechanical properties of carbon nanotube devices. 16

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I joined TU Delft in July 2010 as an assistant professor in the Quantum Nanoscience department. The theme of my research will be Microwave Nanomechanics. Our focus will be to develop high frequency nanomechanical devices based on unconventional materials, such carbon nanotubes and graphene, and to detect their motion directly using RF and microwave measurement techniques at cryogenic temperatures. The combination of high frequency devices and low temperatures will allow us to explore the effects of quantum mechanics on their mechanical motion, and by expanding our current DC measurements to the microwave regime, we can directly listen to their quantum vibrations. Within the department, I collaborate with Val Zwiller on quantum optoelectronics with carbon nanotubes, and with Leo Kouwenhoven on nanotube spin qubits. I recently started a new project with Leo, which we call a “nano-flipchip microscope”. Our goal is to completely decouple nanofabrication from the material itself. We will do this by positioning gates, sensors, and other devices made on a separate chip nanometers above a pristine unprocessed material, such as a high mobility 2DEG. We will first use it to study the 5/2 FQH state, but when developed, it will have a much broader application for making nanostructures in new materials, such as suspended graphene or topological insulators, and for coupling microwave electronic circuits to nanomechanical resonators. Bertus Beaumont (Bionanoscience, Delft) interviewed by Delft Integraal “Suppose you come face to face with a distant ancestor and must fight to find out who is stronger.” This proposition is raised by evolutionary biologist Dr Bertus Beaumont, who conducts experiments with bacteria. “Bacteria enable you to compare two very distant generations. I keep a suspension of bacteria in the freezer and let the rest of the colony evolve. It’s sometimes possible to move on eight generations in a single day. After a few hundred generations, I can compare the bacteria with their distant ancestors.” Bertus Beaumont (36) received his doctorate in molecular biology from VU University Amsterdam in 2004. He went on to work as a postdoc researcher at the University of Auckland and, with a NWO Veni research grant, under Professor Paul Brakefield at Leiden University. “Everything we know about evolution we know through comparative research,” Beaumont states. Comparisons between fossils, comparisons between fossils and current life forms, and between existing species: the entire theory of evolution was developed on the basis of comparative observations. But scientists also like to conduct experiments to test their hypotheses. Current technology makes that possible, although patience and dedication remain essential given the number of successive generations required. Beaumont’s longest experiment involved studying 500 generations. In human terms, that would take 15,000 years, taking us back to the middle of the last Ice Age. “I’m not concerned with showing that bacteria adapt to changing conditions,” Beaumont says. “We know that already. Rather, I am interested in how a complex mechanism such as a bacterium can adapt through random changes to its DNA. That is the real question.” The DNA of the bacteria used in his research has 6 million genetic letters. On each division, the DNA is copied, whereupon there is a 1 in 10,000 chance of a random mutation. Most mutations have no effect whatsoever. 17

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Some make the bacteria grow less quickly, whereupon the mutation itself eventually becomes extinct. Very occasionally, a bacterium will start to grow more quickly, whereupon the entire population eventually has the same characteristic. “In that case, they have evolved one step,” Beaumont summarises. In Delft, Dr Beaumont is to research the evolution of flagella. A flagellum is a sort of tail-like projection which certain bacteria have, and which they use to move around. It is rather like a tiny outboard motor. By studying only the mutations, which affect the flagellum, Beaumont hopes to gain a greater understanding of the evolution of such biological nanomachines. He will also use that knowledge to make certain planned modifications. Evolution designs blind: a scientist likes to know what he is doing. Christophe Danelon (Bionanoscience, Delft) interviewed by Delft Integraal His room in the Applied Sciences faculty building is freshly painted and virtually empty. He formulates his words carefully in English with a slight French accent. Following previous appointments at the university of Toulouse and at the EPFL in Lausanne, Dr Christophe Danelon (33) has now arrived in Delft. He considers it exciting to be part of a new department at a university with such a high reputation in physics and nanoscience. He sees Delft as the ideal setting for his next great project: unravelling the origin of life itself. Most of Danelon’s publications in recent years have been about ion channels. “They are poreforming proteins in the membranes of living cells,” he explains. “They are of major importance in regulating the ion flows, in admitting nutrients and getting rid of waste products.” With his arrival in Delft, Danelon’s research will take ‘a slight change of direction’. Here, he will try to construct protocell models and artificial minimal cells based on the self-assembly of lipid vesicles, called liposomes. “It’s good to set the ambitions high and ask the big questions,” he says. “Even minor progress in this area will be a major achievement,” he adds modestly. One of the most important questions for Danelon is how a myriad of biochemical molecules can remain close enough to each other to efficiently react. Perhaps certain minerals played a part in connecting them thus avoiding their dilution in the vast ocean, or perhaps a membrane to hold the components together is required. Hopefully, five years of research will increase his knowledge in this regard. The underlying question, the molecular origin of life itself, is big enough to keep him busy for the rest of his life, Danelon believes. Alongside this fundamental research, Danelon is also working on the use of liposomes as drug delivery vectors and as nanofactories for the synthesis and targeting of proteins of medical value. David Grünwald (Bionanoscience, Delft) interviewed by Delft Integraal If you ask Dr David Grünwald what his specialism is, he will answer, “Visualising molecules within the nucleus of a living cell.” It takes a few moments for the significance of this simple statement to sink in. You can’t see molecules with an optical microscope, can you? Surely they’re far too small? Seeing molecules within a living cell would enable you to watch various processes of life as they happen. Apparently, it is indeed possible. Through observation, Grünwald has established that it takes 200 milliseconds for an RNA molecule to permeate the cell wall on its way 18

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out, while the average protein finds its way in within 5 to 10 milliseconds. “The best part was seeing how an RNA molecule would try various nuclear pores one after the other. It waits for a second or so at one, then moves on until it finds a pore that will actually let it in.” Grünwald (34) can tell fascinating stories about the world which only a very few people, himself included, can make visible: the living complex that is a cell. Having studied law in Frankfurt for one year (“I wanted to do something good for mankind, but studying law is not easy when you’re dyslexic”), he went on to study physics and was then introduced to biophysics. “How does life work? Not just as a description, but in the quantitative sense. How does a cell function and what principles keep life going? That question grabbed me and has never let me go.” Since then, his glittering scientific career has seen him working at institutes of biophysics, biochemistry, molecular biology and medicine. After four years as a postdoctoral researcher at the Albert Einstein College of Medicine in New York, Grünwald decided it was time to return to physics. With the help of technicians and PhD students, he is now building his own microscope, which ‘How does life work?’ is specially designed to follow individual molecules. The crucial factor is ensuring the maximum possible light. The microscope, which is actually an open arrangement with lasers, a large-aperture lens, a colour-separation mirror and two ultra-sensitive cameras, will enable him to observe the interaction between various molecules with a precision of up to 30 nanometres. Fluorescence labelling can then be used to determine whether a virus is able to penetrate a nuclear pore, or the point at which a particular drug binds itself to the cell and how this affects the functioning of that cell. “All this happens within milliseconds,” Grünwald says. “These processes are taking place in all our cells. We can see them happening. Not just on the outside of the cell – its ‘envelope’ – but in the nucleus itself.” Sander Tans (Bionanoscience, Delft) interviewed by Delft Integraal Professor Sander Tans had his ‘eureka moment’ soon after gaining his doctorate in 1998. At the time, he and his supervisor, Professor Cees Dekker, were writing articles for leading journals such as Science and Nature about the electric charge of carbon nanotubes. “We were able to measure the electricity in individual molecules. That was fantastic! But the nanotools we developed could also be used for other things. They gave us a glimpse into the world of biological processes, all of which are very much more complex than anything we had studied so far. Motor proteins which repair DNA, for example, or which can move in and out of a cell along a special route. They are just as complex as any manmade motor, but at the nanometre scale. When you see this for the first time you think, ‘Wow! How is that possible?’” This prompted Tans to opt for a career in biophysics, a discipline which he now practises as group leader of the biophysics laboratory at the Foundation for Fundamental Research on Matter’s Amolf (Atomic and Molecular Physics) laboratory in Amsterdam, and since 1 January as part-time professor in the new bionanoscience department at TU Delft.

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Tans’ specialism is applying the physicist’s perspective to biological topics. His research field extends from the individual molecule to the level of cells and their evolution. Tans’ ambition is to help develop a more quantitative biology, a science based on testable hypotheses, experiments and predictable results. As an example, he cites his recent research into the mechanisms of evolution, in which a population of bacteria was ‘taught’ a new way of reacting to bacteria through an evolutionary process over some 100 generations. The researchers demonstrated that the bacteria adapted to a variable environment (in which antibiotics were sometimes present and sometimes not) through a combination of random ‘How does complexity evolve?’ mutations and Darwinian selection. “This was the first time that a new reaction was instilled by the process of evolution,” Tans reports. “We were also able to show what determines the success of this process. The research demonstrates that you can only really understand a process if you can reproduce it.” Tans’ follow-up research is concerned with the evolution of complex characteristics. Most biological processes involve several different proteins. How do they change under the pressure of evolution? Or as Tans puts it, “How does complexity evolve? Can we distil some simple basic principles?” It has long been the physicist’s dream that enough study and research will reveal the logic, which underpins our chaotic and complex reality. That dream has not yet been entirely realized in the field of physics itself. Sven Rogge (Appointed full professor at the University of New South Wales in Sydney, Australia) After an MSc degree at the University of Karlsruhe (Germany), a PhD at Stanford University (US), Sven Rogge moved to Delft to become an assistant professor (2003) in the group of Teun Klapwijk. Six years later he was promoted to associate professor. Sven was one the founding fathers of the new Photronic Devices section in 2008, together with Huub Salemink and Jaap Caro. Sven’s background is in the field of Silicon quantum electronics, where he contributed very creatively to a number of projects. In addition, he spent a major effort in setting up a successful research-line on single-dopant transport and he was one of the initiators of the work on photonic crystals in Delft. We are proud that he has been offered the prestigious professorship at the University of New South Wales starting January 2011 and wish him all the best.

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3. Education 3.1. Casimir PhD courses 2010

Casimir PhD Course “Nano- and Mesoscale Electronics” Subject area: Course on the physics of electron transport in nano- and mesoscale structures and devices Number of participants: 13 participants passed the final exam and received the Casimir Certificate. Lecturers: Gerrit Bauer, Yaroslav Blanter (coordinators), Carlo Beenakker and Jan van Ruitenbeek. Description: Electron quantum transport in small structures is a large field of research. Especially for newcomers it is difficult to get an overview over the rapid developments on many fronts. The present course is intended to assist Ph.D. students and postdocs in the process to get to grips with the field by in-depth lectures on crucial concepts and ideas starting from the roots of mesoscopic transport physics with phenomena like quantum point contacts and Anderson localization. Advanced topics such as superconducting and magnetic correlations in nanoscopic circuits, spintronics, molecular electronics, topological insulators, and graphene will be addressed as well.

Casimir PhD Course: “Biology for Physicists” Subject area: The aim of the course is to introduce the participants to the basics of cell and molecular biology. Books: Alberts B., et al., Essential cell biology, 3rd ed., Garland Science, 2009; Alberts, B. Molecular Biology of the Cell, 5th ed., Reference Edition, 2007. Number of participants: 10 participants passed the final exam and received the Casimir certificate. Lecturers: Aviva Joseph, David Grünwald, and Christophe Danelon (TU Delft). Description: Key subjects include the genetic information, protein structure and function, lipids and cell membranes, signaling pathways, the cell cycle, and the immune system. More advanced topics specific to the individual needs or interests will be addressed as well during discussions.

Casimir PhD Course “Electronics for Physicists” Subject area: The course is a must-have for PhD students and post-docs interested in experimental physics Number of participants: 25 participants passed the final exam and received the Casimir certificate Lecturers: Dr V. Zwiller, R. Schouten Description: We will study electronics with a strong focus on practical applications. After reviewing the basics of passive and active components and their practical limitations, we will focus on circuit simulation, systematic troubleshooting and opamp circuits. Signals, noise and interference problems (and solutions!) will also be an important topic. We finish with an overview of microwaves and various measurement techniques, and a day on advanced use of electronic measurement equipment. Several case studies from the physics lab will be used throughout the course to make the theory come alive.

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Casimir PhD Course “Frontiers of Measurement” Subject area: Course on the state of the art in quantum measurement and the detection of weak signals in various area of physics Number of participants: 20 participants passed the final exam and received the Casimir certificate Lecturers: prof.dr G. Nienhuis, prof.dr M. Orrit, dr S. Rogge, prof.dr J.W.M. Frenken/Dr.ir. T. Oosterkamp. Description: The detection of weak signals is an important aspect of modern physics at large. Ultimate limits to these detections are set by the quantum nature of matter, of light and of charge. The course aims at a discussion of these limits as they are encountered in experiments, in various domains of physics. The resulting discussion of the frontiers of measurement will be divided in four blocks, each one with a different lecturer. In the first block quantum detection limits will be discussed, with quantum noise and quantum non-demolition measurements as special topics. The second block deals with interferometric methods for the detection of weak signals, with an emphasis on photodetection. In the third block the central topic is charge detection, where various types of amplifiers are required. The fourth block is devoted to force measurements, with an application to imaging techniques that combine atomic-force microscopy with magnetic resonance. 3.2. Casimir Spring School – Arnemuiden From 14 June till 16 June 2010 the Casimir Spring School took place in Arnemuiden (Zeeland), very close to the sea. The location was different than before, but the idea behind this special event was the same as before: “Sun, sailing and science”. This PhD/postdoc-only event was organized by the Casimir PhD platform. During the Spring School three focus sessions where held with presentation by Casimir PhD-students and postdocs related to major research topics covered within the Casimir Research School. Each session included a introductory lecture, given by international experts: • Biophysics/ soft condensed matter – Tom Duke; • (Quantum) optics – Tobias Kippenberg; • Solid State Physics – Thomas Ihn. The evening sessions consisted of a talk on the HERSCHEL Space Observatory by Rens Water and a workshop by coach Hans Hoogerdijk. In accordance with the motivation of the Spring School of stimulating the interaction between PhD students the workshop dealt with ‘Intercultural Competence’. A poster session was also present. The best poster and oral contribution were awarded with a prize. The prize for the poster was won by Hannes Bernien with his poster entitled ‘Coherent optical control of single NV centers in diamond’ and the prize for best oral contribution was won by Louk Rademaker with his talk on ‘Flux Quantization in Double Layer Exciton Superfluids’. During the Spring School there was enough time for interaction between the PhDstudents and the postdoc, either during the focus sessions and poster sessions or during the social activities. For starters, the weather proved to be good enough for a wonderful afternoon filled with sailing, at which even some of the introductory lecturers attended. Furthermore, the schedule was organised such that one could watch the Soccer World Championship matches. And finally, at the end of each day the bar opened.

