Superconducting circuits for quantum applications
The capacitor creates an electric field
A microwave signal travels across
The inductor creates a magnetic field
The magnetic field interacts with the magnetic vortex
At the centre of the vortex, the core creates a precision spin (the central arrow moves with a circular motion)
The team behind the QFaST project are investigating spin excitations in magnets, using sophisticated sensors to measure their properties, while they are also looking into whether they could be used to perform certain interesting tasks. These could include the detection of dark matter and certain quantum computing applications, as Dr Pepa Martínez-Pérez explains.
The topic of spin excitations in magnets has attracted a lot of attention in research over recent years, borne out of both fundamental interest and also its potential commercial relevance, for example in quantum computing. Much of this research has focused on ferromagnetic objects, in which the magnetisation is uniform, but as Principal Investigator of the QFaST project Dr Pepa Martínez-Pérez is now looking at spin excitations in magnetic textures, which can be thought of as small, discrete magnetic modulations. “One example of a magnetic texture is a vortex in which the spins are naturally arranged, forming a central coil,” she explains. Magnetic textures are interesting for several reasons. “First of all they naturally stabilise, they are very easy to stabilise and to fabricate. They are also very robust, they cannot be destroyed by temperature, or quantum fluctuations” says Dr Martinez-Perez. “These textures bring new spin excitations in the gigahertz regime into play. This regime is very interesting in terms of developing quantum technologies based on superconducting circuits.”
QFaST project
As part of her work in the QFaST project, Dr Martínez-Pérez is studying the fundamental behaviour of spin excitations in magnets, using superconducting quantum interference devices (SQUIDS), which are very sensitive to small variations in magnetisation. These SQUIDS are
typically used to measure very slow or quasi-static variations in magnetisation, now the project team aim to extend these capabilities to AC, and potentially even to the gigahertz frequency range. “SQUIDS are already capable of working at high frequencies, as the Josephson effect holds, but it can be difficult to read-out the notifications. We aim to extend the frequency bandwidth operation of these devices, so that they can be also used not only to detect a magnetic vortex in a particle, but also to measure its dynamics and diversity,” explains Dr Martínez-Pérez.
A material called yttrium barium copper oxide (Yba2Cu3O7 - or YBCO) is being used in the project to develop superconducting quantum nanocircuits which function as SQUIDS. “This is a very well-known, highcritical temperature superconductor,” says Dr Martínez-Pérez.
This material is quite commonly-used in developing photon-conducting circuitry, yet it can be difficult to work with as it is fairly delicate, in the sense that it is very sensitive to defects. However, these
defects also behave as Josephson junctions, which are essential to the functioning of SQUIDS. “We can use these defects to build Josephson junctions, allowing us to fabricate SQUIDS and other devices,” continues Dr Martínez-Pérez. The project’s agenda also includes another line of research, in which Dr Martínez-Pérez and her colleagues are exploring the use of conventional microwaves or high-frequency circuits to address spin excitations in a different way. “This field of research is usually called cavity magnonics,” she says. The project team are investigating the interaction of the spin excitation with photons trapped in cavities, in this case superconducting cavities. This holds fundamental interest to Dr Martínez-Pérez, as a strong coupling between magnetic vortex excitations and photons in a cavity has not yet been achieved, while she says this could also open up wider possibilities. “We want to learn more about these magnetic excitations, and to use them to perform interesting tasks. For instance, vortices in some senses focus the inhomogeneities of a magnetic field in very small regions.
between distant spin qubits,” she outlines.
The project team is collaborating with another group investigating magnetic molecules as candidates for building spin qubits, with a view to their eventual application in quantum computing, which would lead to some significant benefits.
“Quantum computers would not only be much faster than existing technology, but they would also allow us to perform simulations or operations that are not currently possible with classical computers,” explains Dr Martínez-Pérez.
The project team aim to both investigate these more applied possibilities arising from their research, while also exploring many of the fundamental aspects of magnetic textures, which Dr MartínezPérez believes are worthy of attention purely for their own sake. These objects encapsulate a lot of interesting physics, with respect to their boundary conditions, geometry and topology considerations for example, while their excitations are also highly visual and can be understood intuitively. “These magnetic textures are really beautiful, and joining these objects
“We aim to extend the frequency bandwidth operation of SQUIDS, so that they can be also used not only to detect a magnetic vortex in a particle, but also to measure its dynamics and diversity.”
These small regions would then be very sensitive to other excitations of signals,” she says. One interesting possibility is using magnetic excitations as sensors for dark matter. “If you are able to measure the number of magnons you have, you would have a means to sense if dark matter axions are passing through,” continues Dr Martínez-Pérez. “Another possibility would be to couple these magnetic textures to other spins, to sense them.”
Spin qubits
A coupling like this would be extremely useful for the development of quantum computing technology, as spin qubits have been identified as promising candidates for building quantum memories and quantum computing protocols. These spin qubits are very coherent, yet they are also extremely difficult to sense, a topic of great interest to Dr Martínez-Pérez. “Magnons could interact strongly with spin qubits. This would be a very interesting result, as it would allow you on the one hand to sense and read-out spin qubits, but also to mediate interactions
QFaST
Quantum Fast Spin dynamics addressed by High-Tc superconducting circuits
Project Objectives
The QFaST project aims to address the quantum properties of quantized spin waves (or magnons) in magnetic textures. Researchers aim to both develop useful tools for the study of nanoscopic spin excitations based on superconducting circuits, and also to access interesting quantum properties of vortex excitations, such as the characterization of zeropoint magnetization fluctuations in vortices.
Project Funding
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant agreement ID: 948986.
Project Partners
• Dieter Koelle and Reinhold Kleiner at University of Tübingen (Germany)
• Eugenio Coronado at Instituto de Ciencia Molecular (Valencia)
• David Zueco and Irene Lucas at Instituto de Nanociencia de Materiales de Aragón (Spain)
Contact Details
Project Coordinator, Dr. María José (Pepa) Martínez Pérez, PhD
Tenured Researcher
INMA - Instituto de Nanociencia y Materiales de Aragón CSIC - Universidad de Zaragoza. Facultad de Ciencias.
C/ Pedro Cerbuna, 12 - 50009 Zaragoza - España T: +34 976 76 1216
E: pemar@unizar.es
W: https://www.qmad.es/quantummagnonics-with-magnetic-textures/
with quantum photons in cavities is also extremely interesting from a fundamental point of view. We are building a system in which these magnetic excitations are absorbing photons and emitting them, in a quantum coherent state,” says Dr Martínez-Pérez. “I have worked for many years in superconducting circuits, but my real passion has always been magnetism, and I’m very excited about putting these two systems together.”
This research is ongoing, and Dr Martínez-Pérez says that some interesting results have been obtained over the course of the project. Prototype SQUIDS have been developed, which Dr MartínezPérez believes could be used in physics research soon, while progress has also been made in terms of the materials for building superconducting circuits and stabilising magnetic textures. “We are now able to observe quantum polaritons in conventional homogenous magnetisation states. The next step will be to move to textures, which I hope to do in the next year or so,” she says.
Dr. María-José (Pepa) Martínez Pérez is a tenured CSIC Scientist and is deeply involved in the emerging field of quantum magnonics. She is on the board of directors of the Spanish Division on Condensed Matter Physics and the editorial board of the Spanish Journal of Physics.
