# 2019 seminars

## 04/01/2021, Monday, 17:00–18:00 Europe/Lisbon — Online

Long-range, multi-particle quantum entanglement plays a fundamental role in our understanding of many modern quantum materials, including the copper-based high temperature superconductors. Hawking’s quantum information puzzle in the quantum theory of black holes also involves non-local entanglement. I will describe a simple model of randomly entangled qubits which has shed light on these distinct fields of physics.

Subir slides.pdf

## 11/01/2021, Monday, 17:00–18:00 Europe/Lisbon — Online

Michael Berry, University of Bristol
Geometric phases and the separation of the world

The waves that describe systems in quantum physics can carry information about how their environment has been altered, for example by forces acting on them. This effect is the geometric phase. It also occurs in the optics of polarised light, where it goes back to the 1830s; and it gives insight into the spin-statistics relation for identical quantum particles. The underlying mathematics is geometric: the phenomenon of parallel transport, which also explains how falling cats land on their feet, and why parking a car in a narrow space is difficult. Incorporating the back-reaction of the geometric phase on the dynamics of the changing environment exposes the unsolved problem of how strictly a system can be separated from a slowly-varying environment, and involves different mathematics: divergent infinite series.

## 18/01/2021, Monday, 17:00–18:00 Europe/Lisbon — Online

Many-body localised (MBL) phases of matter fall outside the conventional paradigm of equilibrium statistical mechanics and thermodynamics. A natural question thus is, what minimal and generic properties must random many-body Hamiltonians possess for a localised phase to be stable? In this talk, I will address the question by exploiting the exact mapping between a many-body Hamiltonian and a tight-binding problem on the Fock-space graph. In particular, I will present a theory for how the strong correlations in the effective Fock-space disorder play a central role in stabilising an MBL phase. The theory is rooted in analytic but approximate calculations of the propagators on the Fock space. To shed further light on the underlying physics, I will also introduce and discuss a classical proxy for the MBL transition in the form of a percolation transition on the Fock space. Finally, I will discuss a novel class of Anderson localisation problems on correlated trees, to understand the effect of such disorder correlations in a more controlled setting.

## 25/01/2021, Monday, 17:00–18:00 Europe/Lisbon — Online

The ability to create and manipulate optical lattices for cold atoms, with a view towards studying topological matter, has brought renewed focus to the physics of Bloch waves and the role of the lattice in governing their properties. We consider generic tight binding models where particle motion is described in terms of hopping amplitudes between orbitals. The physical attributes of the orbitals, including their locations in space, are independent pieces of information. We identify a notion of geometry-independence: any physical quantity that depends only on the tight-binding parameters (and not on the explicit information about the orbital geometry) is said to be “geometry-independent.” Identification of geometry-dependent vs. independent quantities can be used as a novel principle for constraining a variety of results in both non-interacting and interacting systems. We show, e.g., how Hall measurements based on accelerated lattices or tilted potentials, and those based on applying a chemical potential imbalance between reservoirs, give diffReferenceerent results due to the fact that one is geometry-dependent, while the other is geometry-independent. Similar considerations apply for thermal Hall responses in electronic, cold atomic, and spin systems.

### Reference

Steven H. Simon and Mark S. Rudner, Phys. Rev. B 102, 165148, 2020.

### Video

Steven Simon slides.pdf

## 08/02/2021, Monday, 10:00–11:00 Europe/Lisbon — Online

Rémy Mosseri, LPTMC Sorbonne Université
Liquid crystal director fields in three-dimensional non-Euclidean geometries

This work investigates nematic liquid crystals in three-dimensional curved space, and determines which director deformation modes are compatible with each possible type of non-Euclidean geometry. Previous work by Sethna et al. [1] showed that double twist is frustrated in flat space $\mathbb{R}^3$, but can fit perfectly in the hypersphere $\mathbb{S}^3$. Here, we extend that work to all four deformation modes (splay, twist, bend, and biaxial splay) and all eight Thurston geometries [2]. Each pure mode of director deformation can fill space perfectly, for at least one type of geometry. This analysis shows the ideal structure of each deformation mode in curved space, which is frustrated by the requirements of flat space.

1. Sethna J. P., Wright D. C. and Mermin N. D., 1983 Phys. Rev. Lett. 51 467–70.
2. J.-F. Sadoc, R. Mosseri and J. Selinger, New Journal of Physics 22 (2020) 093036.