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The next Casimir Spring School is scheduled for 2012. It is most likely that it will take place again in Arnemuiden due to all the positive feedback received on the location of this Spring School. - Jan van Ostaay, member of the organizing PhD Platform 3.3. Les Houches Summerschool Frontiers of Condensed Matter (France) This school took place from the 29th of August till the 10th of September, organized jointly by Casimir, the Ecole Doctorale de Physique de Grenoble (France) and the Ecole Doctorale de Physique et d'Astrophysique (PHAST), Lyon (France). Five students from Delft/Leiden participated in the summerschool, situated in a small campus in the French Alps with a beautiful view on the Mont Blanc. The programme of the school contained five main 4,5 hour courses and lots of 1,5 hour seminars from specialists. The subjects of the five main courses were: 1. Molecular magnetism: from classical to quantum nanomagnets, W. Wernsdorfer (Grenoble) 2. Mesoscopic transport and superconductivity, J. Meyer (Grenoble), T.M. Klapwijk (Delft) 3. Quantum information processing with single spins, L.M.K. Vandersypen (Delft) 4. Examples of multi-scale modelling, J.L. Barrat (Lyon) 5. Quantum optics with solid-state artificial atoms, J.M. Gérard (Grenoble) The seminars treated topics such as ‘Bose-Einstein condensation of quasi-particles in a solid-state environment’, ‘Transport and noise at finite frequency in nanostructures’ and ‘Quantum information processing with superconducting circuits’. Examples of participant feedback:

“I met many young and talented scientists working in various fields. We had a lot of inspiring discussions and jokes. Another most interesting event would be hiking in the mountain, which not only challenged my body but also trained my soul.” - Yanting Chen (PhD-student, Delft) “In several ways, this summer school was worthwhile to join. Recently, I started my PhD project on superconducting thin films. With a background in a slightly different field, this school formed a good starting point to learn about different subjects in the so-called nanosciences. Although most topics were not directly relevant for my project, it is very helpful to have knowledge about related work done in the physics of condensed matter. Besides interesting lectures and seminars, the interaction with other PhD students and graduates is very nice and profitable. I would highly recommend this summer school to early PhD students who like to go behind the scope of their own work. This school is also recommendable for excellent graduates who like to meet PhD students from various groups in order to get a taste of how it is to do a PhD. Les Houches is a very attractive place. I believe that an environment like this stimulates the concentration during the courses and can open your mind for new ideas for your research.” - Pieter-Jan Coumou (PhD-student, Delft) “This summer school was well worth the time and I can recommend it to anybody studying Physics. Due to the location and weather the atmosphere was very relaxed and inspiring. Most lecturers made an effort to introduce the subject properly in the first 90 minutes. This way it was possible to follow the more complicated problems in the later sessions. At the end of the respective sessions we were presented with a number of experimental results and the interpretation in regard to the theoretical 23

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framework. Since I already started my PhD my personal goal was to get deeper insight of a number of subjects. I think this goal was achieved as the time devoted to each subject provided enough knowledge to give you a good starting point. Including the forums, which provided further discussion and questions, the school provided a nice mix of subjects. Contacts made during this summer school might proof to be useful in the future of my scientific career.” - Christian Glass (PhD-student, Leiden) “As a pre-PhD Casimir master student I was really glad that I was offered the opportunity to join this school: it gave me the chance to have a better look at which research field I might want to perform my PhD, allowed me to ask in a more informal setting what a PhD is like and I could speak with other master students about their thoughts on where to go next. I hope that next year’s students will be allowed this advantage as well!” - Tim Baart (MSc student, Delft)

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4. Research

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VICI grant for Nynke Dekker (Bionanoscience) Department R3 receives STW Valorisation Grant for luminescence dating

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Leiden student Xin Liu co-author of Science publication on friction in one-atom layers Quantum Nanoscience graduate Rami Barends wins Rubicon grant Eduard Driessen and Timon Idema winners at NTvN contest Royal decorations for Jo Hermans and Jan Schmidt

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Leiden Physics Colloquium named after professor Joan van der Waals

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NeCEN consortium receives 6.1 million euro to establish nanoscopy lab for life sciences in Leiden

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Casimir PhD student selected in National ThinkTank 2010 VENI grant for Akira Endo (Quantum Nanoscience)

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Festive opening of the new Leiden Center for Ultramicroscopy Inaugural symposium of the newly established Department of Bionanoscience in Delft Publication in Nature on RNA-molecules exiting cell nuclei, by David Grünwald (Bionanoscience) Quantum Nanoscience researchers publish on the protection of single spin quantum states in Science VENI grant for Wolfgang Löffler (LION) Hendrik Casimir prize winners announced by Casimir ERC Advanced Grant for Joost Frenken (LION) Delft Bionanoscience researchers reveal new method to dermine DNA torque in Nature Methods Science publication: LION researchers succeed in imaging single molecules by using a fata morgana effect ERC Advanced Grant for Henny Zandbergen (Quantum Nanoscience) VIDI grants for Andrei Parnachev (LION) and Leonardo DiCarlo (Quantum Nanoscience); VICI grant for Dirk Bouwmeester (LION) Jelle Brill and Cosma Fulga (LION) receive Shell Stipendium for Theoretical Physics Delft Bionanoscience researchers reveal in Nature Nanotechnology a new type of biological nanopore with potential for DNA analysis Casimir graduate Zorana Zeravcic (LION) awarded Faculty thesis prize Rubicon grants for Casimir graduates Jan-Willem van de Meent (LION) and Menno Poot (Quantum Nanoscience) Delft Bionanoscience researchers describe in Nature Physics how microcylinders resemble neurons

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New Bionanoscience Department at TU Delft officially started Three Marie Curie fellowships awarded to LION Michiel de Dood and Sander Zandbergen create an isolated conical singularity in photonic graphene, publish in Physical Review Letters In Photosynthesis Research, Leiden biophysicists Marcell Marosvölgyi and Hans van Gorkom demonstrate that the colour of solar cells is determined by energy costs Lorentz Center says goodbye to Wim van Saarloos with a lecture series and party PNAS publication: LION researchers discover new factors in aneurysm rupturing

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4.1. Timeline of research and grant highlights 2010 • • • •

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4.2. NeCEN/NIMIC In 2010, about 12.4 M! was raised in grants from NWO-groot and EFRO, whilst Leiden University initiated the construction of a dedicated building for NeCEN (to be finished early 2011). From the grants, two Titan Krios microscopes were purchases from FEI, after an international tender. One of the microscopes is dedicated to single-particle reconstruction and is equipped with a CS-corrector and an X-FEG, the other microscope is dedicated to electron tomography and is equipped with an energy filter. Both microscopes can be remotely controlled by internet protocols and allow loading up to twelve samples simultaneously. With these capabilities, a revolutionary new facility is being established in the Netherlands. Access to the microscopes will be organized in a similar fashion as access to other major shared infrastructures, such as synchrotrons, telescopes or neutron sources. 4.3. Casimir theses 2010 The PhD theses published in 2010 by Casimir PhD students are listed below. Casimir offers the possibility to act as publisher and provide thesis authors with an ISBN number without charge. The theses that made use of this special arrangement have an additional mentioning of the â&#x20AC;&#x153;Casimir PhD Seriesâ&#x20AC;? in this list. Martin C.: Charge transport through single molecules in two- and three-terminal mechanical break junctions Promotores: Dr.ir. H.S.J van der Zant & J.M. van Ruitenbeek. January 11, 2010 Casimir PhD series 2010-1 Stan R.: Hot-wiring azur ins onto gold surfaces Promotor: Prof.dr. T.J. Aartsma January 27, 2010 Casimir PhD series 2010-2 Flokstra M.: Proximity effects in superconducting spin-valve structures Promotor: Prof.dr. J. Aarts February 17, 2010 Casimir PhD series 2010-4 Habraken S.: Light with a Twist - Ray Aspects in Singular Wave and Quantum Optics Promotor: Prof.dr. G. Nienhuis February 16, 2010 Casimir PhD series 2010-9 Beekman C.: Strain, size and field effects in (La,Ca)MnO3 thin films Promotor: Prof.dr. J. Aarts February 25, 2010 Casimir PhD series 2010-8

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Lansbergen G.: Electron transport through single donors in silicon Promotores: Prof.dr. H.W.M Salemink & Dr. S. Rogge. March 9, 2010 Casimir PhD series 2010-6 Xia T.: Probing spatial heterogeneity in supercooled glycerol and temporal heterogeneity with single-molecule FRET in polyprolines Promotores: Prof.dr. M. Orrit March 25, 2010 Casimir PhD series 2010-3 Borghols W.: Li-insertion in nanostructured titanates Promotores: Prof.dr. F.M. Mulder & dr.ir. M. Wagemaker March 29, 2010 Casimir PhD series 2010-5 Xie H.: Charge transport at organic/dielectric and organic/organic interfaces Promotores: Prof.dr. H.W.M Salemink & Dr. A.F. Morpurgo March 29, 2010 Casimir PhD series 2010-7 Oostinga J.B.: Quantum transport in graphene Promotores: Prof.dr. H.W.M Salemink & Dr. A.F. Morpurgo April 27, 2010 Casimir PhD series 2010-10 Gabrulov A.: Fundamentals of rolling contact fatigue Promotores: Prof.dr. H.W. Zandbergen May 18, 2010 van Kouwen M.: Opto-electronics on single nanowire quantum dots Promotores: Prof.dr.ir. L.P. Kouwenhoven & Dr. V. Zwiller. June 28, 2010 Casimir PhD series 2010-12 Zeravcic Z.: Vibrations in Materials with Granularity Promotores: Prof.dr.ir. W. van Saarloos & Prof.dr. M. van Hecke June 29, 2010 Casimir PhD series 2010-16 van Weert M.: Quantum dots in vertical nanowire devices Promotores: Prof.dr.ir. L.P. Kouwenhoven, Dr. E.P.A.M. Bakkers. June 29, 2010 Casimir PhD series 2010-13 van Tilburg J.: Electron Spins in Nanowire Quantum Dots Promotor: Prof.dr.ir. L.P. Kouwenhoven. July 2, 2010 Casimir PhD series 2010-11

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Goetz G.: Single Electron-ics with Carbon Nanotubes Promotor: Prof.dr.ir. L.P. Kouwenhoven. July 2, 2010 Casimir PhD series 2010-14 Sidorkin A.V.: Resist and Exposure Processes for Sub-10-nm Electron and Ion Beam Lithography Promotores: Prof.dr. H.W.M Salemink, Prof.dr.ir. P. Kruit. July 6, 2010 Casimir PhD series 2010-15 de Groot P.: Coupled flux qubits and double bifurcation readou Promotores: Prof.dr.ir. J.E. Mooij, Dr. C.J.P.M. Harmans & J.W.M. Hilgenkamp. July 8, 2010 Casimir PhD series 2010-17 van de Meent J.W.: Making it Big; How Characean Algae Use Cytoplasmic Streaming to Enhance Transport in Giant Cells Promotores: Prof.dr.ir. W. van Saarloos & Prof.dr. R.E. Goldstein (Univ. of Cambridge). September 16, 2010 Casimir PhD series 2010-22 Liu X.: Quantum dots and Andreev reflections in graphene Promotores: Dr. L.M.K. Vandersypen, Dr. Y.M. Blanter. September 22, 2010 Casimir PhD series 2010-23 Picot T.: On the quantum measurement of superconducting flux qubits Promotores: Prof.dr.ir. J.E. Mooij, Dr. C.J.P.M. Harmans. September 22 Casimir PhD series 2010-24 Nguyen T.: First-order phase transitions and magnetocaloric effect Promotor: Prof.dr. E.H. Br端ck. September 27, 2010 Casimir PhD series 2010-18 Forn-Diaz P.: Superconducting Qubits and Quantum Resonators Promotores: Prof.dr.ir. J.E. Mooij & Dr. C.J.P.M. Harmans. September 27, 2010 Casimir PhD series 2010-21 Mesaros A.: Dislocations in stripes and lattice Dirac fermions Promotor: Prof.dr. J. Zaanen. October 6, 2010 Casimir PhD series 2010-20

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Chen P.: Three-dimensional Nanostructures Fabricated by Ion-Beam-Induced Deposition Promotores: Prof.dr. H.W.M. Salemink & Dr. P.F.A. Alkemade. October 6, 2010 Casimir PhD series 2010-27 van den Hout M.: Forcing DNA and RNA through Artificial Nanopores Promotor: Prof. N.H. Dekker October 21, 2010 Casimir PhD series 2010-26 Nata Atmaja A.:Applications of AdS/CFT in Quark Gluon Plasma Promotores: Prof.dr. J. de Boer (UvA) & co-promotor Dr. K. Schalm October 26, 2010 Casimir PhD series 2010-28 Ament L.: Resonant Inelastic X-ray Scattering Studies of Elementary Excitations Promotor: Prof.dr. J. van den Brink November 11, 2010 Casimir PhD series 2010-25 Groth C.: Anomalous diffusion of Dirac fermions Promotor: Prof.dr. C.W.J. Beenakker December 8, 2010 Casimir PhD series 2010-30 Nadj-Perge S.: Control of Single Electron Spins in Semiconductor Nanowires Promotor: Prof.dr.ir. L.P. Kouwenhoven December 20, 2010 Casimir PhD series 2010-31 Peeters W.: Two-photon interference: Spatial aspects of two-photon entanglement, diffraction, and scattering Promotores: Prof.dr. J.P. Woerdman & co-promotor Dr. M.P. van Exter December 21, 2010 Casimir PhD series 2010-29

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4.4. Highlights of research achievements per Casimir theme In Part II of this yearly report, we put the spotlight on several research highlights as presented by Casimir staff members. These contributions, spanning all six Casimir themes, illustrate the width and diversity of interdisciplinary physics research in the Casimir Research School. Theme 1: Molecular Biophysics

