# View from the arXiv: May 23 - May 27 2022

A summary of new preprints appearing on arXiv during the week of May 23rd to May 27th 2022

Welcome to ‘View from the arXiv’, where each week I’ll put together a short list of new preprints which have appeared on arxiv.org during the week which I’ve found interesting. I’ll focus on the categories ‘*Disordered Systems and Neural Networks*’, ‘*Quantum Gases*’ and ‘*Strongly Correlated Electrons*’, and in particular the first two as these are the main areas of the arXiv which I follow. This is an entirely subjective list of things which appeal to me, and of course there are far too many interesting papers to be able cover all of them so I just choose one or two from each day to highlight here. As these are all preprints which have not yet been peer-reviewed, remember to take any claims and conclusions with a grain of salt and be sure to cast a critical eye over the work if you’re interested in more details. (And let’s face it, this caveat should be applied to any published work too…!)

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**May 23rd**

*Ground-state energy distribution of disordered many-body quantum systems, by Wouter Buijsman, Talía L. M. Lezama, Tamar Leiser, and Lea F. Santos*: In much of modern condensed matter, when we study disordered quantum systems we almost immediately specify to excited states, but there is still a lot to be gleaned from studying the ground state properties. That’s what this paper sets out to do, by comparing the probability distribution for the ground state energy of a disordered system with a distribution obtained from random matrix theory known as the Tracy-Widom distribution. The authors find that certain random models exhibit ground state energy distributions consistent with the Tracy-Widom distribution, while others don’t. Based on a quick read, it looks to me like models which exhibit chaos/thermalisation do exhibit Tracy-Widom-like physics, while models which exhibit non-ergodic behaviour don’t, but this is undoubtedly an oversimplification of the results in this work!

*Nonequilibrium thermodynamics of the asymmetric Sherrington-Kirkpatrick model, by Miguel Aguilera, Masanao Igarashi, and Hideaki Shimazaki*: There has been a resurgence of interest in quantum thermodynamics in recent years, and with it a renewed wider interest in looking again at plain old *thermodynamics* in interesting models that haven’t been widely studied. This paper takes a modified Sherrington-Kirkpatrick model – the unmodified one is a commonly-studied in the context of spin glasses – and looks at the non-equilibrium thermodynamics, particularly in the context of entropy but also including other thermodynamic order parameters. The remarkable feature of this work is that it’s been possible to make a lot of analytical progress in such a complex system. While the model itself seems a little abstract, the results here are intriguing and I’d be very curious to see this extended into the realm of quantum Sherrington-Kirkpatrick models, and quantum spin glasses.

**May 24th**

*Effect of spin-orbit coupling in one-dimensional quasicrystals with power-law hopping, Deepak Kumar Sahu, and Sanjoy Datta*: Quasiperiodic systems offer an interesting intermediate scenario between random disorder and entirely ‘clean’ translationally invariant models. The Aubry-André-Harper model studied here is well-established in this context, but here the authors add a few new ingredients into the mix, namely spinful fermions with long-range hopping and a spin-orbit coupling term. Most of the analysis and quantities studied will be familiar to anyone who’s read recent(ish) papers on quasiperiodic systems, but it’s interesting to see here how the spin-orbit coupling modifies the physics. Specifically, it increases the critical ‘disorder’ strength required for localisation. I suspect this can be understood in terms of the spin-orbit coupling acting a bit like system-bath coupling between the ‘up’ and ‘down’ fermions respectively, increasing the system’s tendency to equilibrate and requiring a stronger quasiperiodic potential to compensate.

*Observation of classical to quantum crossover in electron glass, by Hideaki Murase, Shunto Arai, Takuro Sato, Kazuya Miyagawa, Hatsumi Mori, Tatsuo Hasegawa, and Kazushi Kanoda*: It’s rare for me to include an experimental paper in the weekly round-up (which probably points towards a bias that I should try to address…!) and even though I don’t fully understand the details of this one, it’s worth highlighting here as it investigates a problem close to my heart, namely the classical-to-quantum crossover in glassy systems. Here the authors use Raman spectroscopy to investigate a phase called a ‘charge glass’ rather than a more conventional spin glass, and experimentally demonstrate that increased geometric frustration leads to a crossover from classical to quantum behaviour that can be distinguished in a variety of ways, including the charge density profile and the temperature dependence. This work makes me want to learn more about the theoretical side of charge glasses and find out what else can be done experimentally with these materials!

