Probing quantum many-body dynamics using subsystem Loschmidt echos

Our recent study is online!

February 04, 2025

The Loschmidt echo is a fundamental concept in statistical physics, capturing the probability of a system returning to its initial state following a time evolution. It generally contains key information about a quantum many-body system, relevant across various scientific fields including quantum chaos, quantum many-body physics, or high-energy physics. While widely used in theoretical studies, direct experimental measurements of the Loschmidt echo are typically infeasible since it is exponentially small in system size.

In our work, we demonstrate how this challenge can be overcome by experimentally studying the subsystem Loschmidt echo — a powerful and experimentally accessible probe of quantum many-body dynamics. 

Utilizing our caesium quantum gas microscope, we study the short- and long-time dynamics of the subsystem Loschmidt echo following a sudden quench in a one-dimensional Bose-Hubbard model. In the short-time regime, we use the subsystem Loschmidt echo to observe a dynamical quantum phase transition and show that it arises due to genuine higher-order correlations in the system. In the long-time regime, the subsystem Loschmidt echo allows us to quantify the effective dimension and structure of the accessible Hilbert space in the thermodynamic limit. By measuring the steady-state value of the subsystem Loschmidt echo in an ergodic model and in the presence of emergent kinetic constraints, we provide direct experimental evidence for ergodicity breaking due to fragmentation of the Hilbert space.

Our work highlights how quantum simulators offer new ways to explore complex non-equilibrium dynamics, beyond simple measurements of, e.g., local mean densities or low-order correlations.

Original publication:
Probing quantum many-body dynamics using subsystem Loschmidt echos
Simon Karch, Souvik Bandyopadhyay, Zheng-Hang Sun, Alexander Impertro, SeungJung Huh, Irene Prieto Rodríguez, Julian F. Wienand, Wolfgang Ketterle, Markus Heyl, Anatoli Polkovnikov, Immanuel Bloch, Monika Aidelsburger

arXiv:2501.16995

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