Single molecule fluorescence Elio Abbondanzieri Experimental evolution of bet hedging Bertus Beaumont Fundamental design principles of living systems Christophe Danelon SIMPlex: Single Molecule Approach to Novel Protein Complexes Chirlmin Joo Mini-ferritin protein Dps Anne Meyer Resolving the structure of chromatin by force spectroscopy John van Noort The structure of the chromatin fiber results from a packing problem Helmut Schiessel Directional sensing in Dictyostelium discoideum Thomas Schmidt Detecting a single molecule's absorption at room temperature Michel Orrit Theme 2: Physics of Nanostructures Spin Caloritronics and magnon Seebeck effect Gerrit Bauer New design for a quantum computer Carlo Beenakker Mechanical systems in the quantum regime, and qubit manipulation Yaroslav Blanter Dynamically protecting the quantum state of a single spin at RT Ronald Hanson Molecular devices Sense Jan v/d Molen Electrical pumping a single atom into a direction of choice Sander Otte Shot noise as a probe of molecular junctions Jan van Ruitenbeek Clean Carbon Nanotubes: relativistic QM to quantum nanomechanics Gary Steele Exploring molecular transport using theory and computation Jos Thijssen An all-electric, single-molecule motor Herre van der Zant Playing with single photons Val Zwiller Theme 3: Quantum Matter and Functional Materials Flowing long-range supercurrents through ferromagnetic CrO2 Jan Aarts Mn1-xCrxCoGe compounds: magnetic phase transitions Ekkes Brueck Self-healing in Fe alloys Niels van Dijk Feshbach-Einstein condensates Peter Denteneer Electronic coupling of CdSe nanocrystals Stephan Eijt Physical properties of elongated inorganic nanoparticles Liberato Manna Nanoionic proton conductors Fokko Mulder Metal underpotential deposition in presence of thilo-based additives Marcel Rost The role of surface and interface energy on phase stability of nanoMarnix Wagemaker sized insertion compounds. Generalizing the Cooper mechanism of superconductivity to quantum Jan Zaanen critical metals Theme 4: Quantum Information and Quantum Optics Quantum pumping in a ballistic graphene layer Miriam Blaauboer Characterization of high-dimensional entanglement Martin van Exter Casting light on efficient memory for quantum computers Leo Kouwenhoven Josephson half-lasers/LED, phase-slips and spin-conducting qubits Yuli Nazarov Control of a quantum bitâ&#x20AC;&#x2122;s environment Lieven Vandersypen Theme 5: Universe Physics: theory and instrumentation Significant quantum noise in a terahertz nano-detector Jian-Rong Gao Superconducting resonators to map the cool universe Teun Klapwijk Anti-de Sitter/Condensed Matter Theory Koenraad Schalm Theme 6: Dynamic Complex Systems Flow of Foams Martin van Hecke Curved Space Crystallography Vincenzo Vitelli 30

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5. Outlook to 2011 In 2011 the Casimir Research School in many respects enters a new phase of its existence. New initiatives in Leiden and Delft have permitted hiring of many enthusiastic and energetic young faculty staff, partly supported by the joint investment plan known as the Sectorplan. Notably, the new department of Bionanoscience at Delft is approaching full strength, with a large number of strong hirings. On 26 May 2011 the biannual Casimir Symposium will provide a podium for new staff members at all participating departments to present themselves to the Casimir community. It promises to be an exciting day full of wonderful new perspectives. TU Delft is preparing to bring all its PhD students under the umbrella of one Graduate School. This should not seriously interfere with the ambitions or with the programme under Casimir, which will continue under this umbrella. Just as it has already been practice at Leiden University, the Graduate School will be able to provide support on items such as registration and administration, and by providing general â&#x20AC;&#x153;transferableâ&#x20AC;? skills training programmes. The scientific training and the intellectual community are to be provided by research schools within the graduate school. As a by-product, the formation of the Graduate School has stimulated the interest from other research groups in joining the Casimir Research School and it will be important in the coming year to reconsider our position with respect to adjoining branches of science at Delft and Leiden. From wider, international perspective the European Commission has recognized the importance of building scientific communities as represented by research schools such as Casimir. This will open possibilities for strong partners in Europe to join forces. This year we will see the first group of students completing the Casimir pre-PhD track. The students will compete for prestigious grants that will allow them to pay for their salary for four years and to choose freely their research theme and supervisor. It is our ambition to vigorously continue this program, which should help attracting the best minds internationally to our Masterâ&#x20AC;&#x2122;s programme. 2011 also marks the end of the term of Teun Klapwijk as the first scientific director of the Casimir Research School. After the initiative by Peter Kes and Hans Mooij, it was Teun Klapwijk who quickly became the great source of inspiration for the School and it is by his energy and his leadership that it has grown to the success that it is today. On behalf of all who have enjoyed working and studying under the Casimir Research School I would like to thank him for this fantastic job! From April 1 2011, I will succeed him as scientific director. Those are some of the things that will change. What will remain is our ambition to maintain an intellectually stimulating environment for Master students, PhD students, and post-doctoral researchers. What will remain is our ambition to bring together smart people that share enthusiasm and curiosity, joy and ambition in research. We will continue to strive at bringing together experiment, theory and applied research, with each blooming by itself, and stimulated greatly by the interaction with the other branches of activity. And we will continue to maintain a strong coalition between interdisciplinary physics in Leiden and Delft.

April 1st 2011 Prof. dr. Jan M. van Ruitenbeek, Scientific director 31

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Casimir Research School Delft â&#x20AC;&#x201C; Leiden Report 2010

Highlights of research achievements per Casimir theme

In Part II of this yearly report, we put the spotlight on several research highlights as presented by Casimir staff members. These contributions, spanning all six Casimir themes, illustrate the width and diversity of interdisciplinary physics research in the Casimir Research School. Theme 1: Molecular Biophysics Single molecule fluorescence Elio Abbondanzieri Experimental evolution of bet hedging Bertus Beaumont Fundamental design principles of living systems Christophe Danelon SIMPlex: Single Molecule Approach to Novel Protein Complexes Chirlmin Joo Mini-ferritin protein Dps Anne Meyer Resolving the structure of chromatin by force spectroscopy John van Noort The structure of the chromatin fiber results from a packing problem Helmut Schiessel Directional sensing in Dictyostelium discoideum Thomas Schmidt Detecting a single molecule's absorption at room temperature Michel Orrit Theme 2: Physics of Nanostructures Spin Caloritronics and magnon Seebeck effect Gerrit Bauer New design for a quantum computer Carlo Beenakker Mechanical systems in the quantum regime, and qubit manipulation Yaroslav Blanter Dynamically protecting the quantum state of a single spin at RT Ronald Hanson Molecular devices Sense Jan v/d Molen Electrical pumping a single atom into a direction of choice Sander Otte Shot noise as a probe of molecular junctions Jan van Ruitenbeek Clean Carbon Nanotubes: relativistic QM to quantum nanomechanics Gary Steele Exploring molecular transport using theory and computation Jos Thijssen An all-electric, single-molecule motor Herre van der Zant Playing with single photons Val Zwiller Theme 3: Quantum Matter and Functional Materials Flowing long-range supercurrents through ferromagnetic CrO2 Jan Aarts Mn1-xCrxCoGe compounds: magnetic phase transitions Ekkes Brueck Self-healing in Fe alloys Niels van Dijk Feshbach-Einstein condensates Peter Denteneer Electronic coupling of CdSe nanocrystals Stephan Eijt Physical properties of elongated inorganic nanoparticles Liberato Manna Nanoionic proton conductors Fokko Mulder Metal underpotential deposition in presence of thilo-based additives Marcel Rost The role of surface and interface energy on phase stability of nanoMarnix Wagemaker sized insertion compounds. Generalizing the Cooper mechanism of superconductivity to quantum Jan Zaanen critical metals Theme 4: Quantum Information and Quantum Optics Quantum pumping in a ballistic graphene layer Miriam Blaauboer Characterization of high-dimensional entanglement Martin van Exter Casting light on efficient memory for quantum computers Leo Kouwenhoven Josephson half-lasers/LED, phase-slips and spin-conducting qubits Yuli Nazarov Control of a quantum bitâ&#x20AC;&#x2122;s environment Lieven Vandersypen Theme 5: Universe Physics: theory and instrumentation Significant quantum noise in a terahertz nano-detector Jian-Rong Gao Superconducting resonators to map the cool universe Teun Klapwijk Anti-de Sitter/Condensed Matter Theory Koenraad Schalm Theme 6: Dynamic Complex Systems Flow of Foams Martin van Hecke Curved Space Crystallography Vincenzo Vitelli

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Theme

Molecular Biophysics

> Elio Abbondanzieri (Bionanoscience)

In biochemistry, allosteric regulation describes the modulation of a finely tuned enzymatic protein by the binding of an effector molecule to a regulatory site on the enzyme. The term is derived from the Greek allos (!""#$), "other", and stereos (%&'(')$), "solid (object)", in reference to the fact that the regulatory site of an allosteric protein is physically distinct from its active site. Is it possible to visualize such an allosteric effect? This was one of the questions we sought to answer when I began a project to study HIV reverse transcriptase with single molecule fluorescence several years ago. Reverse trancriptase, or RT, plays a central role in allowing virions to infect new cells. RT has therefore been targeted in efforts to fight HIV: of the four main classes of anti-retroviral drugs used clinically, two of these inhibit the activity of RT. One of these drug classes binds to RT but does not directly block its ability to bind nucleic acids or nucleotides. Instead, it provides an allosteric inhibition of enzyme activity. In order to understand this effect, we used Fรถrster resonance energy transfer (FRET) to measure the binding of individual RT molecules to different nucleic acid targets. This allowed us to visualize subtle changes in the binding conformation of the enzyme, including a shift between an active binding mode and several inactive binding modes [1,2]. The addition of the allosteric drugs did not prevent RT from binding nucleic acids (in fact, it bound more strongly). However, these drugs biased RT towards binding in the inactive modes. These results helped shine light on a process that had been incompletely understood. Moving forward, I plan to continue studying RT and use these tools to understand how drug resistant strains of RT can overcome this inhibition as well as to study new classes of drugs which target RT. [1] Abbondanzieri EA, Bokinsky G, Rausch JW, Zhang JX, Le Grice SF, Zhuang X. Nature. (2008) 453 pp. 184-9. [2] Liu S, Abbondanzieri EA, Rausch JW, Le Grice SF, Zhuang X. Science. (2008) 322 pp. 1092-7.

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> Bertus Beaumont (Bionanoscience)

Experimental evolution of bet hedging In this study, we used bacteria to examine the molecular mechanisms behind biological evolution in real time. The experiment forced bacteria to repeatedly evolve a novel trait: a selection regime that, according to evolutionary theory, can favour the evolution of risk-spreading strategies based on random trait-switching. Initially, the bacteria responded by evolving new traits on the basis of mutation and selection. However, eventually, some bacterial cells evolved the capacity to generate new traits without the need for mutation and selection. These new cells had evolved the capacity to switch randomly between different states and used this as a risk-spreading strategy known as bet hedging. The work provided the first empirical account of the evolution of bet-hedging, which in nature is found in a range of organisms that inhabit unpredictable environments. [1] Hubertus J. E. Beaumont, Jenna Gallie, Christian Kost, Gayle C. Ferguson and Paul B. Rainey. 2009. Experimental evolution of bet hedging. Nature 462: 90-93 > Christophe Danelon (Bionanoscience) In 2008 I worked at the EPFL on different projects in the field of biophysics of biological membranes and cellular signaling. In particular, I developed new fluorescence- and electrical-based assays to measure the activity of ion channels and membrane receptors in model systems [1]. In 2009 I moved to the university of Lausanne, where I have successfully applied a method enabling real-time imaging of cell-cycle progression to probe the temporal dynamics of the Notch signaling pathway. I joined the TU Delft in January 2010 as an assistant professor in the department of Bionanoscience. My group is pursuing two complementary lines of research to understand the fundamental design principles of living systems: (1) The building of protocells from prebiotically relevant compounds. Here we go in search of the origin of cellular life. I have formulated a scenario for the emergence of primordial cells, in which a porous mineral environment could assist the coordinated formation and evolution of a protocytoplasm and lipid vesicle membranes [2]. We are now experimentally exploring the pertinence of this scenario. (2) The construction of artificial minimal cells from contemporary biomolecules, a project at the interface of biophysics and synthetic biology. First, we developed a methodology to assemble liposomes (lipid vesicles) enclosing a minimal gene expression system for the internal synthesis of proteins (Figure, panel B). Next, we demonstrated the capability of using temperature to control membrane permeability, thus enabling to fuel the transcription and translation machinery with the nutrients and energy present in the surrounding medium. We succeeded in imaging the production of fluorescent proteins in surface-immobilized liposomes (Figure, panel A) 34

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and observed prolonged expression by alternating between the sealed and semipermeable states of the liposomes [3].

[1] Danelon C., S. Terrettaz, O. Guenat, M. Koudelka, and H. Vogel. Probing the function of ionotropic and G protein-coupled receptors in surface-confined membranes. Methods 46(2): 104-115, Oct. 2008. [2] Danelon C. A discontinuous protocellular life scenario in a porous mineral matrix. Abstract in Origins2011 International Conference, a full paper is in preparation. [3] Roelofsen W., Z. Nourian, and C. Danelon. Liposome rainbow: Stochastic colorcoding by multiple gene expression inside lipid vesicles. (Manuscript in redaction)

> Chirlmin Joo (Bionanoscience)

SIMPlex: Single Molecule Approach to Novel Protein Complexes Single-molecule techniques enable the study of biological processes with nanometer resolution in real time. However, to date they were only able to examine a subset of biological problems, namely those addressable using pure recombinant proteins. Large eukaryotic proteins, or proteins that are active only in complex with cofactors or require posttranslational modification, are typically not accessed at the molecular level. Here we integrated single-molecule fluorescence microscopy with immunopurification techniques to study human protein complexes directly extracted from human cell lines. We applied this novel method to investigate the regulation mechanism of let-7 microRNA biogenesis. Let-7, which governs differentiation and proliferation in stem cells and cancer cells, is posttranscriptionally suppressed when its precursor is uridylated by TUT4, a polyU polymerase. We analyzed in real time the uridylation of the precursor microRNA by TUT4 immunoprecipitate complexes and revealed that a stem cell marker Lin28 mediates the oligo-uridylation by increasing TUT4â&#x20AC;&#x2122;s processivity. SIMPlex, which allows proteins pulled down with their natural cofactors, will introduce universal platforms of single molecule fluorescence, force and torque spectroscopies for protein complex studies.

