6th Trieste–Ljubljana meeting

10:00 – 18:00, lecture hall P.04, Jadranska 21.

9:30 coffee and discussions
10:15  Márton Mestyán (SISSA):  Molecular dynamics simulation of entanglement spreading in generalized hydrodynamics
10:50 Lev Vidmar (IJS Ljubljana):  Quantum chaos challenges many-body localization
12:25 Tiago Mendes-Santos (ICTP): Entanglement guided search for parent Hamiltonians
12:00 lunch
13:00 discussions
15:15 Spyros Sotiriadis (FMF Ljubljana): Quantum dynamics in the sine-Gordon model
15:50 Jacopo Sisti  (SISSA): Entanglement entropy in higher dimensional CFTs and holography
16:25 Giuliano Giudici (ICTP): Measuring von Neumann entanglement entropies without wave functions
17:00  conclusion

Márton Mestyán: Molecular dynamics simulation of entanglement spreading in generalized hydrodynamics (arXiv:1905.03206)

The so-called flea gas is an elementary yet very powerful method that allows the simulation of the out-of-equilibrium dynamics after quantum quenches in integrable systems. We show that, after supplementing it with minimal information about the initial state correlations, the flea gas provides a versatile tool to simulate the dynamics of entanglement-related quantities. The method can be applied to any quantum integrable system and to a large class of initial states. Moreover, the efficiency of the method does not depend on the choice of the subsystem configuration. We implement the flea gas dynamics for the gapped anisotropic Heisenberg XXZ chain, considering quenches from globally homogeneous and piecewise homogeneous initial states. We compute the time evolution of the entanglement entropy and the mutual information in these quenches, providing strong confirmation of recent analytical results obtained using the Generalized Hydrodynamics approach. The method also allows us to obtain the full-time dynamics of the mutual information after quenches from inhomogeneous settings, for which no analytical results are available.

Lev Vidmar: Quantum chaos challenges many-body localization (arXiv:1905.06345)

Characterizing states of matter through the lens of their ergodic properties is a fascinating new direction of research. In the quantum realm, the many-body localization (MBL) was proposed to be the paradigmatic nonergodic phenomenon, which extends the concept of Anderson localization to interacting systems. At the same time, random matrix theory has established a powerful framework for characterizing the onset of quantum chaos and ergodicity (or the absence thereof) in quantum many-body systems. Here we study a paradigmatic class of models that are expected to exhibit MBL, i.e., disordered spin chains with Heisenberg-like interactions. Surprisingly, we observe that exact calculations show no evidence of approaching MBL while increasing disordered strength in the ergodic regime. Moreover, a scaling analysis suggests that quantum chaotic properties survive for any disorder strength in the thermodynamic limit. Our results are based on calculations of the spectral form factor, which provides a powerful measure for the emergence of many-body quantum chaos.

Tiago Mendes-Santos: Entanglement guided search for parent Hamiltonians

We introduce a method for the search of parent Hamiltonians of input wave-functions based on the structure of their reduced density matrix. The two key elements of our recipe are an ansatz on the relation between reduced density matrix and parent Hamiltonian that is exact at the field theory level, and a minimization procedure on the space of relative entropies, which is particularly convenient due to its convexity. As examples, we show how our method correctly reconstructs the parent Hamiltonian correspondent to several non-trivial ground state wave functions, including conformal and symmetry-protected-topological phases, and quantum critical points of two-dimensional antiferromagnets described by strongly coupled field theories. Our results show the entanglement structure of ground state wave-functions considerably simplifies the search for parent Hamiltonians.

Spyros Sotiriadis: Quantum dynamics in the sine-Gordon model

The study of dynamics in quantum many-body systems is one of the main challenges of modern theoretical physics. While a large amount of intuition has been drawn from weakly interacting systems, linear response theory or semiclassical approximations, the dynamics of strongly interacting systems far from equilibrium remains largely unexplored. We study the dynamics of an interacting Quantum Field Theory, the sine-Gordon model, after an abrupt change of the parameters, a protocol known as “Quantum Quench”. Our focus is on the time evolution of correlation functions, which we study using a combination of numerical and analytical techniques.

Jacopo Sisti: Entanglement entropy in higher dimensional CFTs and holography

Entanglement entropy is a quantity of great interest in diverse fields of physics like condensed matter, statistical and high energy physics. In this talk, I will review some aspects of entanglement entropy in higher dimensional quantum field theories mainly focusing on CFTs that admit a holographic dual. In such theories, the Ryu–Takayanagi formula identifies the entanglement entropy of a spatial region with the area of the minimal hypersurface anchored to the entangling surface and that extends along the holographic direction of the space-time. The holographic duality is a useful tool also to study CFTs with boundary. In particular, I will show some analytical and numerical results on minimal surfaces in space-times dual to (2+1)-dimensional BCFTs.

