# View from the arXiv: Mar 7 - Mar 11 2022

A summary of new preprints appearing on arXiv during the week of Mar 7th to Mar 11th 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 As these are all preprints which have not yet been peer-reviewed, remember to take any claims and conclusions with a grain of salt and be sure to cast a critical eye over the work if you’re interested in more details.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…!)

**Mar 7th**

*Counterdiabatic Optimised Local Driving, by Ieva Čepaitė, Anatoli Polkovnikov, Andrew J. Daley, and Callum W. Duncan*: Wondering about the title? It acronyms to ‘COLD’! This paper is about improving the robustness of time-dependent manipulations of quantum systems. An ideal manipulation of a quantum system should be sufficiently slow and well-controlled that it does not introduce heating or any other undesirable properties, and in principle can be exactly reversed. This ability to reverse the manipulation guarantees that nothing is lost or added to the quantum system. We call call such manipulations ‘adiabatic’. The problem with adiabatic processes is that they are, almost by definition *slow*. This isn’t great for practical purposes, and so it is desirable to come up with ways of making adiabatic manipulations in a fast way. One method is counteradiabatic driving, explored in this paper, which adds time-dependent terms back into the system’s Hamiltonian in order to compensate for deformations induced by fast (non-adiatbatic) manipulations, which is an extremely difficult task. This paper introduces a new technique for the approximate realisation of counteradiabatic driving, allowing fast manipulations to be made to quantum systems without destroying their fragile quantum properties. The authors discuss how this protocol could be realised in experiments, meaning that we could see experiments making use of this technique in the near future. There are many further ways to extend this protocol, any one of which could lead to further very interesting developments!

**Mar 8th**

*On some very rare days, nothing new on arXiv catches my eye, and this was one of those days!*

**Mar 9th**

*Kibble-Zurek scaling due to environment temperature quench in the transverse field Ising model, by Á. Bácsi, and B. Dóra*: On Monday we talked about adiatbaticity, and the idea that most manipulations in condensed matter are made slowly, to avoid introducing heating or other undesirable changes. The Kibble-Zurek mechanism relates to what happens to a system when it is driven across a (continuous) phase transition *quickly*, in a non-adiabatic manner. Such a sudden change leads to the formation of *defects* in the ordered phase, patches of the system which are locally different from the bulk of the system. The formation and number of these defects is described by the Kibble-Zurek theory, and is usually considered at zero temperature. This paper studies a variation on this theme where a system is coupled to a bath, and the bath temperature is suddenly changed in order to drive the system close to a quantum critical point. The authors study the formation of defects in this framework, as well as the entanglement formed between the system and the bath. This is an interesting work, and something I’m almost surprised to have never seen sooner.

*Locobatic theorem for disordered media and validity of linear response, by Wojciech De Roeck, Alexander Elgart, and Martin Fraas*: Continuing the theme of adiabaticity, here we have an extremely long and technical paper which studies the adiabatic theorem in a localized (disordered) system. In general, such systems are not stable to perturbations and so there is no sense of a gradual change in the system properties with respect to small changes in the Hamiltonian, and therefore no adiabatic theorem. This work studies in rigorous mathematical detail what *does* happen when localized systems are perturbed, and the authors discover that while the adiabtic theorem doesn’t hold, the system follows a modified version which they name the *locobatic theorem*. I can’t claim to understand all of the details based on a first quick read, but for those interested in disordered systems and their stability, this is a very intriguing work.

**Mar 10th**

*Resonant superfluidity in the Rabi-coupled spin-dependent Fermi-Hubbard model, by Mathias Mikkelsen, Ryui Kaneko, Daichi Kagamihara, and Ippei Danshita*: It’s unusual, at least in my particular sub-field, to see the study of an *attractive* Fermi-Hubbard model, but that’s exactly what we have here. This work uses density matrix renormalization group (DMRG) methods to study a two-component Hubbard model with *attractive* interactions and an on-site Rabi coupling between both fermion species. The authors show that the Rabi coupling can stabilize superfluidity of the fermions, and investigate how this depends on the doping and the detunement frequency. This has applications towards current generation experiments, although the effects of non-zero temperatures may need to be studied in order to make the comparison more precise. My question when reading this manuscript is whether the Rabi coupling and associated superfluidity is useful beyond its fundamental interest (which I’ll admit is what caught my eye!), but I imagine that later works will shed some light on this, as the focus of this paper is simply demonstrating that ththis intriguing effect exists.

*Snowmass White Paper: New ideas for many-body quantum systems from string theory and black holes, by Mike Blake, Yingfei Gu, Sean A. Hartnoll, Hong Liu, Andrew Lucas, Krishna Rajagopal, Brian Swingle, and Beni Yoshida*: The connection between quantum mechanics and black holes is a long-standing one that has, of late, seen something of a resurgence of interest. This is a speculative ‘white paper’ rather than a review, but its contents are extremely interesting. It lays out the connections between black holes, quantum information and many-body physics, and how the studying of each may inform and shed light on the other. This is well worth a read if any of these topics interest you, as its forward-thinking view is quite inspiring. And don’t be afraid of a 42-page paper: more than half of its length is taken up by references!

**Mar 11th**

*Experimental realization of a fermionic spin-momentum lattice, by Paul Lauria, Wei-Ting Kuo, Nigel R. Cooper, and Julio T. Barreiro*: Ultracold atomic gases have become a fantastic platform for designing highly tunable experiments with a great deal of control over individual atoms, and in recent years have even been able to go beyond direct realisation of quantum systems in real space with the addition of so-called ‘synthetic dimensions’. These can be engineered through the addition of unconventional degrees of freedom in the system, for example periodic drive (a drive systen in *d* dimensions behaves a bit like a static system in *d+1* dimensions) or spin-orbit coupling which enables the exploration of physics beyond directly simulating particles in an optical lattice. This work shows the experimental construction of a spin-momentum lattice, i.e. the emergence of a periodic structure based on momentum-space rather than real-space. This work demonstrates the first realisation of this concept, and it will be very exciting to see the applications it will have for the engineering and study of exotic topological physics: one particular avenue of investigation suggested by the authors is the study of robust fractional quantum Hall states, which can be realised using this scheme in a highly controlled way.

*Universal T-linear resistivity in two-dimensional quantum-critical metals from spatially random interactions, by Aavishkar A. Patel, Haoyu Guo, Ilya Esterlis, and Subir Sachdev*: The title might sound a bit dry and the abstract a bit, well, *abstract*, but there’s some very interesting physics in this paper! The authors study a fermionic model reminiscent of a Sachdev-Ye-Kitaev model, but here use conventional fermions rather than Majorana fermions, with the aim of studying the resistivity in two dimensional metals. In particular, the authors investigate the effects of disorder on the thermodynamic properties of the system, finding a resistivity that is linear in temperature *T* and a specific heat that scales like *T ln(1/T)*. It’s nice to see this sort of study of a disordered low-dimensional fermionic system, where thermodynamic properties are often extremely difficult to compute or interpret. I always find these sort of field-theoretic calculations a little abstract and far from real systems, but I find the results of this work very intriguing. I’d love to see a similar field theoretic approach developed for the study of excited states as well as ground states - perhaps something for future work!