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Figure: Workflow of SIMPlex. (Top) TUT4FLAG-mCherry is immuno-purified with antiFLAG antibody bead and is eluted out with a high concentration of FLAG peptide. (Middle) The eluate is tethered to anti-RFP antibody on a single molecule surface. (Bottom) Interaction of Lin28-bound prelet-7a-1 with the immobilized TUT4-FLAGmCherry is observed via single molecule fluorescence total-internal-reflection microscopy. [1] K.H. Yeom, I. Heo, J. Lee, S. Hohng, V.N. Kim*, C. Joo*, "Direct Observation of Small RNA Modification via Single Molecule Approach to Novel Protein Complexes" EMBO J. (submitted) [2] I. Heo*, C. Joo*, Y.-K. Kim*, M. Ha, M.J. Yoon, J. Cho, K.-H. Yeom, J. Han and V.N. Kim (2009) "TUT4 in Concert with Lin28 Suppresses MicroRNA Biogenesis through Pre-MicroRNA Uridylation" Cell 138, 696-708 *equal contribution [3] I. Heo*, C. Joo*, J. Cho, M. Ha, J. Han and V.N. Kim (2008) “Lin28 Mediates the Terminal Uridylation of let-7 Precursor MicroRNA” Molecular Cell 32, 276-284 *equal contribution > Anne Meyer (Bionanoscience) Bacteria produce a mini-ferritin protein called Dps that provides them with natural resistance to damaging environmental conditions. The protective action of Dps for cellular DNA is modulated through an unusual strategy of proteolytic regulation. Dps is constantly synthesized by cells under all conditions, whether or not it is actively required. During healthy growth, Dps is degraded efficiently by the powerful AAA+ protease ClpXP, so that its cellular concentration is maintained at a low level. When cells encounter hazardous conditions, such as nutrient limitation or harmful levels of reactive oxygen species, ClpXP degradation of Dps is quickly and specifically turned off. Dps then accumulates rapidly to become the third-most abundant protein in the cell, allowing it to exert its protective effects. My recent work has focused on understanding how Dps is recognized by the protease ClpXP at a molecular level. This investigation has revealed that an extended, unstructured segment of Dps makes multiple contacts with ClpXP. Strong interactions with peripheral regions of ClpXP stabilize the overall interaction between the two players, promoting the formation of weaker contacts between Dps and the central, substrate-processing site of ClpXP. These results indicate that the regulation of Dps degradation cannot be performed by a classical ClpXP “adaptor” that would 36

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act to strengthen Dps-ClpXP interactions only when degradation is required, since Dps acts as its own adaptor when interacting with ClpXP. Our new knowledge of the molecular contacts between Dps and ClpXP has allowed us to design variants of Dps that interact either more or less strongly with ClpXP, providing the ability to alter and fine-tune the levels of the DNA-protective Dps. These studies have discovered the fundamental mechanisms underlying a crucial protective response and have created techniques to manipulate and target the therapeutic action of Dps and other similar beneficial enzymes. Since DNA damage caused by adverse environments can lead to tumor formation, our work may have broad significance for the prevention of cancer. > John van Noort (LION)

Resolving the structure of chromatin by force spectroscopy Three quarters of the DNA in the eukaryotic genome is wrapped around histone proteins forming a long string of nucleosomes that are connected by linker DNA. The nucleosomes themselves interact and fold the DNA into a dense chromatin fiber. The structure of the chromatin fiber has been debated for 30 years. By gently pulling on single chromatin fibers, we showed that chromatin organization depends on the DNA linker length and that the fiber is generally folded into a helical structure. Recently identified DNA sequences with very high affinities for histones allow for reconstitution of chromatin fibers with perfectly controlled nucleosome positions. We used an array of these nucleosome-positioning elements to reconstitute chromatin fibers containing exactly 25 nucleosomes and folded these arrays to form a condensed chromatin fiber. In a magnetic tweezers setup we tethered a micron-sized super-paramagnetic bead to a single chromatin fiber and measured force-extension curves of the fiber with nanometer and sub-piconewton resolution.

Figure: Stretching chromatin fibers. Eukaryotic DNA is folded into a flexible chromatin fiber. Forceâ&#x20AC;&#x201C;extension curves of single fibers reveal their structure, which is depends on the length of the linker DNA. Chromatin fibers exhibit an overstretching transition at 4 piconewton that could be attributed to the rupture of nucleosome-nucleosome interactions. Analysis of the kinetics and amplitudes of single rupture events indicate that DNA is partially unwrapped from individual nucleosomes during these rupture events. All forceinduced transitions can be described by a two-state model that includes the compliances of the DNA, the folded chromatin fiber and the unfolded chromatin fiber. The maximal extension of the fiber before rupturing indicates that neighboring nucleosomes stack in a helical conformation when the linker DNA is 50 bp. Fibers with 20 bp linker DNA are stiffer and have larger rupture forces, indicating a zig-zag conformation in which odd and even nucleosomes form two stacks.

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These results are a major step towards a structural understanding of transcription regulation and many other processes that involve DNA. [1] M. Kruithof, F.-T. Chien, A. Routh, C. Logie, D. Rhodes and J. van Noort, Nat Struct Mol Biol., 16, 5, (2009).

> Helmut Schiessel (LION)

The structure of the chromatin fiber results from a packing problem DNA in cells of plants and animals is packaged with the help of proteins into a DNA-protein complex called chromatin. An important level of the hierarchical organization of chromatin is the 33nm wide chromatin fiber. Despite of over 30 years of experiments and model building there is no agreement on the structure of these fibers. We have shown that one can predict a 33nm wide fiber as the result of the dense packaging of its building blocks, wedge-shaped DNA-protein spools called nucleosomes. The total length of DNA per human cell is 2 meters and needs to fit into the micronsized cell nucleus. To make this possible, DNA is complexed with histone proteins into the chromatin complex. As a first step of compaction DNA is wrapped around protein cylinders. The resulting DNA spools, the nucleosomes, are then further compacted into a 33 nm wide chromatin fiber. Standard textbooks on molecular biology show in a every new edition a new model for that fiber but none of those models was ever able to predict the diameter of the fiber, the only observable quantity. We showed that the dense packaging of the nucleosomes leads to a finite number of possible geometries, namely 5 to 8 stacks of nucleosomes that twirl around each other. Since the nucleosomes have an 8 degree splay one can predict the diameter of those fibers. We found that a 5 stacks fiber has 33 nm diameter and a 7 stacks structure has 44 nm diameter, both diameters having been found in experiments. This opens the possibility of building in the future a consistent statistical physics model for chromatin fibers that is currently under development in our group.

Figure: The curves display the effective wedge- or splay angle per nucleosome as a function of the fiber diameter for different numbers of stacks of nucleosomes. The structures A, B and C are 5-, 6- and 7stacks fibers with an 8 degree splay angle. The diameters of fiber A and C coincide with experimentally observed diameters. [1] M. Depken and H. Schiessel, Biophys. J. 96, 777 (2009).

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> Thomas Schmidt (LION)

Directional sensing in Dictyostelium discoideum [1] The process by which cells detect a gradient of a chemical and subsequently initiate movement towards the source is called chemotaxis. This complex cellular response involves several interrelated processes, including directional sensing, polarized cytoskeletal organization and motility. Chemotaxis is involved in processes like neurogenesis, angiogenesis and many morphogenic processes. We used the slime mold Dictyostelium discodeum to study the processes involved in chemotaxis. Upon stimulation by a minute chemical gradient (< 2%) of the small molecule cyclic adenosine mono-phosphate, cAMP, D. discodeum faithfully moves towards the cAMP-source. Intricate intracellular signaling networks subsequently amplify the external gradient into a highly polar behavior. We have followed the initial steps of this signaling network at the single-molecule level in order to obtain a physical and mechanistic insight into the process. Signaling is initiated by binding of cAMP to itâ&#x20AC;&#x2122;s specific receptor, cAR1, that is localized in the membrane of the cell. Activation of the receptor subsequently leads to activation of downstream G protein heterotrimers. One outstanding question in the field was whether the receptor and the G proteins existed in a pre-coupled complex prior to activation or whether such complexes were transiently formed. Our experimental results suggested that ~30% of the G protein heterotrimers exist in receptor precoupled complexes. Upon stimulation in a gradient the complexes dissociate leading to a linear diffusion/collision amplification of the external signal that we were able to follow on a molecule-by-molecule basis. The further observation of partial immobilization and confinement of G!" in an agonist, F-actin and G#2-dependent fashion led to the hypothesis of functional nanometric domains in the plasma membrane that locally restrict the activation signal and in turn lead to faithful and efficient chemotactic signaling.

Figure Left: Fluorescence signals from individual G proteins. Right: Upon binding of cAMP to the receptor (bottom) the G protein heterotrimer is dissociated: the G*2 subunit exchanges GDP for GTP and diffuses into the cytosol where it is free to activate downstream signaling molecules. The previously precoupled cAR1 fraction is engaged in catalytic activation of the large G protein heterotrimer pool (indicated by red arrows). The G+, heterodimeric subunit is immobilized by interaction with F-actin associated structures, which potentially serve to locally enhance chemotactic signaling. Tightening of the membrane-associated F-actin restricts the diffusion of the G proteins to ~600 nm domains.

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[1] Freek van Heemert, Ilena Lazova, B. Ewa Snaar-Jagalska & Thomas Schmidt. "Mobility of G proteins is heterogeneous and polarized during chemotaxis", J. Cell Sci. 2010, 123 2922-30. > Michel Orrit (LION)

Detecting a single molecule's absorption at room temperature by photothermal contrast The single-molecule optics group at LION-MoNOS focuses on the optical detection and study of individual small objects (molecules, nanocrystals, metal nanoparticles, etc.). For example, we can use the fluorescence of molecules to monitor their orientation and position in complex environments. A severe limitation of these studies is bleaching, the photodestruction of the molecule. One possible solution is to replace dye labels by stable gold nanoparticles. Another solution is to detect the absorption of a single molecule, instead of its fluorescence. We have recently pushed the sensitivity of photothermal contrast down to the single-molecule level, at room temperature. Photothermal contrast arises from the slight change of temperature, and of index of refraction, of the surroundings of an absorbing object under heating by a first laser. A second laser probes the thermal lens around the heating object by producing a minute scattered light wave. We use this signal to image single non-fluorescent azo dye molecules in glycerol. The photothermal technique provides contrast for the absorbing molecules only, irrespective of scattering by defects or roughness, with a signal-to-noise ratio of ~10 in an integration time of 300 ms. In the absence of oxygen, virtually no bleaching event was observed, even after more than 10 minutes illumination. In a solution saturated with oxygen, the average bleaching time was of the order of one minute. No blinking was observed in the absorption signal. This result opens the way to detection of non-fluorescing probes (molecules, nanoparticles, nanocrystals, etc.) in complex environments with unprecedented bandwidth and sensitivity.

Figure: Photothermal spots from single azo-dye molecules. Quantum state decay curves for an increasing number of pulses (N). The value on the y-axis quantifies in how far the measured state after a certain waiting period corresponds to the initial state. The higher the number of pulses, the longer it takes before the spin state decays. [1] A. Gaiduk, M. Yorulmaz, P. V. Ruijgrok, M. Orrit, Science 330, 353 (2010).

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Theme

Physics of Nanostructures

> Gerrit Bauer (Quantum Nanoscience)

Spin Caloritronics and magnon Seebeck effect Spin caloritronics is the science and technology of controlling heat currents by making use of the spin degree of freedom, including topics such as spin/magneto Seebeck/Peltier effects, spindependent heat conductance, and nano-scale magnetic heat engines [1]. Figure 1: Spin Caloritronics is about the interaction of heat with the spin angular momentum. Metal-based spintronics has driven technological progress of magnetic data storage technology in the last decades. Fundamental discoveries such as the giant and tunneling magneto resistance are now standardly been used in hard disk drive. The discovery of the spin-transfer torque is likely to lead to revolutionary non-volatile random access memories in the near future. Metal-based spintronics can also help to delay the looming breakdown of Mooreâ&#x20AC;&#x2122;s Law by power-saving magnetic logics and by nanoscale cooling and heat management, viz. spin caloritronics. We recently helped to disentangle the spin Seebeck effect in a magnetic insulator, viz. a thermopower that is fundamentally different from the conventional Seebeck effect in metals [2,3] (see Figure 2). In brief, we find that a thermal gradient excites a non-equilibrium magnon distribution that pumps [4] a spin current into the Pt, in which an electromotive force is generated by the inverse spin Hall effect [5]. We are now exploring possibilities to enhance this â&#x20AC;&#x153;magnon Seebeck effectâ&#x20AC;? and its downscaling to nanoscale devices.

Figure 2: Spin Seebeck effects [1] G.E.W. Bauer, A.H. MacDonald, and S. Maekawa, Solid State Commun. 150, 459 (2010). 41

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[2] K. Uchida, J. Xiao, H. Adachi, J. Ohe, S. Takahashi, J. Ieda, T. Ota, Y. Kajiwara, H. Umezawa, H. Kawai, G. E. W. Bauer, S. Maekawa, and E. Saitoh, Nature Mater. 9, 894 (2010). [3] J. Xiao, G.E.W. Bauer, K. Uchida, E. Saitoh, and S. Maekawa, Phys. Rev. B 81, 214418 (2010). [4] Y. Tserkovnyak, A. Brataas, G. E. W. Bauer, and B. I. Halperin, Rev. Mod. Phys. 77, 1375 (2005). [5] E. Saitoh, M. Ueda, H. Miyajima, and G. Tatara, Appl. Phys. Lett. 88, 182509 (2006). > Carlo Beenakker (LION)

New design for a quantum computer A quantum computer uses the superposition principle from quantum physics to quickly perform calculations that would take a normal computer thousands of years to complete. The zeroes and ones of a quantum computation can be stored in the direction of the current circulating in a superconducting ring: a clockwise current represents 0, counterclockwise represents 1. The coherent superposition of 0 and 1 that is allowed by the laws of quantum mechanics is called quantum bit, or qubit. Unfortunately this superposition turns out to be very fragile, it quickly becomes incoherent as a result of small disturbances. The Leiden team has now proposed a way to store the qubits in a manner, which makes them insensitive to external disturbances. The direction of the circulating current is stored in a semiconducting wire of indium-arsenide, running over the superconductor. An old but little known effect, the so-called Aharonov-Casher effect, is invoked to couple the ring to the wire. The Aharonov-Casher effect has a topological origin (like the more familiar Aharonov-Bohm effect) and does not therefore depend on microscopic details of the system. It is this independence that protects the qubit from decoherence. The topological nature of the Aharonov-Casher effect offers a way to realize the dream of a topological quantum computation, where topology protects the qubit from decoherence.

Figure: Building block of the new design of a quantum computer. A superconducting ring (yellow) is in contact with an indium-arsenide wire. Metal electrodes (grey) can vary the electron density in the wire from low (blue) to high (red). At the transition from low to high density there appear bound states (white), which are coupled via the Aharonov-Casher effect to the direction of the current circulating in the superconducting ring. The superposition of clockwise and counterclockwise circulating current is very sensitive to external disturbances, but the storage of the information in the wire is protected by the topological nature of the Aharonov-Casher effect. [1] F. Hassler, A.R. Akhmerov, C.-Y. Hou, and C.W.J. Beenakker: Anyonic interferometry without anyons: how a flux qubit can read out a topological qubit. New J. Phys. 12, 125002 (2010)

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> Yaroslav Blanter (Quantum Nanoscience)

1. Mechanical systems in the quantum regime NEMS (nanoelectromechanical systems) are nanoscale devices which contain mechanical elements. One of the recent trends in the field is to demonstrate and exploit the quantum regime of mechanical motion. One aims at the same degree of quantum control over phonons as the one currently available in quantum optics. There are several obstacles on the road to this goal, and my theoretical research investigates these obstacles and explores the means to overcome them. One important issue is back-action. A system interacts with a detector, and the effect of the detector on the system (backaction of the detector) in quantum mechanics cannot be made arbitrarily small. Therefore one needs to investigate the back-action. Together with my experimental colleagues from MED group in Delft, we published a study of the back-action in the system of suspended SQUID [1]. We found, both experimentally and theoretically, that the detector shifts the frequency of the mechanical resonator and suppresses its quality factor. Another interesting topic is the readout. Phonons are difficult to measure directly, and a number of suggestions have been made how to organize the readout of mechanical motion in the quantum regime. We proposed [2] to couple phonons with photons in a microwave cavity, and to use the phonon blockade effect â&#x20AC;&#x201C; that only one phonon can be created in a non-linear resonantly driven non-linear resonator â&#x20AC;&#x201C; to detect the quantum regime of mechanical motion.