Giuliano Giudici: Measuring von Neumann entanglement entropies without wave functions

It is nowadays a well known fact that the von Neumann entropy (VNE) of the ground state is a powerful tool to characterize many-body quantum systems, since it provides distinctive information such as length of correlations and universal data of critical systems. Despite its central role as a diagnostic tool for low-energy properties of many-body Hamiltonians, its measurement has so far been elusive both from an experimental and — beyond one dimension — numerical point of view. Here we propose a method to compute the ground state VNE without accessing the many-body wave function. The method is based on the knowledge, from quantum field theory, of the entanglement Hamiltonian of the ground state. We benchmark our technique on critical quantum spin chains, and apply it to several two-dimensional quantum magnets, where we are able to unambiguously determine the onset of area law, together with logarithmic corrections independent of the geometry of the bipartition. We finally focus on one-dimensional critical systems whose large distance behaviour is conformally invariant. We investigate to which extent it is possible to extract the central charge by computing the entanglement capacity, which is simply related to the expectation value of the energy density and thus easily accessible in experiments.

Marko Medenjak: The isolated Heisenberg magnet as a quantum time crystal

Isolated systems consisting of many interacting particles are generally assumed to relax to a stationary equilibrium state whose macroscopic properties are described by the laws of thermodynamics and statistical physics. Time crystals, as first proposed by Wilczek, could defy some of these fundamental laws and for instance display persistent non-decaying oscillations. They can be engineered by external driving or contact with an environment, but are believed to be impossible to realize in isolated many-body systems. Here, we will demonstrate analytically and numerically that the paradigmatic model of quantum magnetism, the Heisenberg XXZ spin chain, does not relax to stationarity and hence constitutes a genuine time crystal that does not rely on external driving or coupling to an environment. We will trace this phenomenon to the existence of periodic extensive quantities and find their frequency to be a no-where continuous (fractal) function of the anisotropy parameter of the chain.

Marko Medenjak, ENS Paris

Dibyendu Roy: An open-quantum system description of Josephson effect in topological superconductors

Using an open-quantum system description, we revisit the Josephson effect in hybrid junctions made of the topological superconductor (TS) and normal metal (N) wires. We consider an X-Y-Z configuration for the junctions where X, Y, Z = TS, N. We assume the wires X and Z being semi-infinite and in thermal equilibrium. We connect the wires X and Z through the short Y wire at some time, and numerically study time-evolution of the full device. For TS-N-TS device, we find a persistent, oscillating electrical current at both junctions even when there is no phase or thermal or voltage bias. The amplitude and period of the oscillating current depend on the initial conditions of the middle N wire indicating the absence of thermalization. This zero-bias current vanishes at a long time for any of X and Z being an N wire or a TS wire near a topological phase transition. Employing properties of different bound states within the superconducting gap, we develop a clear understanding of the oscillating currents.

* Dibyendu Roy, Raman Research Institute, India

Sašo Grozdanov: Pole-skipping as a signature of quantum chaos

In my talk, I will discuss the recently-discovered phenomenon of pole-skipping in thermal correlators of energy and momentum operators. In the presence of low-energy hydrodynamic modes, pole-skipping provides a precise relation between hydrodynamics and the underlying microscopic quantum chaos, as diagnosed by an out-of-time-ordered correlator (OTOC). In the absence of hydrodynamics, as for example in a 1+1 dimensional quantum critical theory (a conformal field theory), pole-skipping can still be used to compute the rate of the exponential growth of the OTOC.

Sašo Grozdanov, MIT

Matija Vidmar: From branching to Lévy processes

The temporal evolution of certain aggregates of particles — of neutrons in a multiplying medium, of electron-photon cascades in cosmic rays, say — can be successfully modeled via so-called branching processes. A brief overview of the most popular classes of branching processes is given. In particular the continuous-time Bienaymé-Galton-Watson processes and continuous-state branching processes are described as time-changed Lévy processes that are “skip-free downwards”.

Márton Mestyán: Spin-charge separation effects in the low-temperature transport of one-dimensional Fermi gases

We study the transport properties of the Yang–Gaudin model – a one-dimensional, integrable, spinful Fermi gas – after a junction of two semi-infinite subsystems held at different temperatures. The ensuing dynamics is studied by analyzing the space-time profiles of local observables emerging at large distances x and times t, as a function of ζ=x/t. At equilibrium, the system displays two distinct species of quasiparticles, naturally associated with different physical degrees of freedom. By employing the generalized hydrodynamic approach, we show that when the temperatures are finite no notion of separation can be attributed to the quasiparticles. In this case, the profiles can not be qualitatively distinguished by those associated to quasiparticles of a single species that can form bound states. On the contrary, signatures of separation emerge in the low-temperature regime, where two distinct characteristic velocities appear. In this regime, we analytically show that the profiles display a piecewise constant form and can be understood in terms of two decoupled Luttinger liquids.

