Quantum Days in Crete 2025

1 Oct 2025, 12:00 → 3 Oct 2025, 17:00 Europe/Athens

“A. Payatakes” Meeting Room (1st floor) (University of Crete & FORTH)

Spyros Sotiriadis (University of Crete), Georgios Siviloglou (University of Crete), Konstantinos Makris (University of Crete)

An international conference on Quantum Many-Body Physics, organised by the University of Crete. 

QDC25 poster

Wednesday 1 October

12:00 RECEPTION

13:00 LUNCH

1 Linear response and exact hydrodynamic projections in Lindblad equations with decoupled Bogoliubov hierarchies

I consider a class of spinless-fermion Lindblad equations that exhibit decoupled BBGKY hierarchies. Importantly these do not describe “free” evolution in the sense that there is no Wick’s theorem and multi-point correlation functions cannot be reduced to two-point functions. In the cases where particle number is conserved, their late time behaviour is characterized by diffusive dynamics, leading to an infinite temperature steady state. Some of these models are Yang-Baxter integrable, others are not. The simple structure of the BBGKY hierarchy makes it possible to map the dynamics of Heisenberg-picture operators on few-body imaginary-time Schrödinger equations with non-Hermitian Hamiltonians. I use this formulation to obtain exact hydrodynamic projections of operators quadratic in fermions, and to determine linear response functions in Lindbladian non-equilibrium dynamics.

Speaker: Prof. Fabian Essler (University of Oxford)

2 The quantum chaos and periodic-orbit perspective in low-dimensional quantum gravity

During the last decade, a new and powerful paradigm has emerged in the context of quantum gravity, here understood as the still elusive quantum theory of gravitational degrees of freedom. This paradigm states that key features of quantum gravity are expected to display signatures of quantum chaos. Following such guiding principle, both compliance with random matrix theory in the long time limit and presence of fast scrambling in short times are now standard checks of any prospect system where quantum gravitational degrees of freedom may appear, either microscopically or as an effective description.

A particular type of abstract models inspired by string theory have gained much attention during the last years due to the possibility of having both a full-consistency quantum theory of gravity and a non-gravitational dual given by matrix models where quantum chaos signatures can be explicitly displayed and analyzed [1]. This models describe gravity in 1+1 dimensions, and the best studied example is the so-called Jackiw-Teitelboim model where 2-dimensional Einstein gravity is coupled with a dilation field.

In this talk I will approach the general idea of quantum gravity being quantum chaotic, but from the fresh perspective of semiclassical methods and periodic orbit theory where the starting point is the existence of a well-defined classical limit and a semiclassical regime and well known methods like the Gutzwiller trace formula and Wigner-Moyal expansions allow then to link quantum observables with classical structures [2]. The interesting aspect is, as I will try to explain, that in the context of quantum gravity neither the classical limit, nor the semiclassical regime and, in fact, not even the Hilbert space of the theory, are known!

[1] P. Saad, S. H. Shenker, D. Stanford, JT Gravity as Matrix Integral , arXiv:1903.11115

[2] F. Haneder, C.A. Moreno, T. Weber, J. D. Urbina, K. Richter, Beyond the ensemble paradigm in low dimensional quantum gravity: Schwarzian density, quantum chaos and wormhole contributions, arXiv:2410.02270. Phys. Rev. D 111 126015 (2025)

Speaker: Prof. Juan Diego Urbina (Universität Regensburg, Institut für Theoretische Physik)

16:30 COFFEE BREAK

3 Thermodynamics, transport, and fluctuations in the sine-Gordon model

The sine-Gordon model is a paradigmatic quantum field theory that provides the low-energy effective description of many gapped 1D systems. Despite this fact, its complete thermodynamic description in all its regimes was lacking. In the talk, I will report the filling of this gap by deriving the Thermodynamic Bethe Ansatz framework that captures the thermodynamics of the model and serves as the basis of its hydrodynamic description. The latter opens the way to the large-scale description of the model’s non-equilibrium dynamics starting from an inhomogeneous initial state, where energy-charge separation can be observed. It also enables the calculation of transport coefficients such as the Drude weight and Onsager matrices. The Drude weight characterising the ballistic transport of the topological charge is found to exhibit a fractal-like dependence on the coupling, and the diffusive corrections also have interesting features. I will also discuss large-scale fluctuations of conserved charges and their currents and show that the distribution of the topological current is another quantity that shows fractal-like behaviour.

