Quantum Many-Body Physics Beyond Ground States
Recently a set of traditional assumptions in quantum condensed matter theory has been upended by the realization that many-body systems can host stable quantum phenomena at infinite temperature. I will discuss recent work suggesting that there is a much richer landscape of such phenomena than has so far been appreciated. First, I will show how strongly disordered quantum many-body systems can spontaneously segregate into delocalized and localized degrees of freedom, leading to a so-called "mobility emulsion." Second, I will discuss how symmetries in otherwise generic systems can protect quantum coherence at infinite temperature through the existence of index theorems more commonly associated with topological phases. Finally, I will discuss "quantum many-body scars," a novel manifestation of high-temperature quantum coherence observed in recent experiments that so far lacks a physical explanation. I will suggest a new picture of this phenomenon wherein the observed coherent dynamics emerges from correlations among low-lying excitations.