View from the arXiv: Jun 13 - Jun 17 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 As these are all preprints which have not yet been peer-reviewed, remember to take any claims and conclusions with a grain of salt and be sure to cast a critical eye over the work if you’re interested in more details.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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June 13th

Quantum heat engine based on a spin-orbit and Zeeman-coupled Bose-Einstein condensate, by Jing Li, E. Ya Sherman, and Andreas Ruschhaupt: I haven’t heard the term heat engine since my undergraduate days, but it’s interesting to see the quantum thermodynamics revolution continue! A heat engine, in general, is a device which converts heat into some mechanical process, and in fact many everyday items work on this principle. This work explores a quantum mechanical version of a heat engine, which implements a particular thermodynamic cycle known as a Stirling cycle in a Bose-Einstein condensate with a tunable spin-orbit coupling. The authors show that the efficiency of the heat engine can be changed by altering the spin-orbit coupling, and that there is an optimal value that leads to particularly high efficiency. It’s a very interesting theoretical study, though one thing that still eludes me is whether quantum heat engines are simply interesting from a fundamental point of view, or whether they could also have any useful applications in the future. (Answers on a postcard…!)

A theory explaining the limits and performances of algorithms based on simulated annealing in solving sparse hard inference problems, by Maria Chiara Angelini, and Federico Ricci-Tersenghi: Inference problems are a class of problem that come up often in modern data science, as they involve working backwards from a set of data to reconstruct properties of the system or model used to generate the data. It’s a hard problem that underpins a lot of different disciplines, from fundamental research to commercial machine learning solutions, and so finding good algorithms which can solve these hard problems is a major challenge. This work focuses on so-called ‘hard inference problems’, which as the name suggests are extremely difficult to solve, and studies in detail the performance of two algorithms based around Monte Carlo Markov Chain methods. In particular, the paper looks as Simulated Annealing and a new approach known as Replicated Simulated Annealing. What I found particularly interesting was that the latter approach changed the nature of the phase transition from discontinuous to continuous. As the concept of a ‘hard’ phase stems from the existence of a discontinuous transition, this method has some interesting implications for both inference problems and other statistical mechanics problems (e.g. studies of glassy physics).

June 14th

Anomalous transport regime in non-Hermitian, Anderson-localizing hybrid systems, by Himadri Sahoo, R Vijay, and Sushil Mujumdar: Normally in physics we work with Hermitian systems, i.e. models which have purely real energy eigenvalues and are isolated from their environments. Recently the study of non-Hermitian systems has attracted growing interest, as models which have complex energy eigenvalues can be thought of as approximations to some kind of dissipative process (e.g. the sort of thing usually studied with Lindblad master equations). This work studies the interplay of non-Hermitian couplings and Anderson localisation, and crucially does so in an experimental setting using hybrid polariton-photon states with dissipation. The authors argue for the existence of an ‘anomalous transport’ regime which occurs at strong disorder, and is due to the formation of what the authors call ’necklace states’, which are basically chains of local resonances that facilitate transport in the system. This is something I haven’t seen before, and to see it here in an experimental system is fascinating.

Second sound with ultracold atoms: A brief historical account, by Hui Hu, Xing-Can Yao, and Xia-Ji Liu: Second sound is a purely quantum mechanical effect where thermal fluctuations propagate in a wave-like manner, and acts as a signature of superfluidity in many systems as the second sound velocity is linked to the superfluid fraction (i.e. fraction of particles in a system which are in a superfluid state). This review describes the history of second sound from both a theoretical and experimental perspective, and is an interesting read for anyone who studies superfluidity or superconductivity, or indeed for anyone who’s never heard of ‘second sound’ and wants to know more about it!

Non-Abelian eigenstate thermalization hypothesis, by Chaitanya Murthy, Arman Babakhani, Fernando Iniguez, Mark Srednicki, and Nicole Yunger Halpern: The eigenstate thermalisation hypothesis (ETH) is commonly used as the explanation for why many-body quantum systems isolated from their environment eventually reach some form of internal thermal equilibrium when left to their own devices. The ETH, however, remains a ‘hypothesis’ and not a proof, and so many outstanding questions remain and the study of when systems obey or do not obey the ETH is a major area of research. This work generalises the ETH to systems with non-Abelian symmetries, which exhibit conserved quantities that do not commute with each other, adding a complication to the usual definition of the ETH (which assumes that any conserved quantities are mutually commuting). This is quite a dense and technical work, which is a little tricky to read if you’re not familiar with some of the mathematical machinery used, but the underlying physics of it is quite intriguing.

June 15th

A dynamical theory for one-dimensional fermions with strong two-body losses: universal non-Hermitian Zeno physics and spin-charge separation, by Lorenzo Rosso, Alberto Biella, Jacopo De Nardis, and Leonardo Mazza: Another non-Hermitian paper, but this time also involving quantum Zeno physics! Here we look at a quantum dengenerate gas of spinful fermions, described by a Hubbard Hamiltonian with some two-body loss term (i.e. when two particles occupy the same lattice site, some might fall out of the system and be lost). The authors propose a theoretical model, valid in the thermodynamic limit (i.e. large systems) which describes the non-equilibrium dynamics of this system and show that it agrees well with numerics. They focus on the Quantum Zeno regime, a counter-intuitive parameter regime where losses (or more general, any form of measurement) can slow the time evolution of a system and act to stabilise transient phases. Future directions for this work could include extending it beyond the Zeno regime to a more general description of loss in one-dimensional quantum systems, and in particular the so-called superradiance transition, which the authors do not focus on here.

June 16th

Energy Transport between Strange Quantum Baths, by Ancel Larzul, and Marco Schirò: The question of how heat flows between two systems prepared at different temperatures has a long history dating back to the time of Fourier, but in quantum systems the question of how heat and energy move between coupled systems has not yet been fully answered. In particular, much of the formalism of non-equilibrium energy transport relies on so-called quasiparticles, well-defined particle-like excitations which move throughout a system and carry heat/energy/charge. But what happens in a system without quasiparticles? That’s the question that the authors of this work set out to answer, by studying two Sachdev-Ye-Kitaev (SYK) ‘strange metal’ thermal baths connected by a quantum dot. Remarkably, they manage to obtain an exact formula for the flow of energy from one bath to another, and show that the properties of these quasiparticle-free SYK models have a dramatic effect on the energy transport. The SYK model is full of surprises, and it always amazes me that a model this complicated can be solved exactly in some situations!

June 17th

Quantum Hall and Synthetic Magnetic-Field Effects in Ultra-Cold Atomic Systems, by Philipp Hauke, and Iacopo Carusotto: This is actually a chapter of an upcoming book, rather than a research paper, but it’s nonetheless a nice overview of synthetic magnetic fields in ultracold atomic matter. The idea here is that ultracold atoms are charge neutral, and standard magnetic fields act only to modify their internal energy levels, so to study magnetic effects in ultracold atomic gases, it’s necessary to construct so-called synthetic magnetic fields that mimic the effect of a charged particle in a conventional magnetic field. This conceptual leap opens up a lot of interesting avenues for the careful construction of exotic quantum matter in a highly controllable environment, and rather than dwell much more on it myself, I instead leave it to this review to explain all of this in much more detail!