Speaker: Prof. Márton Kormos (Budapest University of Technology and Economics)

Thursday 2 October

4 Quantum Mpemba effect

The Mpemba effect is a striking and counterintuitive phenomenon in which, under certain conditions, hotter water cools more quickly than colder water. Although originally observed in classical systems, recent theoretical and experimental studies have uncovered an analogous effect in extended quantum systems.
A specific manifestation of this quantum effect occurs when the system starts in a state that explicitly breaks a given symmetry, yet the time evolution leads to the eventual restoration of that symmetry, sometimes at an unexpectedly fast rate.

To systematically investigate this phenomenon, we introduce the entanglement asymmetry, a quantity which quantifies the degree of symmetry breaking in a quantum state. This measure is inspired by concepts from entanglement theory in many-body systems and provides a powerful tool to track the restoration of symmetry over time. By leveraging entanglement asymmetry, we gain new insights into non-equilibrium quantum dynamics and the fundamental mechanisms governing symmetry restoration.

This talk will explore the theoretical foundations of the quantum Mpemba effect, recent experimental observations, and the implications of entanglement asymmetry for understanding non-equilibrium processes in quantum many-body physics.

Speaker: Prof. Pasquale Calabrese (SISSA - International School for Advanced Studies)

5 Entanglement in an expanding universe

In the first part I discuss the evolution of entanglement entropy for a massless field within a spherical region in an expanding background. The formalism is applied to the inflationary period and the subsequent era of radiation domination, starting from the Bunch-Davies vacuum. Each field mode evolves towards a squeezed state upon horizon exit during inflation, with additional squeezing when radiation domination sets in. This results in the enhancement of the entanglement entropy. A volume term develops in the radiation-dominated era, and becomes the leading contribution to the entanglement entropy at late times. In the second part I discuss the form of the entanglement entropy in various gravitational backgrounds (de Sitter and anti-de Sitter space, the Einstein universe) focusing on the structure of the divergences. Universal coefficients are determined for ultraviolet and infrared divergent terms. A remarkable conclusion is that the entanglement entropy of sub-horizon regions in de Sitter space displays a logarithmic dependence on the size of the total system, which may extend beyond the horizon. In the third part I discuss the use of the finite part of the entropy for the calculation of c- and a-functions.

Speaker: Prof. Nikolaos Tetradis (National and Kapodistrian University of Athens)

11:30 COFFEE BREAK

6 Computing c- and a-functions from entanglement

In this talk, I will examine the direct connection between entanglement entropy and the notion of irreversibility in the renormalization-group flow in the context of a simple free massive scalar theory. The change of the entanglement entropy for a spherical entangling surface as its radius grows from zero to infinity corresponds to the flow from the UV to the IR. I will provide details regarding the numerical and analytical computation of the entanglement entropy using correlation functions in order to deduce a c-function in 1 + 1 dimensions and an a-function in 3 + 1 dimensions. I will show that both these functions are monotonic and vary continuously between one and zero, as expected for this simple theory.

Speaker: Mr Konstantinos Boutivas (University of Athens)

TALK BOUTIVAS.pdf

7 Exceptional stationary state in a dephasing many-body open quantum system

The eigenstate thermalization hypothesis (ETH) [1] is a cornerstone of condensed matter physics,offering a simple and physical framework to explain the emergence of thermal features in the late-time dynamics of closed quantum systems. Nevertheless, the presence of rare eigenstates, known as quantum many-body scar states, that escape thermalization and violate ETH has been recently pointed out in relevant physical systems. While for closed systems these exceptional eigenstates have been thoroughly studied, their counterparts in open dynamics have been less explored.

In this work [2], we study a dephasing many-body open quantum system that hosts, together with the infinite-temperature state, another additional stationary state, that is associated with a non-extensive strong symmetry. This state, that is a pure dark state, is exceptional in that it retains memory of the initial condition, whereas any orthogonal state evolves towards the infinite-temperature state erasing any information on the initial state.

We discuss the approach to stationarity of the model focusing in particular on the fate of interfaces between the two stationary states. A simple model based on a membrane picture helps developing an effective large-scale theory, which is different from the usual hydrodynamics since no extensive conserved quantities are present. The fact that the model reaches stationary properties on timescales that diverge with the system size, while the Lindbladian gap is finite, is duly highlighted. We point out the reasons for considering these exceptional stationary states as quantum many-body scars in the open system framework.