2. Qubit manipulation

A protocol of weak measurement known in quantum optics was designed to highlight the features of quantum measurement, which are usually not emphasized. It was shown that if a quantum system is preselected in a given state, then measured weakly, and then postselected, the weak measurement result can on average provide values which lie outside the bound of the measured quantity, for instance, spin of $ particle can be measured to be 100. We proposed [3] the first weak value protocol with solid-state qubits, demonstrating indeed that a weak measurement of electron spin (via spin blockade) in a double-dot spin qubit can yield values above $. Entanglement is essential for realization of quantum information protocols. Therefore the knowledge of the evolution of entanglement of qubits is important. We considered [4] the systems of two qubits coupled via a non-ideal cavity and discovered that if the qubits are entangled in the beginning, the concurrence (the measure of the entanglement) can be a periodic function of time, which manifests periodic entanglement revival.

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[1] Tunable Backaction of a DC SQUID on an Integrated Micromechanical Resonator, M. Poot, et al., Phys. Rev. Lett. 105, 207203 (2010). [2] Detecting phonon blockade with photons, N. Didier, S. Pugnetti, Ya. M. Blanter, and R. Fazio, arXiv:1007.4714 [3] Weak Values of Electron Spin in a Double Quantum Dot, A. Romito, Y. Gefen, and Ya. M. Blanter, Phys. Rev. Lett. 100, 056801 (2008). [4] Periodic revival of entanglement of two strongly driven qubits in a dissipative cavity, M. Dukalski and Ya. M. Blanter, Phys. Rev. A 82, 052330 (2010).

> Ronald Hanson (Quantum Nanoscience)

Dynamically protecting the quantum state of a single spin at room temperature Single solid-state spins are promising building blocks for new quantum technologies like a quantum computer, but uncontrolled interactions of the spins with their environment have been a major hurdle. By flipping the spin of a single electron with very short pulses we have succeeded to mitigate these effects, effectively rendering the spin decoupled from its environment [1]. The research is performed on single electrons in diamond, a material that has recently become very popular with quantum scientists. Unique to diamond is that quantum mechanical properties manifest themselves even at room temperature, offering a great advantage for future applications. Previously, we succeeded in measuring the spin state of a single electron in diamond and probing its environment. By using high-frequency pulses of only a few nanoseconds we have achieved control over the state of a single spin with a record-high accuracy. We periodically rotated the spin with very high precision so that the effects of the environment completely averaged out. This made it seem as though the spin was completely decoupled from its environment. The more often we flipped the spin, the longer its quantum state was preserved. With 130 pulses the state was already protected 25 times longer than previously measured. In addition, we proved that the protection works for any arbitrary spin state. These results are a breakthrough for quantum science and engineering, where uncontrolled interaction with the environment have thus far proved to be the major bottleneck for new fundamental experiments and for applications.

Figure: Protecting the quantum state of a single spin. Quantum state decay curves for an increasing number of pulses (N). Y-axis quantifies in how far the measured state after a certain waiting period corresponds to the initial state. With increasing N it takes longer before the spin state decays. [1] G. de Lange, Z. Wang, D. RistĂ¨, V.V. Dobrovitski, and R. Hanson, Science 330, 60 (2010). 44

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> Sense Jan van der Molen (LION)

Quantum effects determine the conductance properties of molecular junctions, even at room temperature. We recently obtained first evidence for destructive interference effects in molecular charge transport. We also demonstrated lightcontrolled switching of a molecular device. Our general approach is to scrutinize series of molecules with subtle differences in their electronic structure [1,2]. Figure 1a-b, shows two so-called diarylethenes. The top molecule (1a, ‘onstate’) is fully conjugated, i.e., it has delocalized !-electrons over the full molecule. The bottom one (1b, ‘off’) is similar, but has broken conjugation. Remarkably, it is possible to switch between both states by light (visible: on to off; UV: vice versa). Moreover, in Fig. 1c we demonstrate a room-temperature switchable device, fully based on the properties of the molecules involved.1 More exactly, we control the conductance of a 2D-network of gold nanoparticles connected by diarylethenes. Upon illuminating the initial, ‘on’-molecules with visible light, the conductance drops. Upon irradiating with UV, the reverse happens. The experiment is repeated 5 times. We stress that this result is far from trivial, since molecules tend to lose their functionality after connection to electrodes [3]. Another fascinating molecular pair is seen in Fig. 1d-e. Here two organic molecules are shown without (1d) and with (1e) two O-atoms. Both molecules have approximately the same energy levels. However, for the bottom molecule, destructive interference is predicted broadly around the Fermi level. In Fig. 1f, we show preliminary evidence for this, for experiments on selfassembled monolayers of both molecules: the conductance properties differ dramatically for these apparently similar molecules. This provides a strong indication for the essential role of interferences in molecular systems, even at 300 K. More experiments are currently carried out to investigate this further. [1] S.J. van der Molen et al. Nano Lett. 9, 76-80 (2009) [2] C.M. Guédon, J. Zonneveld, H. Valkenier, J.C. Hummelen, S.J. v.d. Molen, Nanotech. 22 125205 (2011) [3] S.J. van der Molen, P. Liljeroth, J.Phys.: Condens. Matter 22, 133001 (2010) (Topical Review)

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> Sander Otte (Quantum Nanoscience)

Electrically pumping a single atom spin into a direction of choice The possibility to control the spin state of a single atom represents the ultimate limit in downscaling magnetism-based information processing. By injecting spin-polarized electron currents directly into a magnetic atom, we can now controllably pump the atomâ&#x20AC;&#x2122;s spin direction along or opposite to the direction of spin-polarization. Individual atoms can be probed by means of low-temperature scanning tunneling microscopy (STM). In recent years, two complementary techniques have evolved that gain access to the magnetic states of such atoms. Through a method named spin-polarized STM (SP-STM), the magnetic orientation of nanoscale objects can be visualized. Alternatively, inelastic electron tunneling spectroscopy (IETS) gives insight into the energy levels of the spin through well-defined excitations, but without direct knowledge of the spinâ&#x20AC;&#x2122;s directional orientation. By combining these two methods, we were able to choose which excitation to make and which not, thus controlling the orientation of the spin. In our experiments we did not perform SP-STM the conventional way, using a bulk magnetic or magnetically coated probe tip. Instead we could switch on and off the spin-polarization by attaching or releasing a single magnetic atom to/from a nonmagnetic metal tip. By pumping the surface atom spin antiparallel to the tip magnetization, we found that the total current passing through the junction dramatically decreased due to increased magneto-resistance. This observation suggests that we have built a single-atom spin valve.

Figure: Creating a spinpolarized probe tip by picking up a single magnetic atom. Left: STM topography and cartoon showing a nonmagnetic (Cu atom terminated) probe tip. Right: After releasing the Cu atom and picking up a magnetic Mn atom, we have created a spin-polarized tip that we can use to probe magnetic surface atoms.

[1] S. Loth, K. von Bergmann, M. Ternes, A. F. Otte, C. P. Lutz and A. J. Heinrich, Nature Physics 6, 340 (2010) > Jan van Ruitenbeek (LION)

The information is in the noise: shot noise as a probe of molecular junctions Single organic molecules are being explored as the smallest possible building blocks for electronic circuits. Testing even the simplest devices is still a challenge because imaging of a molecular device is extremely difficult. Fortunately, the current through 46

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the device carries a lot of information. For example, one may exploit the effects of inelastic electron scattering on characteristic vibration modes of the molecule, which is observed as a small step in the differential conductance at the voltage bias corresponding to the vibration mode energy. More recently, we have introduced shot noise, the intrinsic noise due to the discreteness of the electron charge, as a characterization tool. It gives information on the number conductance channels allowed by the molecule, and their transmission probability. Combining the two tools we were able to show that benzene can form a nearperfectly conducting bridge between Pt electrodes, without the need for chemical anchoring groups [1]. Remarkably, the signal in the differential conductance was observed to have a sign that depends on the transmission of the contact. A detailed study on molecular junctions of water, H2O, showed this more prominently (see Figure below) and the sign change was observed to happen at a conductance of 0.65 in quantum units [2]. The crossover had been predicted at 0.5, and it was possible by analyzing shot noise that allowed us to show that H2O carries two conductance channels, for which the main channel crosses the predicted value of 0.5 when the change of sign occurs. This observation unites two fields of physics, inelastic electron tunneling spectroscopy (IETS) and point contact spectroscopy.

Figure: Differential conductance for two realizations of a conducting bridge formed by a single molecule of H2O between Pt leads. At high conductance (G =1.02G0, left) inelastic scattering on a vibration mode is seen as a step to lower conductance. At low conductance (G =0.23G0, right) it is seen as an increase. One can take a step further and ask what happens to shot noise above a bias corresponding to a vibration mode. We have recently detected the first inelastic corrections to shot noise [3], for which again a change of sign occurs, but apparently for different reasons and at a different transmission value. This opens the perspective for studying dissipation in molecular junctions from the statistics of the occupation of the vibration modes, but revealed by electron shot noise. [1] M. Kiguchi, O. Tal, S. Wohlthat, F. Pauli, M. Krieger, D. Djukic, J. C. Cuevas, and J. M. van Ruitenbeek, Phys. Rev. Lett. 101, 046801 (2008). [2] O. Tal, M. Krieger, B. Leerink, J.M.van Ruitenbeek, Phys. Rev. Lett., 100, 196804, (2008). [3] M. Kumar, R. Avriller, A. Levy Yeyati, and J.M. van Ruitenbeek, in preparation

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> Gary Steele (Quantum Nanoscience)

Clean Carbon Nanotubes: From relativistic quantum mechanics to quantum nanomechanics By developing a type of nanofabrication technology, we have dramatically improved both the electrical and mechanical quality of nanotube devices. Using these “clean” nanotubes, we have achieved a new level of control over the confinement of single electrons and holes, revealing a novel type of tunnelling analogous to the Klein paradox. Studying the mechanical properties of the nanotube, we find it can act as a resonator with an ultra-high quality factor exceeding 105, and that it’s mechanical motion is strongly coupled to single electrons tunneling on and off the device. The breakthrough in the nanotechnology was essentially to “do the fabrication backwards”, starting with electrodes and gates on the substrate, and growing nanotubes over top in the last step. The crucial development is that the carbon nanotube was not in any way exposed to the fabrication process, preserving the characteristics intrinsic to their perfect molecular structure. Using these new clean devices, we were able to confine both single electrons and single holes for the first time in a tunable double quantum dot in a carbon nanotube [1]. Studying the tunnelling of a single electron from one dot to the other as we increased the barrier between the two, we observed that tunnelling initially decreased as expected, but subsequently increased again for very large barriers, as shown in Figure 1. This increase of tunnelling as occurs as a virtual process through filled states in the valance band, analogous to tunnelling in the Klein paradox of relativistic quantum mechanics.

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An important part of the fabrication of these clean nanotubes is that they are free hanging across a trench. As such, they also have the possibility to vibrate, like violin string. To our surprise, the mechanical resonance associated with this motion had a remarkable quality factor exceeding 150,000 [2], two orders of magnitude higher than previous measurements. Furthermore, we found a striking back-action between our detector, a quantum dot in the suspended nanotube, and the mechanical motion. The force from a single electron shifted the resonance frequency by more than 100 times its linewidth, shown in Figure 2, and mechanical motion was dominated by the strong coupling to the quantum dot [3]. The high frequency of the mechanical resonance also indicates we have already cooled to its quantum mechanical ground state: future research will focus on detecting and studying this quantum motion. [1] G.A. Steele, G. Gotz, L.P. Kouwenhoven, Nature Nano.4, 363 (2009) [2] A.K. Huettel, G.A. Steele, B. Witkamp, M. Poot, L.P. Kouwenhoven, H.S.J. van der Zant, Nano Lett.9 (7),2547 (2009) [3] G.A. Steele, A.K. H端ttel, B. Witkamp, M. Poot, H.B. Meerwaldt, L.P. Kouwenhoven, H.S.J. van der Zant, Science 325, 1103 (2009)

> Jos Thijssen (Quantum Nanoscience)

Exploring molecular transport using theory and computation The exciting field of molecular electronics lies at the interface between chemistry and physics. Different experimental designs and different molecular structures define a plethora of possible functionalities, although the extreme difficulty involved in fabrication leaves many challenges ahead. Equally challenging is the description of the experimental phenomena using theory and computation. If the details of the molecules used are to be incorporated in this description, a good deal of quantum chemistry is a necessary ingredient of the description. Our computational tools are therefore based on quantum chemistry codes, in particular the ADF (Amsterdam Density Functional) code. We have implemented a non-equilibrium Green's function (NEGF) code within ADF/Band. This enables us to tackle transport through large molecules in the strong coupling and in the off-resonant regime.

Figure 1: Charge distribution of a porphyrin molecule between gold contacts (C. Verzijl)

Figure 2: Top: Our method, using DFT and Green function methods.

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Another important regime is that in which the coupling between the molecule and leads is relatively weak. We have developed a density-functional theory (DFT) based scheme, which describes the weak to intermediate coupling regime. Our method gives a full description of the transport as long as there are no more than two orbital levels in between the bias window. The figure below shows the agreement we achieve with DMRG calculations on a Hubbard chain of spinless fermions. [1] F. Mirjani and J. M. Thijssen, Phys. Rev. B 83, 035415 (2011) > Herre van der Zant (Quantum Nanoscience)

An all-electric, single-molecule motor Molecular electronics, which aims at using single molecules as active device components, is a promising technological concept that has stirred fast-growing interest. It offers the promise of reduced energy consumption, and ultrahigh density and speed through low-cost device fabrication with self-assembly and selforganization processes. The fundamental challenge is to condense the functionality of an electronic device into a single molecule and to exploit the functional versatility offered by the chemical diversity of molecules for electronic device purposes: Single molecules could then serve as interconnects, transistors, data-storage cells, rectifiers, switches or even miniature motors. Different from the usual approaches that use light, temperature or magnetic fields as external stimuli, we use electric fields to control molecular functionality on the singlemolecule level. For example, we have measured the magnetic states of a singlemolecule magnet trapped in a threeterminal device geometry and shown that by the electric field on the gate the molecule becomes a stronger magnet. Another example includes the electric control over the low-spin to high-spin transition in spin-crossover compounds.

Figure: Design of a molecular motor with a permanent electric dipole moment. The motor consists of anchoring groups connecting the conjugated backbone to the leads, allowing the measurement of the low-bias conductance, and a dipole rotor which can be driven by the oscillating gate field underneath. Recently, we have also proposed the design of a single-molecule motor based on electric field actuation and electric current detection. In this all-electric motor, the motion of a dipolar moiety of a conjugated molecule, the â&#x20AC;&#x2DC;rotorâ&#x20AC;&#x2122;, is driven by the alternating gate field. Depending on the rotor position, the degree of conjugation in the molecule varies, so that the motion can be detected through read-out of the current through the molecule. This completely new device can also be operated as a switch and, by varying the temperature and chemical structure, allows for probing the behavior of a singlemolecule electromechanical system in the classical and in the quantum regime. 50

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[1] J.S. Seldenthuis, F. Prins, J.M. Thijssen and H.S.J. van der Zant, ACS Nano 4 (2010) 6681-6686. > Val Zwiller (Quantum Nanoscience)

Playing with single photons My group studies optical processes at the single photon level. We have recently coupled the emission from a single quantum dot to a rubidium vapor [1]. By merging a solid-state system with an atomic system, we were able to generate single photons on demand with a single quantum dot and couple the photons to a rubidium vapor to perform a slow light experiment. Slow light at the single photon level represents an important step towards the realization of a single photon quantum memory.

