On Thursday November 14, 2024, Jožef Stefan Institute (JSI) will host the 13th Trieste–Ljubljana-Zagreb meeting. This is a regular event attended by researchers from SISSA and ICTP (Italy), FMF UL and JSI (Ljubljana), and Ruđer Boškovič Institute (Zagreb).

The venue is the Main Lecture Hall of the Jožef Stefan Institute.

Programme

9:55-10:00	Welcome remarks
10:00-10:30	Gabriele Perfetto (Universität Tübingen): Quantum walks and many-body first detection probability
10:30-11:00	Anna Delmonte (ICTP and SISSA): The Post-Selection Problem in Infinite and Long-Range Systems
11:00-11:30	Coffee break
11:30-12:00	Gianluca Lagnese (JSI): Positive Operator Valued Measures Neural Networks for simulation of light-matter coupled system
12:00-12:30	Pavel Orlov (FMF): Methods of Adiabatic Transformations in (non)Hermitian Quantum Systems
12:30-14:30	Lunch and discussions
14:30-15:00	Michele Fossati (SISSA): Boundary quench in CFT
15:00-15:30	Gianpaolo Torre (IRB): Quantum Coherence in Topologically Frustrated Quantum Systems
15:30-16:00	Coffee break
16:00-18:00	Poster session
18:00-	Discussions and Social dinner

 

Abstracts

Gabriele Perfetto, Universität Tübingen

Quantum walks and many-body first detection probability

In the talk, I will discuss the statistical properties of the time needed to first detect a certain quantum state. This topic is inherently related to monitoring of the quantum-unitary dynamics and quantum walks. 

In the first part of the talk, I will therefore introduce the topic of quantum walks: a single quantum particle undergoing unitary hopping dynamics on a lattice is subject to projective measurements performed at stroboscopic times. The first time the particle is detected on a target site defines the first detection probability. The latter displays long-time universal algebraic behavior with the associated exponent being different from the classical counterpart.

In the second part of the talk, I will discuss the formulation of the first-detection problem in the more challenging scenario of many-body quantum systems. I show that the first detection return probability of a certain quantum state can be mapped to the equilibrium partition function of classical noninteracting magnetic domains. This mapping allows to classify and understand different long-time universal decays of the first detection probability in terms of different thermodynamic phases of the associated classical spin model. Ferromagnetic ordering of the classical spin model maps to algebraic decay at long times of the quantum first-detection probability, while paramagnetic behavior maps to a faster exponential decay of the first detection probability. We illustrate this mapping considering the example of N adjacent fermions on a lattice. This analysis thereby provides an overarching connection between non-equilibrium measurement-induced quantum fluctuations and equilibrium thermodynamic phases.

[1] B. Walter, G. Perfetto, A. Gambassi, arxiv:2311.05585 (2023)

 

Anna Delmonte, ICTP and SISSA

The Post-Selection Problem in Infinite and Long-Range Systems

A key challenge in observing measurement-induced phase transitions is mitigating the post-selection barrier, which causes the reproducibility of quantum trajectories—sequences of measurement readouts—to be exponentially small in system size and observation time. Recent studies, however, suggest that entanglement measurement-induced phase transitions in certain classes of collective models may reduce this overhead, as quantum correlations tend to saturate rapidly. In this talk, I will explore the monitored dynamics of an infinite-range cavity model and a long-range spin model, demonstrating the emergence of measurement-induced phase transitions in entanglement. I will analyze the dynamical properties of quantum correlations to identify regions with favorable post-selection properties, providing insight into which experimental platforms may be most suitable for observing monitored phases.

 

Gianluca Lagnese, JSI

Positive Operator Valued Measures Neural Networks for simulation of light-matter coupled systems.

Recent advances in quantum simulation are focused on combining matter and light to engineer new types of interactions, typically characterized by long-range effects, requiring the development of advanced numerical simulation techniques. For instance, ordered arrays of atoms placed at distances smaller than the wavelength of light display photo-mediated long-range interactions and a peculiar correlated emission. The main features observed when starting from a highly excited initial state are a superradiant burst at short times, followed by a non-trivial “subradiant” critical regime with a slow power-law relaxation. By integrating out the photonic degrees of freedom, the dynamics are effectively described by a Lindblad equation with long-range interactions and dissipation. To simulate these dynamics, we employ a recently proposed numerical approach [1] that combines a positive operator-valued measure (POVM) description of the density matrix—approximated by a neural network—with a time-dependent variational principle (TDVP) to project the evolution of the state onto the neural network manifold. We explore upscaling to larger system sizes as a complementary tool to standard tensor network techniques, especially for long-range interactions and two-dimensional setups. From a more physical perspective, by applying a time-dependent Generalized Gibbs ensemble Ansatz, we uncover the role of (approximate) integrability at long times, which leads to the observed polynomial decay

 

Pavel Orlov, FMF

Methods of Adiabatic Transformations in (non)Hermitian Quantum Systems.

Many quantum phenomena can be studied through the prism of adiabatic deformations. In this talk, I would like to review recent advances in this research branch, focusing on my studies.  I will discuss two applications of the adiabatic approach. One of them is detecting critical points of quantum phase transitions. Another is the identification of different types of perturbation in the context of quantum chaos. Starting from closed Hamiltonian systems, I will move on to a discussion of dissipative and non-Hermitian systems, arguing that the adiabatic approach can be very useful in this case as well.

 

Michele Fossati, SISSA

Boundary quench in CFT

We consider critical systems with a discrete group symmetry that is explicitly broken by the boundary conditions. First we study the entanglement asymmetry in the groundstate, finding universal results that depend only on the cardinality of the group and the dimension of a boundary changing operator. Then we focus to the the Ising model and we time evolve the state, quenching the boundary condition to the symmetric one. We monitor the magnetization, entanglement entropy and entanglement asymmetry, finding periodic patterns that come from reflections of the perturbations at the boundary.

 

Gianpaolo Torre, IRB

Quantum Coherence in Topologically Frustrated Quantum Systems

It has been proposed that the asymptotic behavior of every entropic resource for the ground states of topologically frustrated systems, in the thermodynamic limit, can be expressed as the sum of the contributions from topological frustration and non-frustrated ones. Indeed, it has been shown that the presence of delocalized excitations within the system induces non-local, size-dependent correction terms both in the entanglement and in the Stabilizer Entropy. In this talk, we explore this hypothesis by examining the behavior of quantum coherence. We found that the presence of these excitations introduces a non-local logarithmic correction to the volume law of the non-frustrated model. This result is also more general, being valid for any system with a finite number of excitations. From a technical standpoint, we were able to efficiently compute quantum coherence, a quantity difficult to calculate for large systems, using a combination of DMRG and Tensor Cross Interpolation algorithms.

 

Local organizers

Lev Vidmar and Zala Lenarčič