[1] M. Srednicki, Journal of Physics A: Mathematical and General, 32, 1163 (1999).
[2] A. Marché, G. Morettini, L. Mazza, L. Gotta, and L. Capizzi, PRL 135, 020406 (2025).

Speaker: Gianluca Morettini (LPTMS, Université Paris-Saclay)

13:00 LUNCH

8 Quantum Many-Body Systems and their Information Content

Quantum Many Body Systems are at the basis of many Quantum Simulations. It is therefore of upmost interest to understand their information content and structure and how it can be manipulated and extracted or measured. One avenue, based on our understanding of quantum field theories, is based on correlation functions, which reveal the accessible structure [1] and their effective descriptions either directly [2] or through learning algorithms [3]. A different approach is through many-body tomography [4] which then allows to extract of von Neuman entropies. This allowed us to verify the area law for mutual information [5] in quantum many body systems. The tomography approach is limited to Gaussian effective models. Currently we are developing a new model agnostic approach, which allows to study also strongly correlated and topological systems [6]. Finally, I will ask the question what it takes to erase information in a many-body system and present our experiments probing Landauer's principle in the quantum many-body regime [7].
Work performed in collaboration with the groups of P. Zoller (Innsbruck), Th. Gasenzer und J. Berges (Heidelberg), Jens Eisert (FU Berlin), E. Demler (Harvard/ETH) and Silke Weinfurtner (Nottingham). Supported by the DFG-FWF SFB ISOQUANT, and the ERC-AdG Emergence in Quantum Physcs (EmQ)

[1] T. Schweigler et al., Nature 545, 323 (2017), arXiv:1505.03126
[2] T. Zache et al. Phys. Rev. X 10, 011020 (2020)
[3] R. Ott et al. Phys. Rev. Res. 6, 043284 (2024)
[4] M. Gluza et al., Communication Physics 3, 12 (2020)
[5] M. Tajik et al., Nature Physics 19, 1022 (2023)
[6] F. Moller et al. arXiv:2509.13821
[7] S. Aminet et al. Nature Physics 21, 1326 (2025)

Speaker: Prof. Jörg Schmiedmayer (Vienna Center for Quantum Science and Technology (VCQ), Atominstitut, TU-Wien)

9 Quantum transport of matterwaves: Fundamental Limits

Mattewaves are promising candidates for the realization of extremely sensitive sensors. Some of the most sensitive and precise measurements to date of gravity [1], inertia [2], and rotation [3] are based on matter-wave interferometry with free-falling atomic clouds. A critical requirement to achieve very high sensitivities is the long interrogation time, which consequently leads to experimental apparatus up to a hundred meters tall or the requirement for experiments to be performed in microgravity in space[4–7]. To tackle this problem, the gravitational acceleration must be cancelled, e.g. by manipulating atomic waves in time-changeable traps and waveguides [8]. In the past, we demonstrated near-perfect smooth and controllable matter- wave guides by transporting Bose-Einstein condensates (BECs) over macroscopic distances without any heating or decohering their internal quantum states [9]. A neutral-atom accelerator ring was utilized to bring BECs to very high speeds (up to 16 times their sound velocity) and transport them in a magnetic matter-wave guide for 15 centimetres whilst fully preserving their internal coherence.
If this represents a ‘perfect’ waveguide then what is a non-perfect waveguide? How much imperfection can a waveguide tolerate before it starts to harm quantum-transport? In this presentation, we will introduce the basics of TAAP waveguides and explore experimentally how strong an obstacle has to be in order to disturb the travelling matterwave. We propose a simple fundamental limit which depends only on the transverse trapping frequencies.

