# View from the arXiv: Jun 20 - Jun 24 2022

A summary of new preprints appearing on arXiv during the week of June 20th to June 24th 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 As these are all preprints which have not yet been peer-reviewed, remember to take any claims and conclusions with a grain of salt and be sure to cast a critical eye over the work if you’re interested in more details.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 20th**

*Effects of auto-correlated disorder on the dynamics around the many-body localization transition, by Isaías Vallejo-Fabila, and E. Jonathan Torres-Herrera*

Often when studying disordered systems, we work with disorder which is generated entirely randomly. It’s unusual to see works consider correlated disorder (except in the special case of *quasiperiodic* systems), but that’s what we have here - an example of a disordered system where the disorder is *not* entirely random, but instead exhibits some degree of spatial correlation. One might naively expect correlations to give rise to delocalising resonances (e.g. spatially separated lattice sites are more likely to be close in energy than in a totally random potential), and indeed that seems to be the case here, as confirmed by a variety of different static and dynamic quantities. The main interesting thing about this work is the unusual choice of disorder distribution, and I hope it leads to further interest in novel types of disorder, as this is something I’ve also been getting interested in recently.

*Delayed Thermalization in Mass-Deformed SYK, by Dillip Kumar Nandy, Tilen Cadez, Barbara Dietz, Alexei Andreanov, and Dario Rosa*

This paper wasn’t quite what I expected from the title, and I almost didn’t include it, but it has some interesting features nonetheless. It considers a mixed Sachdev-Ye-Kitaev (SYK) model with both 2-body and 4-body random couplings (known in the field as SYK2 + SYK4). It’s not what I’d normally think of a ‘mass-deformed’ or ‘random mass’ model, as the quadratic term is off-diagonal and doesn’t include an on-site potential (i.e. what I’d call a ‘mass’ term), but terminology aside this paper still draws some interesting parallels between the SYK model and many-body localization. While I think most of the results won’t be new to the SYK community, who have studied SYK2 + SYK4 models extensively, this is still worth a read for anyone interested in keeping up to date with numerical studies of how disordered many-body systems approach thermal equilibrium.

**June 21st**

*There was no arXiv announcement today, and I’m not sure why…!*

**June 22nd**

*Localization detection based on quantum dynamics, by Kazue Kudo*

It’s always nice to see new approaches to established problems which are stuck in a bit of a rut, and that’s exactly what this paper proposes. Rather than using exact numerics or analogue quantum simulators, this paper proposes the use of current-generation (i.e. noisy!) quantum computing hardware to investigate many-body localization via a novel parameter called the ’twist overlap’. The basic idea is that magnetisation is something that’s experimentally easy to measure, so defining a new observable based on magnetisation could be a useful/desirable thing to do. The author introduces a ’twist operator’ which rotates the spins in a one-dimensional chain, and then computes the expectation value of this operator in an excited eigenstate of a transverse Ising Hamiltonian. As the model isn’t the ‘standard model’ of many-body localisation, it’s not entirely clear to me how well this observable fares in comparison to other standard probes, but it’s nice to see a fresh attempt at shedding light on the problem of localisation in many-body quantum systems.

*On-Site Potential Creates Complexity in Systems with Disordered Coupling, by Igor Gershenzon, Bertrand Lacroix-A-Chez-Toine, Oren Raz, Eliran Subag, and Ofer Zeitouni*

This one’s pretty technical, but quite interesting, so bear with me here! Often in complex disordered systems, we talk about the ’energy landscape’. This is an abstract high-dimensional landscape that described how the energy of the system changes as we vary some parameter, such as magnetic field or some form of interaction between particles. The minima of the energy landscape are important, as they usually represent stable (or metastable!) states. In glasses, these minima are separated by large energy barriers, like high walls standing between each of the minima. Characterising the topography of this ’landscape’ is one way of gaining insights into the behaviour of glassy states of matter, and that’s what this paper is about. It computes the number of ‘critical points’ in the free energy landscape, and shows that this can be associated to a complexity measure. The authors go on to show that when an on-site potential is added into the model, the complexity can dramatically increase. This opens a window onto how complex energy landscapes and glassy behaviour can arise from simpler models, and offers a different perspective than the sorts of papers I usually include in these lists which focus on dynamics and other experimentally-measurable quantities.