Fig 1. Slow light calculations for light emitted from a single quantum dot stored in a rubidium vapor. The dot emission can be delayed by 7 ns in a 7 cm long rubidium cell over a frequency range of 1 GHz. Our current work centers on the development of quantum light emitting diodes where single electron-hole pairs are efficiently and coherently transferred into photons. We are also developing quantum plasmonics where quantum optics experiments are performed in allon-chip circuits based on superconducting detectors. [1] Hybrid semiconductor-atomic interface: slowing down single photons from a quantum dot, N. Akopian, L. Wang, A. Rastelli, O. G. Schmidt, V. Zwiller, Nature Photonics (2011). [2] Quantum optics: A spooky light-emitting diode, V. Zwiller, Nature Photonics 4, 508 (2010).

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Theme

Quantum Matter and Functional Materials

> Jan Aarts (LION)

Flowing long-range supercurrents through ferromagnetic CrO2 It has long been believed that superconducting correlations can only penetrate ferromagnets over very short lengths, of the order of nm, because the exchange field in the magnet will quickly flip one of the two spins of the Cooper pair. It turns out, however, that such correlations can easily exist, not as spin singlets but as spin triplets. We have recently shown how this can lead to the flow of a supercurrent through a full micrometer of CrO2, which is a fully spinpolarized ferromagnet. For this we grow thin films of CrO2 by chemical vapor deposition and deposit superconducting contacts on the films by a lift-off technique. The superconductor is amorphous MoGe, with a Tc of about 6 K. Growth was performed on two different substrates, TiO2 and Al2O3 (sapphire), which leads to very different film morphologies. On TiO2 the films are quite flat and consist of unidirectional platelets with a long axis of about 2 Âľm. On sapphire the film is rougher, with crystallites growing along the 6 equivalent directions of the underlying hexagonal lattice. In the TiO2-based devices we do not find a supercurrent, but it is detected in the sapphire-based ones. The Figure shows the layout of the sample; current-voltage characteristics at various temperatures with the value of the critical current Ic indicated; and the temperature dependence of Ic for different devices, all with gaps between the electrodes around 700 nm. The penetration length of spin singlets would be of the order of 1 nm, and the observations are therefore a strong indication for the existence of so-called odd-frequency spin triplets.

Figure: (left) sample layout (upper) and gap between the electrodes (lower); (middle) current-voltage characteristics at various temperatures; (right) temperature dependence of the critical current for three different devices. [1] M.S. Anwar, F. Czeschka, M. Hesselberth, M. Porcu, and J. Aarts, Phys. Rev. B 82, 100501(R) (2010)

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> Ekkes Brueck (Radiation, Radionuclides and Reactors) Mn1-xCrxCoGe compounds: from single to double first-order magnetic phase transitions

About 15% of the total worldwide energy-consumption is spent on cooling. Magnetic cooling utilizes the so-called magnetocaloric effect in solids. In a magnetocaloric material, entropy can be transferred between the lattice and magnetic moments. Removing an external magnetic field leads to disordering of magnetic moments. Consequently, the transfer of entropy from the magnetic system to the lattice results in cooling. As a solid-state cooling technology, magnetic cooling is environmentally friendly, because it doesnâ&#x20AC;&#x2122;t use ozone-depleting chemicals and global-warming gases. It has also potential to achieve higher energy-efficiency than conventional vapor-compression cooling. Obviously efficient magnetocaloric materials are key ingredients for this evolving technology. Substitution of some Cr for Mn atoms in MnCoGe was employed to control the magnetic and structural transitions in this alloy to coincide, leading to a single first-order magneto-structural transition from the ferromagnetic to the paramagnetic state with a giant magnetocaloric effect observed near room temperature. Further increase in the Cr content in the Mn1-xCrxCoGe alloys can induce another first-order magnetoelastic transition from the antiferromagnetic to the ferromagnetic state occurring at lower temperature. The giant magnetocaloric effect as well as the simultaneous tunability of the two magnetic transitions make these materials promising for future cooling applications.

Figure: Phase diagram of Mn1-xCrxCoGe (0 - x - 0.25) alloys as derived from the magnetic and structural measurements. The FM-AFM and PM-FM transitions on heating (filled symbols) and cooling (open symbol) processes occur at Tt and Tc, respectively.

[1] N. T. Trung, V. Biharie, L. Zhang, L. Caron, K. H. J. Buschow and E. BrĂźck. Applied Physics Letters 96 (2010) 162507

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> Niels H. van Dijk (Radiation, Radionuclides and Reactors) Self-healing in Fe alloys studied by neutron scattering and positron annihilation

Recently it was realised that, in analogy with biological systems, metals can autonomously repair damage. This mechanism is known as self-healing. Under stress metal alloys may accumulate open volume defects. In some cases mobile solute atoms can partly fill these open volume defects with stable nanoscale clusters and thereby heal the damage, resulting in a significant improvement in the lifetime of the material. In order to study the physical mechanism responsible for self-healing in detail, we have designed high purity iron-based model alloys with added Cu, B and N. The creation of defects during deformation and the formation nanoscale clusters during healing was studied within the bulk by in-situ small-angle neutron scattering and positron annihilation spectroscopy in combination with transmission electron microscopy. Small angle neutron scattering probes the size distribution of the healing clusters in the range of 1-100 nm. Positron annihilation spectroscopy characterises velocity spectrum of the electrons in the open volume defects and in the healing clusters that annihilate with the trapped positrons.

Figure 1: Small-angle neutron scattering characterising the nanoscale clusters in the Fe alloy. The anisotropic pattern arises from the spin dependent scattering of the magnetic contrast between the Cu cluster and the Fe alloy.

Figure 2: Positron annihilation spectrum as a function of the electron momentum pL. The deformed Fe-Cu alloy response is composed of the reference curves for pure Cu, Fe and defects in Fe.

[1] S.M. He et al., Phys. Rev. B 81 (2010) 094103 [2] S.M. He et al., Phys. Rev. B 82 (2010) 174111

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> Peter Denteneer (LION)

Feshbach-Einstein condensates Bose-Einstein condensation of ultracold atoms is often achieved with the aid of Feshbach resonances, to tune the interatomic interaction, and optical lattices, to reach the strong-interaction regime. The many-body description is usually simplified by using a Hubbard model, with a single interaction energy (for atoms in the same potential well) that can be varied. We have added a conversion term to the Bose-Hubbard model by which two atoms can be converted into a bound state of two atoms ("molecule"), describing more faithfully what happens at a Feshbach resonance. This two-species Bose-Hubbard model with conversion term for atoms and molecules on a lattice has been studied numerically using a newly developed quantum Monte Carlo (QMC) algorithm: the stochastic Green function (SGF) algorithm. The SGF algorithm allows to efficiently simulate a much wider class of Hamiltonians and obtain a more general set of correlation functions than existing QMC methods. Amongst other properties, superfluid densities and momentum distribution functions are computed, which are the main indicators of superfluidity and condensation. The most notable results are: (1) the existence of an exotic "special-Mott" phase, a Mottinsulator in which the two individual species are very mobile (without becoming superfluidic), (2) the occurrence of both atomic and molecular condensates (which we term "Feshbach-Einstein condensates" because of the presence of a Feshbach resonance, and because Bose never envisioned the possibility of condensation of massive indistinguishable bosons), including compelling indications for a phase with an atomic condensate, but no molecular condensate. The latter phase was not found in meanfield studies and believed to be impossible based on general symmetry arguments. [1] V.G. Rousseau and P. J. H. Denteneer, Feshbach-Einstein condensates, Phys. Rev. Lett. 102, 015301 (2009). [2] V.G. Rousseau and P. J. H. Denteneer, Quantum phases of mixtures of atoms and molecules on optical lattices, Phys. Rev. A 77, 013609 (2008).

> Stephan Eijt (Radiation, Radionuclides and Reactors) Electronic coupling annihilation

CdSe

nanocrystals

monitored

positron

Colloidal semiconductor nanocrystals are of considerable interest to the development of future generation highly-efficient solar cells, since their optical properties are highly tunable as a function of their size and shape. Thin-film positron annihilation methods emerge as sensitive probes to investigate the electronic structure and surface chemistry of nanocrystal surfaces at the interior of multi-layer nanocrystal thin films. We employ the sensitivity of positron annihilation methods to probe the electronic structure of CdSe colloidal nanocrystals present in sub-micrometer thin films with thicknesses relevant for thin-film solar cells. Upon gently heating the films in a vacuum, the pyridine ligand molecules are removed from the surfaces of the 55

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nanocrystals, leading to the formation of interfaces between neighbouring nanocrystals. The induced electronic coupling between the nanocrystals is revealed in ACAR distributions by a strong reduction of the confinement peak at ~1 a.u. (1 a.u.=7.29%10-3 moc). Depth-resolved positron-electron momentum density methods thus show great promise for uncovering electronic properties in nanocrystal composite layers, superlattices and heterostructures.

Figure: Left: 1D-ACAR positron-electron momentum distributions before (blue line) and after (red line) removal of pyridine ligand molecules, presented as ratio curves with respect to bulk CdSe. The dotted line represents the estimated ratio curve for a CdSe surface. Right: Schematic representation of electronic coupling of valence electron states of two neighbouring CdSe nanocrystals induced by the formation of an interface. [1] S.W.H. Eijt, P.E. Mijnarends, L.C. van Schaarenburg, A.J. Houtepen, D. Vanmaekelbergh, B. Barbiellini, and A. Bansil, Appl.Phys.Lett. 94, 091908 (2009). > Liberato Manna (Quantum Nanoscience)

Physical properties of elongated inorganic nanoparticles Understanding the size and shape dependence of physical properties in nanoscale particles is a fundamental step towards the design, fabrication and assembly of materials and devices with predictable behavior. In recent years, there has been a remarkable advancement in the ability to fabricate shape-controlled nanoparticles, for example rods, wires, and nanoparticles with branched shapes, especially via synthetic approaches in solution. Shape-controlled inorganic nanoparticles are among the most promising candidates as building blocks in nanoscale materials and devices, both because their physical properties are modified considerably compared to those of spherical nanoparticles and because their intrinsic geometry opens many new opportunities for their assembly into organized superstructures. In [1] we review the physical properties of elongated inorganic nanoparticles, with particular emphasis on the transition in these properties when the shape of the nanoparticles evolves from a sphere to a rod, but we will consider in many cases also 56

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nanowires. From the point of view of specific properties and materials, we cover the optical properties of semiconductors and noble metals, the electrical properties of semiconductors, the magnetic properties of various metals and metal oxides, the catalytic properties of various classes of materials and the mechanical properties of metals and metal alloys.

Figure from [1]: (a) Examples of heterostructured nanorods: dot/rod, Co-tipped, collinear nanorods and nanobarbells; (b) Band alignment of type-I/type-II heterostructures. Optical and electrical properties of elongated semiconductor nanocrystals. Semiconductor nanocrystals are among the most studied materials in nanoscience nowadays, due to the large number of potential applications employing these materials, for example in optical devices or biological labeling, to cite a few. Elongated, rod-shaped semiconductor nanocrystals possess interesting physical properties which depend on their size, aspect ratio and chemical composition, and these nanoparticles have been proposed as active materials in lightemitting devices, photocatalysis, optically induced light modulation, photovoltaics, wavefunction engineering, and optical memory elements exploiting the exciton storage process. More in general, these nanoparticles have been considered as replacement for spherical nanocrystals (quantum dots) in all those studies in which the elongated shape could in principle add new or improved properties. Optical properties of elongated metal nanocrystals. Metallic nanocrystals have been proposed in a wide range of applications in various fields, among them sensing, biosensing, photodynamic therapy, photovoltaics, (nano-)optics and nano-electronics (for example plasmonic waveguides). Metal nanostructures can interact strongly with light in the visible and near infrared region of the spectrum, due to the presence of free electrons, which can be promoted both to empty energy levels in the same band or to levels of an empty overlapping band. An incident electromagnetic field can elicit collective oscillations of these free electrons, which cause a displacement of the electrons from the nuclei, leading to the formation of various possible distributions in the surface charges. This creates Coulomb interactions between positive and negative charges, which induce restoring oscillating forces acting on free electrons. Each type of surface charge distribution is characterized by a collective oscillation mode, also termed as localized surface plasmon resonance. Various factors influence the possible types of SPRs in nanostructures and the frequencies at which they are observed and the shape of metal nanoparticles is certainly one of them. We also review the magnetic properties of elongated nanoparticles (of importance to the life sciences and biomedicine), and the catalytic properties of nanoparticles. Mechanical properties The miniaturization of micro electro-mechanical devices and the fabrication of thin films in the electronic industry have started to raise questions already decades ago about the mechanical behavior of confined systems. Early experiments on tensile 57

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testing of metal whiskers with micrometer transverse sides have evidenced strengths much higher than the bulk value, and recently pure metals and alloys with at least one dimension in the micro- and nanoscale range have been investigated, thanks to advances in the fabrication of new generations of samples suitable for mechanical testing (for example micropillars prepared by focused ion beam) and in various techniques for studying their stress and deformation properties. Those studies have revealed a marked deviation in the mechanical properties of samples from bulk-like behavior already when their size is of the order of a few micrometers, which is comparable to the length scale of many plasticity mechanisms based on dislocation nucleation and propagation. The increased strength of single nanocrystals could be useful for applications of these materials as active probes in nanoindentation, scanning-probe microscopy and field emission. [1] R. Krahne, et al., Physics Reports (2011), doi:10.1016/j.physrep.2011.01.001 > Fokko Mulder (Radiation, Radionuclides and Reactors) Nanoionic proton conductors

Nanostructuring composites of a solid acid and a proton accepting material can enhance the proton conduction strongly, leading to better fuel cell electrolytes. The goal is to develop anhydrous fuel cell electrolytes suitable for application at elevated temperatures (150-250Â°C), while still a cold start of the fuel cell is possible. Advantages of the elevated temperatures are in the enhanced energy efficiency, lower poisoning of catalysts and easier cooling. Using neutron scattering and NMR the origin of these enhancements was found to be in so-called space charge effects. A model combining DFT and analytical calculations is established that reproduces the large concentration variations in the proton densities that are observed. For a composite of TiO2 anatase and solid acid CsHSO4, the strong enhancement of the ionic conductivity at the nanoscale can be assigned to this space-charge effect. In an experimental study using neutron diffraction surprisingly high hydrogen concentrations in the order of 1021 cm3 in TiO2 are measured, which means that about 10% of the available sites for H+ ions are filled on average. In addition the protons in the solid acid in the composite show high mobility over the temperature range from RT to 250oC in quasielastic neutron scattering and solid-state NMR experiments.

