Observation of Strong-to-Weak Spontaneous Symmetry Breaking in a Dephased Fermi Gas

TL;DR

This paper reports the first experimental observation of strong-to-weak spontaneous symmetry breaking in a dephased Fermi gas, revealing a new phase transition in mixed quantum states using quantum gas microscopy and machine learning.

Contribution

It demonstrates the experimental detection of SW-SSB in a dephased Fermi liquid, linking symmetry breaking to information transitions in open quantum systems.

Findings

Observation of long-range Rnyi order in dephased Fermi liquid

Detection of a sharp SW-SSB phase transition driven by a superlattice

Use of machine learning to access nonlinear correlators diagnosing symmetry breaking

Abstract

Symmetry-based classification of quantum phases of matter is one of the most foundational organizing principles in physics; however, an analogous framework for mixed, decohered quantum states has only begun to emerge. A central new concept is strong-to-weak spontaneous symmetry breaking (SW-SSB), a sharp transition in mixed quantum states that is invisible to any observable linear in the density matrix and that has since been predicted across a broad class of open and monitored quantum systems. It also provides a unifying language for phenomena as disparate as the decodability of topological quantum memories and the emergence of classical hydrodynamics from decohered quantum dynamics. Here we report the first experimental observation of SW-SSB, in dephased single-component fermionic matter imaged by a quantum gas microscope. A quantum-classical estimator built on a machine-learned Gaussian reference state gives direct access to the nonlinear R\'enyi-1 and R\'enyi-2 correlators that diagnose SW-SSB, and reveals long-range R\'enyi order in the dephased Fermi liquid. Adding a commensurate superlattice drives the underlying fermions through a metal-to-insulator transition that, after full dephasing, manifests as a sharp SW-SSB phase transition. Our results uncover the symmetry principle behind information-theoretic transitions in open quantum systems, and extend Landau's symmetry paradigm into the regime of real, decohering quantum devices.