Materials development
Quantum states for tomorrow’s materials It has become evident over recent years that some quantum many-body systems do not thermalise when excited by laser pulses, which raises new questions around their behaviour and opens new opportunities. We spoke to Professor Dmitry Abanin about his work in investigating the behaviour of these systems and developing new theoretical tools to describe them. It has long been thought that complex physical systems thermalise when they are initialized in a non-equilibrium state, meaning that they reach a state of thermal equilibrium, which can be described using statistical physics methods. For example, the temperature of a cup of coffee will change when milk is mixed in, before settling into a more stable state. “There will be some exchange of energy between different sub-systems and then the system will reach a thermal equilibrium,” says Dmitry Abanin, Professor of Physics at the University of Geneva. However, over recent years it has become clear that certain quantum many-body systems in fact do not behave in this way, raising new questions. “There is a whole world that we need to understand beyond conventional statistical physics,” says Professor Abanin. Quantum many-body systems This area forms a central part of Professor Abanin’s agenda in a new project based at the University of Geneva, in which researchers aim to probe deeper into the behaviour of quantum many-body systems, building on experimental data. These systems are defined quite broadly within the project, with researchers looking at a wide variety of systems. “Let’s say there are 1020 electrons in a system, which all interact strongly with atoms through Coulomb interaction. This is a many-body system which can lead to a full range of different behaviours,” outlines Professor Abanin. “The focus in this project is on better understanding how to control many-body states, and on investigating what new, essentially quantum phenomena we can get when we disrupt the equilibrium of a system.” The primary interest here lies in the states of matter which can arise when a system is stirred out of equilibrium, so when it does not thermalise. Researchers in the project aim to develop detailed descriptions of these many-body systems when they have been excited, for example by a laser or some other method. “If these systems don’t reach
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By building a deeper picture of the behaviour of many-body systems, researchers hope to lay the foundations for the development of new materials with interesting properties in future. In the long-term, Professor Abanin plans to apply what has been learned about systems out of equilibrium to help develop materials with specific properties. “We want to see whether by nudging a many-body state out of equilibrium, we can achieve certain desired properties,” he explains. This is a fast-moving area, and recent experimental breakthroughs could open up new avenues of investigation. “A new class of systems which do not thermalise was recently discovered, and in time we hope to have a much better understanding of the underlying mechanisms,” says Professor Abanin.
able to do certain useful things, like quantum computing and sensing.” A deeper understanding of the quantum physics of many-body systems is an essential step towards realising this potential, a point which underlines the wider relevance of Professor Abanin’s research. While a lot of investment and resources have been poured into research in areas like quantum computing, Professor Abanin says more work is required to translate this into tangible progress. “A much better understanding of the quantum physics of many-body systems is really an essential building block before we can truly realise the potential of these systems,” he stresses. “There’s been a lot of progress in the development of quantum technologies, but it’s not yet clear how to build further on these foundations in the most fruitful way.”
The focus in this project is on better understanding how to efficiently control many-body states, and on investigating what new types of unique phenomena we can get when we disrupt the equilibrium of a system.
(top left) A cloud of atoms can be controlled by lasers, realizing a tunable quantum many-body system. (top right) In traditional systems, a non-equilibrium state (with non-uniform density of particles) relaxes to thermal equilibrium state described by statistical mechanics. (bottom left) This project studies systems which do not thermalize, and are outside the realm of statistical mechanics. (bottom right) Experimental system of interest: Nitrogen-Vacancy (NV) spins in diamond. Their entangled states may be used to build better sensors.
thermal equilibrium, how do they behave?” asks Professor Abanin. Part of this work involves developing theories to describe the behaviour of many-body systems, with theorists and experimentalists working closely together. “As theorists we may predict something, then the experimentalists work on it. Or experimentalists may see something unusual, then we come up with a theory to explain it. Synergy between theory and experiment is essential in this field,” explains Professor Abanin. A number of new tools have been developed over recent years to make very controllable quantum systems, such as cold atoms, opening up new possibilities in this area of research. These quantum optics tools enable researchers to put particles in interesting initial states outside of their equilibrium, from which new insights can then be drawn. “You put the particles in an initial state, and then you let them interact and evolve,” explains Professor Abanin. “With cold atom systems there are typically enough particles to see the
most interesting phenomena that can stem from the collective interactions between them. With modern experimental techniques, it is possible to control how some of the particles interact, and to tune the properties of the lattice.” The aim is to achieve a greater degree of control over these many-body systems, which will enable researchers to study interesting phenomena like high-temperature superconductivity in greater detail. A lot of computational resources are required to describe these quantum many-body systems however. “They work differently to classical systems,” stresses Professor Abanin. One of the many body systems that Professor Abanin and his colleagues are working with is black diamond, which hosts degrees of freedom that may serve as quantum bits. “These are nitrogen vacancy spins, which are very coherent and can be controlled,” he continues. “For example, I might want to engineer an entangled state of the spins, in order to measure the magnetic field very precisely.”
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This research holds important implications for materials science, providing the foundations on which future development will be based. For his part, Professor Abanin plans to continue investigating these many-body states in future, work which will help researchers build a deeper understanding of certain interesting phenomena. “One of the reasons why people are interested in these quantum systems is because they open a window into probing fundamental scenarios that we couldn’t probe before,” he explains. “A second major reason why people are interested in these quantum systems is the expectation that they will be
This is a challenge for both theorists and experimentalists, and close collaboration will be central to efforts to harness the potential of non-equilibrium quantum many-body states. It’s possible to run experiments today with a much higher degree of control, allowing researchers to probe theoretical predictions much quicker than previously, and Professor Abanin hopes the project’s work will yield some exciting results over the coming years. “I’m hoping that we will have some new results in terms of better understanding the potential applications of a non-thermalising state. This is a very exciting time in research,” he says. The project team
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Ergodicity breaking and non-equilibrium quantum many-body states Project Objectives
The project aims to develop a theoretical framework for non-equilibrium quantum systems which are studied experimentally in systems such as cold atoms. Based on our recent work, we will focus on systems which do not thermalize as they can host unique phenomena not possible in equilibrium. This research is expected to guide efforts to control many-body states, with potential applications in quantum computing and material design.
Project Funding
This project is funded by the Swiss National Science Foundation. Project start date: October 1, 2019
Project Partners
Dr. Michele Filippone, Dr. Louk Rademaker (Ambizione Fellows, University of Geneva)
Contact Details
Project Coordinator, Prof Dmitry Abanin University of Geneva UNIGE Department of Theoretical Physics 24 quai Ernest-Ansermet, 1211 Geneva, Switzerland T: +41 22 379 631 E: Dmitry.Abanin@unige.ch W: https://www.unige.ch/sciences/ physique/theorique/en/research/
Professor Dmitry Abanin
Prof. Abanin received PhD from MIT in 2008, followed by a fellowship at Princeton University. He joined University of Geneva as a professor of physics in 2015. He theoretically studied quantum twodimensional materials such as graphene. His current research focuses on unique phenomena found in non-equilibrium quantum systems not described by statistical mechanics.
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