## 15/02/2021, Monday, 17:00–18:00 Europe/Lisbon — Online

Quantum chaos, especially when caused by particle interactions, is closely related with topics of high experimental and theoretical interest, from the thermalization of isolated systems to the difficulties to reach a localized phase, and the emergence of quantum scars. In this talk, various indicators of quantum chaos will be compared, including level statistics, structure of eigenstates, matrix elements of observables, out-of-time ordered correlators, and the correlation hole (ramp). These indicators are then employed to identify the minimum number of interacting particles required for the onset of strong chaos in quantum systems with short-range and also with long-range interactions.

### References

NOTE

For unforeseeable technical reasons were not able to record Lea's talk.

However, some of her previous talks cover part of the topics she talk about.

You can find her previous talks at:

Lea Santos slides.pdf

## 22/02/2021, Monday, 17:00–18:00 Europe/Lisbon — Online

Maciej Koch-Janusz, University of Zurich
Statistical physics through the lens of real-space mutual information

Identifying the relevant coarse-grained degrees of freedom in a complex physical system is a key stage in developing effective theories. The renormalization group (RG) provides a framework for this task, but its practical execution in unfamiliar systems is fraught with ad hoc choices. Machine learning approaches, on the other hand, though promising, often lack formal interpretability: it is unclear what relation, if any, the architecture- and training-dependent learned "relevant" features bear to standard objects of physical theory.

I will present recent results addressing both issues. We develop a fast algorithm, the RSMI-NE, employing state-of-art results in machine-learning-based estimation of information-theoretic quantities to construct the optimal coarse-graining. We use it to develop a new approach to identifying the most relevant field theory operators describing a statistical system, which we validate on the example of interacting dimer model. I will also discuss formal results underlying the method: we establish equivalence between the information-theoretic notion of relevance defined in the Information Bottleneck (IB) formalism of compression theory, and the field-theoretic relevance of the RG. We show analytically that for statistical physical systems the "relevant" degrees of freedom found using IB compression indeed correspond to operators with the lowest scaling dimensions, providing a dictionary connecting two distinct theoretical toolboxes.

### Video

Koch-Janusz slides.pdf

## 01/03/2021, Monday, 17:00–18:00 Europe/Lisbon — Online

Simon Becker, University of Cambridge
Mathematics of magic angles for twisted bilayer graphene

Magic angles are a hot topic in condensed matter physics: when two sheets of graphene are twisted by those angles the resulting material is superconducting. Please do not be scared by the physics though: I will present a very simple operator whose spectral properties are thought to determine which angles are magical. It comes from a recent PR Letter by Tarnopolsky–Kruchkov–Vishwanath. The mathematics behind this is an elementary blend of representation theory (of the Heisenberg group in characteristic three), Jacobi theta functions and spectral instability of non-self-adjoint operators (involving Hoermander’s bracket condition in a very simple setting). The results will be illustrated by colourful numerics which suggest some open problems. This is joint work with M. Embree, J. Wittsten, and M. Zworski.

### Video

Simon Becker slides.pdf

## 08/03/2021, Monday, 17:00–18:00 Europe/Lisbon — Online

Hannah Price, University of Birmingham
Exploring 4D topological physics in the laboratory

Spatial dimensionality plays a key role in our understanding of topological physics, with different topological invariants needed to characterise systems with different numbers of spatial dimensions. In a 2D quantum Hall system, for example, a robust quantisation of the Hall response is related to the first Chern number: a 2D topological invariant of the electronic energy bands. Generalising to more spatial dimensions, it was shown that a new type of quantum Hall effect could emerge in four dimensions, but where the quantised response was related to a four-dimensional topological invariant, namely the second Chern number. While systems with four spatial dimensions may seem abstract, recent developments in ultracold atoms and photonics have opened the door to exploring such higher-dimensional topological physics experimentally. In this talk, I will introduce the theory of 4D topological phases of matter, before discussing recent experiments in cold atoms, photonics and electrical circuits that have begun to probe aspects of this physics in the laboratory.