Illustration: Local charge profiles occurring in a nanocomposite of a proton donor, in this case the solid acid CsHSO4, and a proton acceptor, here TiO2. For the small particles the charged red and blue areaâ&#x20AC;&#x2122;s start to overlap and space charge enhanced conductivity occurs throughout the material. The concentrations of space charge induced protons in the proton acceptor TiO2 are about 3 orders of magnitude larger than what is known from other space charge systems. It is shown that ionic defects with negative formation enthalpy indeed reach such extremely high concentrations near the interfaces and throughout the material. By performing first-principles density functional theory calculations, it is found that proton insertion from CsHSO4 into the TiO2 particles is preferred 58

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compared to neutral hydrogen atom insertion and indeed that the formation enthalpy is negative. Moreover, the average proton fractions in TiO2, estimated by the theoretical ionic density profiles, are in good agreement with the experimental observations. [1] L.A. Haverkate, W.K. Chan, F.M. Mulder, Adv. Funct. Materials 20 (2010) 4107. > Marcel Rost (LION)

General model of metal underpotential deposition in presence of thilobased additives based on an in-situ STM study Electrodeposition is a low-cost electrochemical process, in which a conducting surface is plated to serve as an electrode. The industry uses so-called additives (certain atoms, organic molecules, and even larger chemical complexes) that are added to the electrochemical bath, in order to influence the precise structure and, therefore, also the quality of a coating. Although the plating industry strongly relies on the function of these additives, the underlying atomic scale processes that alter the growth are unknown in most cases. The research is performed at our special electrochemical scanning tunneling microscope (EC-STM), which allows us to change the potential of the sample in-situ, while continuously scanning. Depending on the sample potential, deposition and desorption can be followed online. In addition, we developed a flow cell for this microscope that allows electrolyte exchange, again, while continuously scanning. This provides us with the possibility to switch from an electrolyte without additive to an additive containing electrolyte, while continuously observing the growth on the sample at an atomic scale.

Figure: Different stages of the Cu UPD layer formation during a potential sweep from -145mV to -200mV. Arrow indicates scan and sweep direction. In order to shed a light on the effects of thilobased additives we studied Cu underpotential deposition (UPD) on a Au(111) surface modified by Bis(3-sulfopropyl)disulfide (SPS), a thiol-based additive widely used in commercial Cu electroplating baths. In contrast to the additivefree case, where a flat monolayer of Cu is formed that nucleates at the step edges, we observed vacancy islands, patches that nucleate on the terraces, and islands that are higher than the patches although they nucleate in the holes of the coalescing patches (see figure). Based on our in-situ observation, we derived a model for the function of the SPS. By analyzing the literature, we come to the conclusion that our model is of a more general character and can explain several other observations of metal (i.e. Cu, Agâ&#x20AC;Ś) UPD on metallic electrodes (Au, Ag, Cu, Ptâ&#x20AC;Ś) modified by thiols. We believe that our model can help to prevent further confusion in explaining experimental results on metal UPD on thiol-modified surfaces. [1] Y. Yanson, J.W.M. Frenken, and M.J. Rost, Phys. Chem. Chem. Phys. submitted 59

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> Marnix Wagemaker (Radiation, Radionuclides & Reactors)

The role of surface and interface energy on phase stability of nano-sized insertion compounds Nano-sizing of Li-ion battery electrode materials is an active area of research that seeks to improve storage and kinetic performance of Li-ion materials. Up to date particle size has largely unknown impact on the material properties. Here we reveal the impact of surface and interfaces on the thermodynamics opening the road for nano-architecturing of optimal electrode materials for Li-ion batteries. In nano-scale particles, the influence of surfaces and interfaces cannot be neglected in any analysis of Li insertion thermodynamics. Here we derive thermodynamic equilibrium criteria for two-phase coexistence within and between nano-crystallite insertion compounds that accommodate Li as interstitial guest ions. Using representative values for surface and interface energies calculated from first principles for LixFePO4, a state of the art electrode material, we show that both particle size and shape can have a pronounced effect on equilibrium compositions at the nano-scale. Especially interface energy contributions play an important role and their effect explains observations of the narrowing of electrochemically measured miscibility gaps in nano-electrodes. An important difference with bulk thermodynamics is the prediction that surface and interface contributions in a nanocrystallite make the equilibrium compositions of the coexisting phases dependent on the overall guest ion concentration. This is expected to have large impact on the ionic mobility and nucleation rate of these materials that should enable larger (dis)charge rates of Li-ion batteries.

Figure: Schematic figure illustrating the increasing impact of the interface energy between the coexisting phases with decreasing particle size on the Gibbs free energy. The consequence is that the miscibility gap of the first order phase transition is reduced (range to ).

[1] M. Wagemaker, F. M. Mulder, and A. van der Ven, Adv. Mater. 21 (2009) 1

> Jan Zaanen (LION)

Generalizing the Cooper mechanism of superconductivity to quantum critical metals Since a quarter of a century physics is facing the mystery of superconductivity at a high temperature. This appears to have some traits in common with conventional superconductivity as described by the BCS theory. In the last few years a new understanding of the nature of the pairing mechanism has been emerging: nonFermi liquid metals that are ruled by quantum criticality turn out to be subjected to a very muscular version of the Cooper mechanism which is at the heart of BCS. 60

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The celebrated BCS theory of conventional superconductivity prescribes that when a Fermi-gas is subjected to an attractive interaction fermion pairs will form at some low temperature and superconductivity sets in. Something similar appears to happen in the high Tc superconductors, with the big difference that the metallic state is not a Fermi gas but some strongly interacting quantum critical state. We proposed recently a simple scaling theory revolving around the quantum physical scale invariance of the normal state and the idea of the Cooper instability demonstrating that superconducting Tc’s in such systems should be strongly enhanced [1]. This idea was picked up by Jarrel and coworkers, having the best numerical machinery (“DCA”) for the study of the Hubbard model: amazingly, their numerics indicate that our “quantum critical BCS” is behind the superconductivity that is detected at the phase separation quantum critical end point [2]. Even more excitingly, this turns out to be closely related to the recently discovered mechanism of holographic superconductivity [3] coming from string theory (see also the contribution of K. Schalm). Theory has now advanced to a stage that we can tell what needs to be done experimentally to find out whether such physics is at work.

Figure: Observing the difference between conventional and quantum critical superconductivity. Predictions for the imaginary part of the dynamical pair susceptibility (false colors) in the normal state of a conventional BCS- (FLBCS), a quantum critical BCS- (QCBCS) and holographic (HS) superconductor as function of energy ( ) and reduced temperature . This quantity can in principle be measured in special superconducting junctions [4]. According to Scalapino and Ferrel the dynamical pair susceptibility in the normal state can be measured via the second order Josephson effect in a strong superconductor-insulator-weak superconductor junction and this quantity directly reveals the gross differences in the fundamental physics behind the pairing instability (see Figure). [1] J.-H. She and J. Zaanen, Phys. Rev B 80, 184518 (2009) [Editor’s choice]. [2] S.X. Yang, H. Foiso, S.Q. Su, D. Ganalikis, E. Khatami, J.-H. She, J. Moreno, J. Zaanen, and M. Jarrell, Phys. Rev. Lett. 106, 047004 (2011). [3] J. Zaanen, Nature 462, 15 (2009). [4] J.-H. She, PhD thesis “Fermions, Criticality and Superconductivity, Leiden University, 2011

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Theme

Quantum Information and Quantum Optics

> Miriam Blaauboer (Quantum Nanoscience)

Quantum pumping in a ballistic graphene bilayer Quantum pumping of charge refers to the generation of a dc electrical current in the absence of an applied bias voltage by periodic (ac) modulation of two or more system parameters, for example the shape of the confining potential or a magnetic field [1]. The idea of adiabatically generating a flow of particles in a moving periodic potential is due to Thouless [1], and has been investigated during the last decade in a diverse range of meso- and nanoscale systems. We have investigated quantum pumping of massless Dirac fermions in an ideal (impurity free) double layer of graphene [2]. The pumped current is generated by adiabatic variation of two gate voltages in the contact regions to a weakly doped double graphene sheet, see Figure 1.

Figure 1: Schematic picture of the graphene bilayer. Top panel: Two stacked honeycomb lattices of carbon atoms in a strip of width W between metallic contacts (blue and red regions). Bottom panel: Variation of the electrostatic potential across the two layers. We found that at the Dirac point and for a wide bilayer with width W >> length L the pumped current Ip scales linearly with the interlayer coupling length l for L/l <<1, exhibits a maximum for L/l ~1, and crosses over to a ln(L/l )/(L/l )-dependence for L/l >> 1. This scaling behavior with l is markedly different from the behavior of the conductance in the same system, which is independent of l and equal to the conductance across two monolayers in parallel [3]. In practice, this different behavior of Ip and G as a function of l and L could be used to distinguish between the conductance and the pumped current. For typical experimental parameters the pumped current is predicted to be on the order of 10-100 pA. "

[1] D.J. Thouless, Phys. Rev. B 27, 6083 (1983). [2] G.M.M. Wakker and M. Blaauboer, Phys. Rev. B 82, 205432 (2010). [3] I. Snyman and C.W.J. Beenakker, Phys. Rev. B 75, 045322 (2007). 62

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> Martin van Exter (LION)

Characterization of high-dimensional entanglement Quantum entanglement in more than two dimensions can carry more quantum information and can be made more robust against perturbations than 2D quantum-bit entanglement. Spatial entanglement of photon pairs is ideally suited for this job. We have performed the first complete and unbiased characterization of the entanglement of photon orbital angular momenta l, associated with their generic azimuthal exp(il ) field dependence, and developed a tool to increase the dimensionality of this form of entanglement. We generate photon pairs via the nonlinear optical process of spontaneous parametric down-conversion, in which a high-frequency photon splits in a pair of lowfrequency photons, and analyze their OAM entanglement with two-photon interference in a Mach-Zehnder interferometer. When both interferometer beams are aligned the two-photon interference, first discussed by Hong, Ou, and Mandel, is such that the two photons always exit from the same output port of the interferometer. We have shown that this interference effect disappears when one of the beams is rotated with respect to the other. Fourier analysis of the angle dependence of the interference yields the full distribution of the modal weights Pl of the (l,&l) photon pairs that we generate with our rotationally-symmetric pump beam. We have also developed a technique (on-purpose phase mismatching) that increases the width and flattens the distribution, thus leading to a more useful form of entanglement with an even larger dimension. The effective dimension of OAM entanglement is typically Kaz = 10 & 40 and can be increased even further by small modifications of the geometry.

Figure: Characterization of high-dimensional spatial entanglement. The model probability Pl versus the orbital angular momentum l of one photon out of an entangled pair, as measured (blue bars) and calculated (dashed red curve) for a particular geometry. The effective dimension of the OAM entanglement is given by the azimuthal Schmidt number Kaz = 21.4 Âą 0.5. [1] H. Di Lorenzo Pires, H.C.B. Florijn, and M.P. van Exter, Phys. Rev. Lett. 104, 020505 (2010).

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> Leo Kouwenhoven (Quantum Nanoscience)

Casting light on efficient memory for quantum computers Scientists in our group, working in collaboration with German colleagues, have succeeded in controlling individual photons in a way that brings an efficient memory for quantum computers a step closer. Recently Dr. Nika Akopian and his colleagues published an article about their research in the online edition of Nature Photonics. The researchers were able to control the speed of individual light particles (photons) and to reduce it to less than 4 percent of the speed of light in vacuum. They achieved this by guiding photons one by one through a vapour of rubidium atoms that has precisely the right physical properties to decelerate the photons. By drastically reducing this speed, the individual photons can be â&#x20AC;&#x2DC;keptâ&#x20AC;&#x2122; with their properties in a controlled manner for a short period. This temporary storage of information is a first step towards the creation of a quantum memory based on slow photons. A quantum memory has the same function as a usual computer memory but can be applied within a quantum computer: a (future) super-efficient computer that functions on the basis of the physical laws in quantum mechanics. There are major advantages in working with single photons in this type of quantum application. Indeed, in current technology most information is transmitted via photons (in optical fibres). Moreover, individual photons can transmit quantum information over very large distances in a manner that is 100 percent secure and cannot be hacked. The photons are generated and emitted by tiny objects known as quantum dots. Since they are fabricated from well-known semiconductor material, they can easily be integrated in modern electronics.

[1] N. Akopian, L. Wang, A. Rastelli, O.G. Schmidt & V. Zwiller, Nature Photonics (2011) 20 February 2011

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> Yuli Nazarov (Quantum Nanoscience)

In my group, the research is mostly concentrated in the framework of three projects, each has a â&#x20AC;&#x153;superconductingâ&#x20AC;? component. 1. Josephson half-laser and Josephson LED [1] We describe a superconducting device capable of producing laser light in the visible range at half of the Josephson generation frequency with the optical phase of the light locked to the superconducting phase difference. It consists of two single-level quantum dots embedded into a p-n semiconducting heterostructure and surrounded by a cavity supporting a resonant optical mode. We study decoherence and spontaneous switching in the device. We access the suitability of the recently proposed Josephson LED for quantum manipulation purposes. We show that the device can both be used for on-demand production of entangled photon pairs and operated as a two-qubit gate. Besides, one can entangle particle spin with photon polarization and/or measure the spin by measuring the polarization.

2. Phase-slips [2] Non-linear effects on driven oscillations are important in many fields of physics, ranging from applied mechanics to optics. They are instrumental for quantum applications. A limitation is that the non-linearities known up to now are featureless functions of the number of photons N in the oscillator. Here we show that the non-linearities found in an oscillator where superconducting inductance is subject to coherent phase-slips, are more interesting. They oscillate as a function of number of photons N with a period of the order of square root of N, which is the spread of the coherent state. We prove that such nonlinearities result in multiple metastable states encompassing few photons and study oscillatory dependence of the responses of the resonator. The experimental realization of our proposal can deliver an unambiguous verification of coherent quantum phase-slips. In order to illustrate the emergence of Coulomb blockade from coherent quantum phase-slip processes in thin superconducting wires, we propose and theoretically investigate two elementary setups, or "devices". The setups are derived from Cooper-pair box and Cooper-pair transistor, so we refer to them as QPS-box and 65

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QPS-transistor, respectively. We demonstrate that the devices exhibit sensitivity to a charge induced by a gate electrode, this being the main signature of Coulomb blockade. Experimental realization of these devices will unambiguously prove the Coulomb blockade as an effect of coherence of phase-slip processes. We analyze the emergence of discrete charging in the limit strong phase-slips. We have found and investigated six distinct regimes that are realized depending on the relation between three characteristic energy scales: inductive and charging energy, and phase-slip amplitude. For completeness, we include a brief discussion of dual Josephsonjunction devices.