References
1. Rosi, G., Sorrentino, F., Cacciapuoti, L., Prevedelli, M. & Tino, G. M. Precision measurement of the Newtonian gravitational constant using cold atoms. Nature 510, 518–521 (2014).
2. Geiger, R. et al. Detecting inertial effects with airborne matter-wave interferometry. Nat. Commun. 2, 474 (2011).
3. Dutta, I. et al. Continuous cold-atom inertial sensor with 1 nrad/sec rotation stability. Phys. Rev. Lett. 116, 183003 (2016).
4. Kovachy, T. et al. Quantum superposition at the half-metre scale. Nature 528, 530–533 (2015).
5. van Zoest, T. et al. Bose–Einstein condensation in microgravity. Science 328, 1540–1543 (2010).
6. Barrett, B. et al. Dual matter-wave inertial sensors in weightlessness. Nat. Commun. 7, 13786 (2016).
7. Soriano, M. et al. Cold atom laboratory mission system design. In 2014 IEEE Aerospace Conference 1–11 (IEEE, 2014).
8. Wang, Y. J. et al. Atom Michelson interferometer on a chip using a Bose–Einstein condensate. Phys. Rev. Lett. 94, 090405 (2005).
9. Saurabh Pandey, Hector Mas, Giannis Drougakis, Premjith Thekkeppatt, Vasiliki Bolpasi, Georgios Vasilakis, Konstantinos Poulios, and Wolf von Klitzing Hypersonic Bose--Einstein condensates in accelerator rings Nature 570:7760 205--209 (2019)
10. Saurabh Pandey et al. Atomtronic Matter-Wave Lensing Phys. Rev. Let. 126 17 (2021)

Speaker: Prof. Wolf von Klitzing (FORTH)

16:30 COFFEE BREAK

10 Quantum and classical dynamics with random permutation circuits

Understanding thermalisation in quantum many-body systems is among the most enduring problems in modern physics. A particularly interesting question concerns the role played by quantum mechanics in this process, i.e. whether thermalisation in quantum many-body systems is fundamentally different from that in classical many-body systems and, if so, which of its features are genuinely quantum. I will discuss this question by considering minimally structured many-body systems that are only constrained to have local interactions, i.e. local random circuits. In particular, I will introduce random permutation circuits (RPCs), which are circuits comprising gates that locally permute basis states, as a counterpart to random unitary circuits (RUCs), a standard toy model for generic quantum dynamics.

Speaker: Prof. Bruno Bertini (University of Birmingham)

Friday 3 October

11 Measuring weak integrability breaking in a Rydberg-atom quantum machine

I will present some experimental results produced at Institut d'Optique in Palaiseau with a Rydberg-atom machine studying the melting of a domain-wall. I will discuss a theoretical protocol that shows how to detect the fact that the unitary dynamics implemented in the model is not integrable, as it is theoretically expected. I will extensively comment on the results and their theoretical implications. The theory is a joint work with Luca Capizzi and Maurizio Fagotti.

Speaker: Prof. Leonardo Mazza (Université Paris-Saclay)

12 Transport in Dipole–Dipole Coupled Rydberg Atom Chains: A Work in Progress

I consider transport properties of an extended-range XX model, a candidate framework for describing dipole–dipole coupled Rydberg atom chains. With the goal of capturing the properties of the system at the intermediate time scales accessible in experiments, I discuss the possibility of approximating the system with an integrable one.
Two complementary approaches are considered. First, I develop an effective thermodynamic Bethe Ansatz (TBA) description, the associated generalized hydrodynamic (GHD) equations, and discuss their inherent limitations. Second, I construct a mean-field framework that closely parallels the TBA/GHD and appears to face similar issues. I then outline a possible route to overcoming those obstacles and highlight the striking physical properties that seem to be responsible for the difficulties in developing a complete theoretical description.

Speaker: Prof. Maurizio Fagotti (CNRS, LPTMS Orsay)

11:30 COFFEE BREAK

13 Confinement and localisation in out-of-equilibrium quantum spin chains

The non-equilibrium dynamics of quantum spin chains is significantly influenced by confining forces, as was demonstrated using the Ising quantum spin chain in a longitudinal magnetic field. Depending on the setup, they can suppress thermalisation either through real-time confinement, analogous to strong interactions, or via Wannier-Stark localisation caused by Bloch oscillations. Both mechanisms limit the light-cone spreading of correlations and restrict the growth of entanglement entropy. Furthermore, Bloch oscillations interfere with the decay of a false vacuum by hindering the expansion of nucleated true vacuum bubbles. We recently extended these investigations to the 3-state Potts quantum spin chain, which shows richer behaviour due to baryonic excitations and various possible alignments between the initial magnetisation and the applied longitudinal field. The latter lead to novel features such as partial localisation in certain quench scenarios, for which some correlations still spread unsuppressed.

Speaker: Prof. Gábor Takács (Budapest University of Technology and Economics)

13:00 CONCLUDING REMARKS