### Video

Hannah Price slides.pdf

## 15/03/2021, Monday, 17:00–18:00 Europe/Lisbon — Online

In this talk I will discuss how one can detect quantum chaos in generic interacting models using adiabatic transformations, specifically the fidelity susceptibility. In particular, I will show that it exhibits a very sharp crossover behavior from the algebraic to the exponential scaling form with the system size in the presence of a small integrability breaking parameter. This sensitivity allows one to identify tiny integrability breaking perturbations, not detectable by conventional methods. I will also discuss that generically integrable and chaotic regimes are separated by a universal regime of “maximal chaos” where the fidelity susceptibility saturates its upper bound and the system exhibits exponentially slow, glassy dynamics. I will illustrate how this probe works using several examples of both clean and disordered systems and, in particular, will argue that numerical results indicate absence of a continuous many-body localization transition in the thermodynamic limit.

### Video

Anatoli Polkovnikov slides.pdf

## 22/03/2021, Monday, 17:00–18:00 Europe/Lisbon — Online

Markus Heyl, Max-Planck Institute for the Physics of Complex Systems, Dresden
Quantum many-body dynamics in two dimensions with artificial neural networks

In the last two decades the field of nonequilibrium quantum many-body physics has seen a rapid development driven, in particular, by the remarkable progress in quantum simulators, which today provide access to dynamics in quantum matter with an unprecedented control. However, the efficient numerical simulation of nonequilibrium real-time evolution in isolated quantum matter still remains a key challenge for current computational methods especially beyond one spatial dimension. In this talk I will present a versatile and efficient machine learning inspired approach. I will first introduce the general idea of encoding quantum many-body wave functions into artificial neural networks. I will then identify and resolve key challenges for the simulation of real-time evolution, which previously imposed significant limitations on the accurate description of large systems and long-time dynamics. As a concrete example, I will consider the dynamics of the paradigmatic two-dimensional transverse field Ising model, where we observe collapse and revival oscillations of ferromagnetic order and demonstrate that the reached time scales are comparable to or exceed the capabilities of state-of-the-art tensor network methods.

### Video

Markus Heyl slides.pdf

## 29/03/2021, Monday, 17:00–18:00 Europe/Lisbon — Online

Alexander Altland, University of Cologne
Spectral density of weakly disordered Weyl semimetals

Weyl semimetals contain an even number of singular points in their Brillouin zone around which the dispersion is linear and the density of states (DoS) vanishes. How does the density of states change in the (inevitable) presence of impurities? This question has been the subject of an intensive and partially controversial discussion in the recent literature. In particular, it has been suggested that below a critical disorder strength the DoS remains zero, and that the system supports a phase transition separating an intrinsically clean from a disordered phase. In this talk, I discuss this problem on the basis of several effective models. All these support the integrity of the Weyl node and hence are compatible with the above scenario. I will also comment on the (tricky) comparison to numerics and point out a puzzle whose solution invites mathematical research.

## 12/04/2021, Monday, 17:00–18:00 Europe/Lisbon — Online

David J. Luitz, Max Planck Institute for the Physics of Complex Systems, Dresden
Hierarchy of Relaxation Timescales in Local Random Liouvillians

To characterize the generic behavior of open quantum many-body systems, we consider random, purely dissipative Liouvillians with a notion of locality. We find that the positivity of the map implies a sharp separation of the relaxation timescales according to the locality of observables. Specifically, we analyze a spin-$1/2$ system of size $\ell$ with up to n-body Lindblad operators, which are n local in the complexity-theory sense. Without locality ($n=l$), the complex Liouvillian spectrum densely covers a “lemon”-shaped support, in agreement with recent findings [S. Denisov et al., Phys. Rev. Lett. 123, 140403 (2019), L. Sa et al., JPA 53, 305303].However, for local Liouvillians ($n < l$), we find that the spectrum is composed of several dense clusters with random matrix spacing statistics, each featuring a lemon-shaped support wherein all eigenvectors correspond to $n$-body decay modes. This implies a hierarchy of relaxation timescales of n-body observables, which we verify to be robust in the thermodynamic limit. Our findings for n locality generalize immediately to the case of spatial locality, introducing further splitting of timescales due to the additional structure.

To test our theoretical prediction, we perform experiments on the IBM quantum computing platform, designing different "waiting circuits" to inject two body dissipative interactions by two qubit entangling gates. We find excellent agreement with our theory and observe the predicted hierarchy of timescales.

[1] K. Wang, F. Piazza, D. J. Luitz ” Hierarchy of relaxation timescales in local random Liouvillians “ Phys. Rev. Lett. 124, 100604 (2020)

[2] O. E. Sommer, F. Piazza, and D. J. Luitz “Many-body Hierarchy of Dissipative Timescales in a Quantum Computer” arXiv:2011.08853