Julien Barrier

Writing software is a big part of experimental physics. Even a moderately complex experiment involves several instruments that need to talk to each other and stay synchronised, so sooner or later we all have to decide how to control them all.

For a long time, LabVIEW was the answer. It is easy to pick up, but is awkward to version-control, and clumsy when you just want to write a quick scripts. Fewer and fewer students learn as part of the community has moved to Python. Written in Python, QTLab was popular for a while, with a simple interface, but it is no longer supported. QCoDeS and LabRAD have since become common in mesoscopic physics and quantum transport. They are really powerful, but they are not easy to learn for a new graduate student.

When I was a PhD student in Manchester, I looked at several of these options. With my friend Servet Ozdemir we tried a few different things to try to replace the aging LabView interface. Eventually I settled on QCoDeS, and started writing my own functions, drivers for the instruments we needed, station files and scripts.

I carried that set of scripts, commands and station files with me from Manchester to ICFO during my postdoc, and it showed. I ended up with scripts nobody else could run, overnight queues that were lost when something crashed, and Jupyter notebooks so large they could no longer be opened on a modest laptop. It was also difficult to train students this way. Everyone has their own idea of what good code looks like, and most people do not read the QCoDeS documentation. Some students end up writing fixes for problems that existing functions already solve, which complicates the analysis, because datasets measured by different people have their own nomenclature and conventions.

In the end, nothing was as easy as the plain old LabVIEW interface. It got me started quickly, and let me learn the physics before learning how to write advanced functions. Now that I am starting my own group and have PhD students to train, I wanted a tool that makes the learning curve gentle without giving up the functionality that makes QCoDeS useful.

For this reason, I have released mesoscopy, an open-source graphical interface for running experiments in quantum transport and mesoscopic physics. It is built on top of QCoDeS, so it works with the instrument drivers and databases many transport labs already use.

• Code: github.com/mpiLDE/mesoscopy
• Documentation: mpilde.github.io/mesoscopy

What it does

• Sweeps. Linear, logarithmic and “together” sweeps (several parameters moving in lockstep), with live plotting while the measurement runs. Takes advantage of the dond function of QCoDeS and its modularity.
• Safety first. Configurable limits and maximum ramp rates on every parameter, so a typo cannot blow up a device.
• A queue that survives crashes. Queue up measurements and leave them running overnight. If something goes wrong, the queue can be resumed.
• Monitoring. Alarms and persistent logs for the quantities you care about (temperature, leakage current, and so on).
• Reproducibility Set experiment parameters based on those recorded for a previous run.
• Standard data format. Everything is written to regular QCoDeS databases, so your existing analysis code keeps working.

Getting started

I wrote it and tested it using Python 3.14+ and QCoDeS 0.60.0+. Installation instructions and a walkthrough are in the documentation. The test suite runs on simulated instruments, so you can try the software without any hardware attached.

Open source

Mesoscopy is released under the MIT license. Bug reports, feature requests and pull requests are welcome on GitHub. If you use it in your lab, I would be glad to hear about it.

A bit late announcing this formally. I will be joining the Max Planck Institute for Solid State Research next Spring as an Independent Research Group Leader.

Our research group will focus on exploring low-dimensional quantum materials using a combination of electronic transport and nano-photonics techniques. I am looking forward to working with a talented team of scientists on this journey.

PhD and postdoctoral positions will be available starting next summer − feel free to reach out for more details! Prospective PhD candidates should apply through the Max Planck Graduate Center for Quantum Materials (deadline for this call is 10 December 2025). Prospective postdocs can contact me by e-mail with a CV. I am also able to support potential applications to Humboldt or MSCA-PF fellowships.

This is a press release for my recent paper of 1D proximity superconductivity along graphene domain walls

Manchester Scientists Found Novel One-Dimensional Superconductor In a significant development in the field of superconductivity, researchers at The University of Manchester have successfully achieved robust superconductivity in high magnetic fields using a newly created one-dimensional (1D) system. This breakthrough offers a promising pathway to achieving superconductivity in the quantum Hall regime, a longstanding challenge in condensed matter physics. Superconductivity, the ability of certain materials to conduct electricity with zero resistance, holds profound potential for advancements of quantum technologies. However, achieving superconductivity in the quantum Hall regime, characterised by quantised electrical conductance, has proven to be a mighty challenge.

The research, published this week (25 April 2024) in Nature, details extensive work of the Manchester team led by Professor Andre Geim, Dr Julien Barrier and Dr Na Xin to achieve superconductivity in the quantum Hall regime. Their initial efforts followed the conventional route where counterpropagating edge states were brought into close proximity of each other. However, this approach proved to be limited.

“Our initial experiments were primarily motivated by the strong persistent interest in proximity superconductivity induced along quantum Hall edge states,” explains Dr Barrier, the paper’s lead author. “This possibility has led to numerous theoretical predictions regarding the emergence of new particles known as non-abelian anyons.”