3. Spin-superconducting qubits [3] We propose and theoretically investigate spin superconducting qubits. Spin superconducting qubit consists of a single spin confined in a Josephson junction. We show that owing to spin-orbit interaction, superconducting difference across the junction can polarize this spin. We demonstrate that this enables single qubit operations and more complicated quantum gates, where spins of different qubits interact via a mutual inductance of superconducting loop where the junctions are embedded. Recent experimental realizations of Josephson junctions made of semiconductor quantum dots in contact with superconducting leads have shown that the number of electrons in the quantum dot can be tuned by a gate voltage. Spin superconducting qubit is realized when the number of electrons is odd. We discuss the qubit properties at phenomenological level. We present a microscopic theory that enables us to make accurate estimations of the qubit parameters by evaluating the spin-dependent Josephson energy in the framework of fourth-order perturbation theory. [1] P. Recher, Y.V. Nazarov , L.P. Kouwenhoven, Phys.Rev.Lett. 104, 156802 (2010) [2] A.M. Hriscu and Y.V. Nazarov, Phys. Rev. Lett. 106, 077004 (2011) [3] C. Padurariu C, Y.V. Nazarov, Phys. Rev. B 81 144519 (2010)

> Lieven Vandersypen (Quantum Nanoscience)

Control of a quantum bitâ&#x20AC;&#x2122;s environment [1] One of the unique properties of quantum particles is that they can occupy multiple states at the same time. For instance, the so-called 'spin' of an electron can point in two different directions simultaneously. In terms of bits: a 'quantum bit' can be both 0 and 1 at the same time, as opposed to the usual 'classical' bits which or always either 0 or 1. These quantum particles would allow for superfast quantum computations. However, keeping a quantum particle under control for a long enough time is extremely difficult, because the environment - also consisting of quantum particles - constantly disturbs the state. In this experiment, we were able to partly control the environment of a single electron spin confined to a semiconductor quantum dot, a nanoscale sized box. Earlier, we had shown that it is possible to manipulate the spin of an electron using a quantum dot. The problem was that all atomic nuclei in the host material also possess spin. Since these nuclear spins behave as tiny magnets, they constantly

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push and pull on the spin of the electron in the box. The electron, however, also pushes and pulls back. Precisely this interaction between the electron spin and the spins of the surrounding nuclei enabled us to fix the state of the spins. We sent an electric current through the nanobox and thereby influenced the spin direction of the nuclear spins. Due to the interaction between the electron and nuclear spins we managed to create a situation where the nuclear spins no longer fluctuated randomly, but actually became relatively stable. The evidence was that when RF excitation was applied to drive the electron spin by magnetic resonance, the electron spin remained locked in its magnetic resonance condition even when the driving frequency or static magnetic field were shifted away from the nominal resonance condition. This locking was caused by a build-up of nuclear spin polarization at precisely the right rate and in the right direction through electron-nuclear feedback.

Figure: High current indicates that the electron spin is on resonance with a fixed frequency excitation. As the magnetic field (right axis) is swept back and forth, the current remains high, indicating that feedback on the nuclear spin environment locks the electron spin to its magnetic resonance condition. [1] I.T. Vink, K.C. Nowack, F.H.L. Koppens, J. Danon, Yu.V. Nazarov and L.M.K. Vandersypen, â&#x20AC;&#x153;Locking electron spins into magnetic resonance by electronnuclear feedbackâ&#x20AC;?, Nature Physics 5, 764 (2009)

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Theme

Universe Physics: theory and instrumentation

> Jian-Rong Gao (Quantum Nanoscience) Significant quantum noise in a terahertz nano-detector Superconducting detectors play a key role in astrophysics in the terahertz (THz) frequency region (1 THz=1012 Hertz, 300 micrometer in wavelength). Approximately half of the total luminosity of the Universe and 98% of all the photons emitted since the Big Bang fall in this frequency band. Observations of the spectral lines in this region of the spectrum provide crucial and unique information on the history of the universe. Such observations require a heterodyne detection technique, which down converts an astronomic signal at THz frequency with a local oscillator to a signal at GHz frequency, to achieve sufficient sensitivity and spectral resolution. Hot electron bolometer (HEB) heterodyne detectors are developed for the next generation of balloon-, air-, and space-borne telescopes in the upper THz frequency range (2-6 THz). HEB mixers are based on a superconductor Niobium-nitride nanobridge and make use of a sensitive way of the change of the resistance. It has been known that a coherent amplifier must add a minimum amount of noise to its output that corresponds to a minimum input noise temperature of hf/k, where h is Planck’s constant and f the frequency. HEB mixers measure the incoming radiation by preserving both amplitude and phase information, and thus are subject to a quantum mechanical limit (quantum noise limit) to their sensitivity. People in the detector community have long believed that HEB mixers have too much noise from “classical sources”, and therefore there is no need to worry about the contribution of the quantum noise. In a close collaboration with an international team consisting of SRON (Netherlands Institute for Space Research), Purple Mountain Observatory, China, Chalmers University of Technology, Sweden, and University of Massachusetts, USA, we have measured and modeled the noise in terms of “receiver noise temperature” as a function of frequency from 1.6 to 5.3 THz. The key results are shown in the below Figure, together with the expected quantum noise contribution. The latter is found to be significant and responsible for about half of the total noise at the highest frequency. The ultimate sensitivity of a HEB mixer is crucial for practical observation instruments. Our sensitive nanodetectors are planned for the 4.7 THz 16-pixel arrays in a Spectroscopic/Stratospheric THz Observatory (GUSSTO) which is a long duration (~100 days) balloon observatory, and will potentially be used in SOFIA's (Stratospheric Observatory for Infrared Astronomy) 2nd-Generation Science Instruments.

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[1] W. Zhang, P. Khosropanah, J. R. Gao, E. L. Kollberg, K. S. Yngvesson, T. Bansal, R. Barends, and T. M. Klapwijk â&#x20AC;&#x153;Quantum noise in a terahertz hot electron bolometer mixerâ&#x20AC;?, Appl. Phys. Lett. 96, 111113 (2010). > Teun Klapwijk (Quantum Nanoscience)

Superconducting resonators to map the cool universe Superconductors have the nice property that the electrons do behave in two different ways. At very low temperatures all of them are bound in pairs and only a few are free, and called quasi-particle. A superconducting wire can act as a resonator with a resonance frequency set by the length and dielectric embedding, reaching quality factors in the order of 1 million. This high quality factor reflects the fact that the superconductor is lossless except for the few quasi-particles left at low temperatures. An attractive feature of these resonators is that many of them can be stacked together, each with a different resonance frequency, and read-out with one transmission-line. Weak, low energy photons will break the electrons pairs creating extra quasi-particles, leading to a small increase in loss which is detectable as a small reduction in quality-factor. This arrangement has the potential to revolutionize astronomical instrumentation allowing an ultrasensitive multi-pixel camera working at THz frequencies to study the cool submillimeter universe. However, entering this range of few quasi-particles, high quality factors and aimed for sensitivity the superconductor is tested to an unprecedented level. The expected ideal properties are not found in reality, which is due to the substrate, an oxide on the surface, and/or spurious signals. In addition in order to achieve the highest sensitivity one is dependent on material parameters, which are usually given and many times not engineerable. The below Figure shows a recent result [1], obtained in a collaboration with the National Institute for Space research (SRON), of the measured number of quasiparticles in aluminium at low temperatures as predicted theoretically (green curve). The black squares show the data following from the measured noise, which reflects the dynamic equilibrium between the reservoir of electron-pairs and that of the quasiparticles, socalled generationrecombination noise, measured here for the first time. The inverted red triangles show similar data following from the measured recombination time after exposure to photons. Obviously the number of quasiparticles is higher than expected theoretically, which limits the current sensitivity.

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This work has implications for quantum computation with superconducting tunnel junctions as well as for envisioned new telescopes such as the Cerro Chajnantor Atacama Telescope (CCAT). [1] P.J. de Visser, J.J.A. Baselmans, P. Diener, S.J.C. Yates, A. Endo, and T.M. Klapwijk, Number fluctuations of sparse quasiparticles in a superconductor, arXiv:1103.0758v1 [3 March 2011]

> Koenraad Schalm (LION) Within theoretical physics the explanation of high-temperature superconductivity is one of the big unsolved problems. The most important stumbling block is the Pauli principle responsible for the characteristic Fermi-energy of valence-electrons. At optimal doping --- the location in the phase diagram where the onset of superconductivity occurs at the highest temperature --- this energyscale mysteriously disappears. Experiments show that electrons form baffling collective “quantum-critical” states that behave identically on both microscopic and macroscopic scales. All conventional theoretical techniques are unable to escape the demands of the Pauliprinciple and hence fail to explain this quantum-critical phenomenology. The revolutionary developments in string theory since the late ‘90s have given rise to a completely new avenue to approach this question. Through the “Anti-de Sitter string theory/Conformal Field Theory” dictionary this collective behavior of electrons into and out of a scale-invariant quantum-critical state can be translated into a computation of the absorption of an electron into a black hole [1,2]. Additional string theoretic computations support this miraculous “dual” perspective on quantum-critical systems as black holes: existent empirical models of high-Tcsuperconductors are naturally seen to emerge from an analytical computation in string theory. Moreover given this black-hole point of view, one can explain the nonperturbative stability of the Fermi-liquid as the generic ground state of a many-body electron system.

Figure 1: The “Anti-de Sitter/ Conformal Field Theory” dictionary, (Image Source: A.T. Kamajian, in Information in the Holographic Universe by J. Bekenstein, Scientiﬁc American 2003) These three remarkable breakthroughs in our understanding of strongly correlated quantum mechanical electrons are accelerating research in “Anti-de Sitter/Condensed Matter Theory” through both the string theory and condensed matter community, as it may finally yield an understanding of the strange metal state in high-Tcsuperconductors.

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Figure 2: From a quantum-critical state (top) the normal state of electrons in metals emerges (The Fermi-liquid; bottom ďŹ gure), as computed with string theory. (Image taken from [1]) [1] M. Cubrovic, J. Zaanen, K. Schalm. String Theory, Quantum Phase Transitions and the Emergent Fermi-Liquid. Science 325 439 (2009). [arXiv:0904.1993] [2] Thomas Faulkner, Nabil Iqbal, Hong Liu, John McGreevy, David Vegh. Strange metal transport realized by gauge/gravity duality. Science 329 1043 (2010). [arXiv:0903.2477, arXiv:1003.1728]

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Theme

Dynamic Complex Systems

> Martin van Hecke (LION)

Flow of Foams Foams, emulsions, suspensions and granular media exhibit an intriguing mix of solid-like (jammed) and liquid-like behavior, which is well known but poorly understood. The main flow phenomenology of such glassy materials can be encoded in a rheological curve: a nonlinear relation between applied stress and observed flow rate, which is usually interpreted as an empirical fit-formula. In recent years we have developed a novel, powerful and predictive description of the complex flows of foams and grains, where the rheological curves are scaling functions of the associated jamming transition, scaling is governed by diverging fluctuations near the critical jamming point and these fluctuations are governed by an energy cascade, reminiscent of turbulence. The first step in this work was to experimentally probe the flow of twodimensional foams. By comparing the flow properties and dragforces of disordered foams (see figure), ordered (crystalline) foams and individual bubbles, we found that while ordered foams behave as given by a simple coarse graining of the local bubble interactions, for disordered foams the global drag forces scale very differently from the local drag forces, making a naïve coarse graining procedure impossible here and pointing at a nontrivial mechanism [1].

Left: Snapshot of a 2D foam, consisting of millimetric bubbles trapped below a glass plate. Right: Bubble trajectories reveal swirly, highly complex flow patterns

In subsequent work we also found that the amount of fluctuations, as given by the self-diffusion of bubbles in flow, becomes stronger, the slower the foam flows. To be precise: a naïve prediction for slow flowing materials is that there is a quasi-static limit – so that the bubble motion becomes independent of flow rate. In other words, a sped-up or slowed-down version of a movie of such flows would in all aspects look physically correct, since the only thing that is relevant is the amount of deformation, not its rate. We find, however, that this is not true: for the same deformation, the amount of fluctuations grows without bound, the slower the flow is! [2] Recently, we have been able to rationalize these findings in a theoretical model. We combine three ingredients. First, we note that power balance dictates that fluctuations in the relative motion of pairs of contacting particles (bubbles, grains) should diverge when the flow rate goes to zero. The second crucial ingredient is that the diverging fluctuations lead to nontrivial relaxation scale. Third, the relaxation scale governs the stresses in these materials through an effective elastic relation – combining these three ingredients leads to three equations for three unknowns. We 72

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are now in the position that, given the local interactions between bubbles, grains, droplets, we have a fully predictive model which describes both the diverging fluctuations and the non trivial rheology of a range of materials (our own foams, recent experiments at Penn on colloidal particles, several simulations) [3]. By predicting rheology from local interactions we have thus provided a new perspective on flows of disordered media. [1] G. Katgert, M. E. Mรถbius, M. van Hecke Rate Dependence and Role of Disorder in Linearly Sheared Two-Dimensional Foams Phys. Rev. Lett. 101 058301 (2008) [2] M.E. Mรถbius, G. Katgert and M. van Hecke Relaxation and Flow in Linearly Sheared Two-Dimensional Foams EPL 90 44003 (2010) [3] B.P. Tighe, E. Woldhuis, J.J.C. Remmers, W. van Saarloos, and M. van Hecke, Model for the Scaling of Stresses and Fluctuations in Flows near Jamming Phys. Rev. Lett. 105 088303 (2010) > Vincenzo Vitelli (LION)

Curved Space Crystallography When soft matter orders on a curved surface, intriguing patterns of elastic distortions arise from the competition between bending and compression. Often, these phenomena can be understood by combining elasticity with a simple geometrical principle: curvature causes geodesics and elastic energy to focus, in analogy to the gravitational lensing of light by the curvature of space-time. In this work, we study experimentally and theoretically the frustrated elasticity of colloidal monolayers confined on curved capillary bridges [1]. Cylindrical capillary bridges coated by colloids are stretched to produce a surface of negative curvature similar to a catenoid (see Figure). As a result of the imposed deformation, we observe a sequence of defect unbinding instabilities triggered by curvature rather than temperature. Topological defects first nucleate in the form of isolated dislocations above a critical curvature threshold that we calculate within the framework of continuum elastic theory. The dislocations subsequently proliferate and organize into pleats, neutral dislocation lines that vanish on the surface and resemble fabric pleats in clothing.

Figure: Two-dimensional curved crystals. A cylindrical capillary bridge coated by colloidal particles is stretched to form a curved surface shaped as a catenoid. As a result, the inital crystalline arrangement of the colloidal particles is disrupted by the nucleation of topological defects that relax the geometrical frustration. The detailed understanding and experimental control of topological defects on curved surfaces demonstrated in this study provides a firm basis to explore order and disorder in curved space. In terms of practical applications, we have identified design principles useful to engineer structures with curvature (like waisted nanotubes and vaulted architecture) as well as to develop novel techniques for soft lithography and directed self-assembly. [1] W. Irvine, V. Vitelli and P. Chaikin, Nature 468, 947 (2010). 73

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