**June 23rd**

*One-Dimensional Disordered Bosonic Systems, by Chiara D’Errico, and Marco G. Tarallo*

As regular readers will know, disordered quantum systems are something that interest me quite a lot. One-dimensional systems are particularly interesting, as it’s here that the interplay between quantum mechanics and randomness can have the most dramatic effects. This paper reviews the state of the art in experimental observations of disordered ultracold quantum gases, with an emphasis on one-dimensional bosonic systems, and also surveys the necessary theoretical background required to understand the experiments. This is a nice primer for anyone interested in disorder but not sure where to start, particularly if you already have some background knowledge of ultracold atoms.

*Microscopic 3D printed optical tweezers for atomic quantum technology, by Pavel Ruchka, Sina Hammer, Marian Rockenhäuser, Ralf Albrecht, Johannes Drozella, Simon Thiele, Harald Giessen, and Tim Langen*

This one’s not remotely within my wheelhouse, but I couldn’t resist including it this week as it’s just plain cool. Optical tweezers are a way of controllably moving single atoms and building ‘designer’ quantum systems on the atomic level, enabling all kinds of incredible research and development work, but they are typically bulky and expensive pieces of kit. The authors of this work have designed a new type of optical tweezers by 3D printing tiny microscopic lenses onto the ends of optical fibres, potentially enabling optical tweezer technology to become much cheaper, easier to build, and much more portable. All of these points are vital in getting this technology out of the lab and into the wider world, and while I’m absolutely no expert on this stuff, this is too big a development not to include in this week’s list!

**June 24th**

*Temperature-Induced Disorder-Free Localization, by Jad C. Halimeh, Philipp Hauke, Johannes Knolle, and Fabian Grusdt*

This is a really interesting one, and frankly if it was by any other group of people I’d probably have a hard time believing it! Normally when we study quantum mechanics, we work at very low temperatures, as this lets us avoid encountering thermal fluctuations which can destroy the delicate quantum effects we’re interested in. This work shows a highly counter-intuitive phenomenon, namely the enhancement of quantum localisation effects by *higher* temperatures, which at first glance sounds like it should be impossible. To reconcile this seemingly paradoxical claim, it’s important to note that the role of temperature here is only in the preparation of the initial state: the authors prepare their system as a thermal ensemble (with respect to some preparation Hamiltonian) before quenching it and computing the dynamics using a different Hamiltonian. At this point, ’temperature’ is not necessarily well-defined (as the system is far from equilibrium), and so the usual intuition of ‘high temperature destroys quantum physics’ breaks down here. In fact, I’d be very curious to know if any measures of effective temperature (e.g. as used in spin glass studies) could be used here, as I would guess that the effective temperature of the relevant degrees of freedom post-quench is actually rather small, consistent with more ’traditional’ ideas of quantum localisation - that’s purely a guess though. In any case, this is an extremely interesting bit of work, and I’m sure it’ll lead to a lot of follow-ups on the topic!

*Quantum Many-Body Scars: A Quasiparticle Perspective, by Anushya Chandran, Thomas Iadecola, Vedika Khemani, and Roderich Moessner*

Another highly unusual way for quantum systems to avoid reaching thermal equilibrium, quantum many-body scars are a (relatively) recently discovered phenomenon whereby certain states of a system fail to equilibrate, even though the majority do. This preprint reviews quantum scar states from the point of view of quasiparticles, which are a key component to the descriptions of many systems in condensed matter physics, and argues that scar states arise due to quasiparticle modes which are unusually stable, in contrast to the naive expectation that quasiparticles at high energy densities should decay rapidly. This is a nice review for anyone wanting to learn more about the state of the art in quantum many-body scars, including the current understanding of them as well as the remaining challenges and problems still to be addressed!