Strong-to-weak $U(1)$ Symmetry Breaking in Open System Dynamics

          @misc{ scivideos_PIRSA:26050026,
            doi = {10.48660/26050026},
            url = {https://pirsa.org/26050026},
            author = {Fisher, Matthew},
            keywords = {Quantum Information, Quantum Matter},
            language = {en},
            title = {Strong-to-weak $U(1)$ Symmetry Breaking in Open System Dynamics},
            publisher = {Perimeter Institute for Theoretical Physics},
            year = {2026},
            month = {may},
            note = {PIRSA:26050026 see, \url{https://scivideos.org/pirsa/26050026}}
          }
          

Abstract

n generic open quantum systems, universal dynamical processes tend to exhibit classicality - dissipation washes out quantum effects. Here I consider open quantum dynamics in the presence of a global U(1) symmetry. In mixed states, this symmetry can be either strong or weak, and distinct dynamical phases can be characterized by the spontaneous breaking of either symmetry. The strong-to-weak spontaneous symmetry breaking (SW-SSB) transition is characterized by various information theoretic quantities which are non-linear in the system's mixed state density matrix. Within a Lindblad framework for U(1) dynamics, we show that a finite-time SW-SSB transition occurs in two and three dimension, whereas in one dimension the correlation length grows linearly with time and diverges only in the infinite-time steady state. Moreover, with carefully chosen Lindblad dynamics, the SW-SSB transition signals a finite time breakdown of quantum dynamics, giving rise to classical dynamics described by a stochastic Langevin equation (Model F of Halperin-Hohenberg). When the weak symmetry is spontaneously broken, the system enters a superfluid state with classical hydrodynamics. I will describe observation of a SW-SSB phase and phase transition in cold atomic gas simulators, as revealed by an appropriately chosen quantum-classical cross-correlator.