The team then explored a new strategy inspired by their earlier work demonstrating that boundaries between domains in graphene could be highly conductive. By placing such domain walls between two superconductors, they achieved the desired ultimate proximity between counterpropagating edge states while minimising effects of disorder.

“We were encouraged to observe large supercurrents at relatively ‘balmy’ temperatures up to one Kelvin in every device we fabricated,” Dr Barrier recalls.

Further investigation revealed that the proximity superconductivity originated not from the quantum Hall edge states propagating along domain walls, but rather from strictly 1D electronic states existing within the domain walls themselves. These 1D states, proven to exist by the theory group of Professor Vladimir Fal’ko’s at the National Graphene Institute, exhibited a greater ability to hybridise with superconductivity as compared to quantum Hall edge states. The inherent one-dimensional nature of the interior states is believed to be responsible for the observed robust supercurrents at high magnetic fields. This discovery of single-mode 1D superconductivity shows exciting avenues for further research. “In our devices, electrons propagate in two opposite directions within the same nanoscale space and without scattering”, Dr Barrier elaborates. “Such 1D systems are exceptionally rare and hold promise for addressing a wide range of problems in fundamental physics.” The team has already demonstrated the ability to manipulate these electronic states using gate voltage and observe standing electron waves that modulated the superconducting properties.

“It is fascinating to think what this novel system can bring us in the future. The 1D superconductivity presents an alternative path towards realising topological quasiparticles combining the quantum Hall effect and superconductivity,” concludes Dr Xin. This is just one example of the vast potential our findings holds.”

20 years after the advent of the first 2D material graphene, this research by The University of Manchester represents another step forward in the field of superconductivity. The development of this novel 1D superconductor is expected to open doors for advancements in quantum technologies and pave the way for further exploration of new physics, attracting interest from various scientific communities.

Bibliographic reference: J. Barrier et al., “One-dimensional proximity superconductivity in the quantum Hall regime” Nature 628, 741-745, (2024)

I had the chance to present my research at the American Physical Society’s (APS) March meeting 2024. The event brought together over 13,000 physicists from across the globe. All talks were recorded, which allowed me to post mine on Youtube. If you missed it, you may hear about my recent research about inducing proximity superconductivity along graphene domain walls in the quantum Hall effect regime:

A preprint is available on arXiv.

I have recently been asked by a journalist to comment my work on Brown-Zak fermions (see my previous blog here ). As part of the discussion, I was asked to introduce the concept of a quasiparticle.

In condensed matter physics, simple equations are usually used to describe complex behaviours. For example, it is very simple to consider the equations of an electron not interacting with its surroundings, because the number of atoms or any other particle is usually in the order of $N_A \approx 10^{23}$, which would make analytical resolutions impossible to solve. When one tries to make the model a tad more realistic, in semiconductors for example, it is possible to treat the effect of surrounding electrons as “perturbations” on the free electron. By adding a different mass to that free electron, its behaviour will be different, slowed down or accelerated (basic understanding with newton’s law: $\Sigma F = ma$). That way, electrons in semiconductors, for example, can be modelled as a “quasiparticle”, that is, move like a heavy or light free particle.

The idea for first proposed by the Russian physicist Lev Landau in the early 1950s. He was studying the properties of liquid helium, a superfluid that can flow without any friction. He found that the behaviour of liquid helium could be explained by treating it as if it were made up of not He atoms, but of a different kind of particles that were not actually present in the liquids. These particles, which he called quasiparticles, were excitations of the underlying quantum system that could be described by their own equations of motion. This was a major breakthrough in our understanding of condensed matter physics. It has been used to explain the behaviour of a wide variety of materials, including superconductors, semiconductors, insulators. Quasiparticles are now an essential part of the theoretical framework for understanding condensed matter physics.   Let us give important examples of how the idea of quasiparticles has been used to explain the behaviour of materials:

• Superconductors. In a superconductor, the electrons are able to move without any resistance. This is because they are paired up into what are called Cooper pairs. These Cooper pairs can be thought of as quasiparticles that are different from the electrons that make up the normal state of the material.
• Semiconductors and metals. In semiconductors and metals electrons can move through the ideal crystal without any resistivity, and in real crystals all the resistivity appears due to scattering of electrons on defects, and due to the oscillations of the crystal lattice (when the position of atoms is deflected from their equilibrium).

In this picture, electrons just ignore the atoms of the crystal, while one would think that those atoms also should be an obstacle for electrons. This is happening because electrons in solid state conductors are not the same electrons which we usually discuss in vacuum. Electrons in crystals are quasiparticles, which have a very different mass from the mass of the free electron, and in some exotic cases the even have a different charge (e.g. fractional quantum Hall effect).

The idea of quasiparticles is a powerful tool that has helped us to understand a wide variety of phenomena in condensed matter physics. It is likely to continue to play a major role in our understanding of this fascinating field of science.