* Márton Mestyán, SISSA, Trieste

[1] M. M., B. Bertini, L. Piroli, P. Calabrese, Phys. Rev. B 99, 014305 (2019)

Michele Filippone: Identical Suppression and Universality of the Hall Response in Interacting Quantum Lattices

The Hall effect is known as the voltage drop V perpendicular to the current I and magnetic field B (B⊥ I) measurable in electric conductors. Its importance relies in the fact that, at weak field, the Hall constant R = V/(IB) ≈ -1/(nq)   and measures the charge q and density n of charge carriers. Nevertheless, its calculation remains an open problem in strongly correlated systems. We demonstrate the identical suppression of the Hall response in quasi two-dimensional ballistic lattices, which is robust  to large variations of magnetic field, Fermi level, temperature, disorder and absence of particle-hole symmetry. This suppression is unexpected and we show its relation to the topological properties of the Fermi surface: namely its central charge.  We show that this suppression is commonly achieved in standard (Landauer) quantum transport settings and we rely on DMRG to show that our results equally apply to strongly interacting regimes [1]. Time permitting, I will also discuss how, in some particular cases, interactions do not affect the Hall response at all, universally leading to the non-interacting result R = -1/(nq).

*  Michele Filippone, Université de Genève

[1] M. Filippone, C.-E. Bardyn, S. Greschner and T. Giamarchi, To appear soon.

[2] S. Greschner, M. Filippone and T. Giamarchi, arXiv:1809.10927.

Tony Jin: Equilibrium Fluctuations in Maximally Noisy Extended Quantum Systems

I will introduce and study a class of models of free fermions hopping between neighbouring sites with random Brownian amplitudes. These simple models describe stochastic, diffusive, quantum, unitary dynamics. I focus on periodic boundary conditions and derive the complete stationary distribution of the system. It is proven that the generating function of the latter is provided by the Harish-Chandra-Itzykson-Zuber integral which allows us to access all fluctuations of the system state. The steady state is characterized by non trivial correlations which have a topological nature. In the thermodynamic large system size limit, the system approaches a non random equilibrium state plus occupancy and coherence fluctuations of magnitude scaling proportionally with the inverse of the square root of the volume. The large deviation function for those fluctuations is determined. Although decoherence is effective on the mean steady state, we observe that sub-leading fluctuating coherences are dynamically produced from the inhomogeneities of the initial occupancy profile.

* Tony Jin, LPT ENS Paris

Adrian Ortega: Efficient quantum transport in disordered interacting many-body networks

We give an account of efficient quantum transport over disordered networks in fermionic inter- acting systems. The ingredient necessary to achieve good transport rates is centrosymmetry. We study transport efficiency in the time-dependent [Ann. Phys. 527 (2015)], as well as in the time- independent [Phys. Rev. E 94 (2016)] regimes. Later, we show how robust is the system against several types of perturbations. Finally, we show that in the presence of noise, centrosymmetry sets an upper bound for transport when compared to popular dephasing assisted transport [Phys. Rev. E 98 (2018)].

* Adrian Ortega, Centro Universitario de Ciencias Exactas e Ingeniería, Guadalajara, México

Eric Ragoucy: Integrability in out-of-equilibrium systems

Out-of-equilibrium systems have nowadays an important role in 1d statistical physics. Although an equilibrium state obviously doesn’t exist for such systems, one looks for a steady state (that is stationary in time). It is defined as the zero-eigenvalue eigenstate of the Markov matrix that describe the evolution of the system. Its exact computation is at the core of many researches.
In some cases, the matrix product state ansatz (matrix ansatz for short) allows to compute this steady state. However no general approach for this ansatz is known. On the other hand, many 1d statistical models appear to be integrable, which allows to get eigenstates of the Markov matrix through Bethe ansatz. The goal of this presentation is to show how integrability gives a natural framework to construct the matrix ansatz for 1d systems with boundaries.
It can be done on very general grounds, allowing to construct the matrix ansatz when it is not known, and also to define new models and/or to find boundary conditions ‘adapted’ to the model under consideration. We will illustrate the technique on some examples.

Eric Ragoucy, Laboratoire d’Annecy-le-Vieux de Physique Théorique, Annecy, France