Abstract: Hydrodynamics provides a remarkably universal description of matter in terms of a few continuous fields. But how does this continuum description emerge from the dynamics of discrete quantum particles? I will address this question in open quantum systems of bosons with a conserved particle-number symmetry. In mixed states this symmetry can be either strong or weak, and distinct dynamical phases can be characterized by the spontaneous breaking of these symmetries. A strong-to-weak spontaneous symmetry-breaking (SW-SSB) transition can be diagnosed by information-theoretic quantities that are nonlinear in the mixed-state density matrix. Remarkably, a  nite-time SW-SSB transition can occur, separating quantum from classical dynamics - after the transition classical hydrodynamics emerges. I will discuss observations of a SW-SSB transition in cold atomic gas experiments.
 

Matthew P. A. Fisher is a professor of physics at the University of California, Santa Barbara. He received his Ph.D. from the University of Illinois in 1986 and subsequently worked at IBM T. J. Watson Research Center before joining UCSB in 1993. He later spent several years at Microsoft Station Q and a year at Caltech before returning to UCSB in 2010. His research broadly explores quantum many-body physics and condensed matter theory, with more recent interests in quantum dynamics and information. Fisher received the NSF Alan T. Waterman Award, the National Academy of Sciences Award for Initiatives in Research, and the APS Oliver E. Buckley Prize in Condensed Matter Physics. He is a member of the National Academy of Sciences and the American Academy of Arts and Sciences.