**May 25th**

*Real-time correlators in chaotic quantum many-body systems, by Adam Nahum, Sthitadhi Roy, Sagar Vijay, and Tianci Zhou*: This is a long and very technical paper studying the evolution of correlation functions in systems exhibiting quantum chaos. It’s a very careful study of how these correlation functions evolve in time, and reads like a collection of insights compiled over quite a few years. I won’t say much more about this as it’s a dense and difficult read, but it’s worth highlighting as a very thorough reference that will likely be of a lot of use to people working on non-equilibrium dynamics in the near future.

*Benchmarking Quantum Simulators using Quantum Chaos, by Daniel K. Mark, Joonhee Choi, Adam L. Shaw, Manuel Endres, and Soonwon Choi*: I’m not sure I’ve ever seen a paper with a Supplemental Material long enough to have its own table of contents, but here we are: a four-page paper with a twenty seven page supplement! This paper tackles the problem of benchmarking quantum systems. The idea is that if quantum hardware can do things that classical hardware can’t, how can we first test the quantum hardware to make sure that it’s doing what we think it’s doing? Many benchmarking protocols already exist, but they’re not always practical, and so there is still demand for improved methods for the testing of quantum systems. This work presents a new way to compute the fidelity of a given quantum state prepared in current-generation quantum simulators. In other words, it’s a way of checking how exactly a desired quantum state is produced in real experiments, and crucially this approach is claimed to be highly efficient and ’easy to implement’ in existing hardware platforms. If this turns out to be true, I can see this being an extremely useful piece of work!

**May 26th**

*Anderson and many-body localization in the presence of spatially correlated classical noise, by Stefano Marcantoni, Federico Carollo, Filippo M. Gambetta, Igor Lesanovsky, Ulrich Schneider, and Juan P. Garrahan*: Fully characterising and understanding the stability of quantum localised phases of matter remains an open question. This work looks at two types of localised phase (Anderson localised and many-body localised) in the presence of spatially correlated classical noise. The authors find that memory-like localisation effects can be preserved for long times, with a timescale linked to the correlation length of the noise, leading to a metastable localised phase that disappears in the long-time limit. Most of the analysis focuses on the Anderson localised model, with a brief look at the many-body case towards the end. The abstract makes mention of a possible connection with glassy physics that I’d love to have seen more about, but alas the manuscript doesn’t dig into this any further.

**May 27th**

*Simulation Complexity of Many-Body Localized Systems, by Adam Ehrenberg, Abhinav Deshpande, Christopher L. Baldwin, Dmitry A. Abanin, and Alexey V. Gorshkov*: Borrowed from the quantum information community, complexity theory is fast becoming a very interesting way to study and understand many-body quantum matter, giving an interesting and complementary perspective that’s quite different to the way most many-body theorists think about things, i.e. rooted in the concept of observables and order parameters rather than information content. This work studies complexity in the context of many-body localisation, providing an interesting counterpart to studies of complexity in systems which exhibit chaotic behaviour. In particular, the authors find that many-body localised phases are ’less complex’ than chaotic systems (defined here as exhibiting a sublinear growth of complexity, as opposed to a linear growth). This is an interesting application of complexity to disordered systems, although the disorder itself doesn’t enter explicitly as far as I can see, which makes me wonder if any of the conclusions would be modified in a realistic system with rare regions and Griffiths effects. I’ll be curious to see more work using complexity theory to study localisation in the future! (With thanks to Dr Sumeet Khatri for drawing this paper to my attention, as it appeared in quant-ph which is an area of the arXiv that I don’t read…!)