Colloquium: Quantum Matter out of Equilibrium

Quantum Matter out of Equilibrium

Abstract: A central goal of condensed matter physics is to study the universal emergent properties of macroscopic quantum systems with large numbers of interacting particles. Due to a variety of conceptual and experimentally motivated reasons, the traditional approach of many-body physics is largely built around the study of low-temperature and near-equilibrium properties of time independent Hamiltonians.  A confluence of developments across a range of subfields --- particularly experimental advances in building programmable quantum devices --- have opened up a vast new territory of studying many-body phenomena in completely novel regimes: highly excited, "post Hamiltonian", and far from equilibrium.  A unifying theme in this enterprise has been the study of many-body quantum dynamics in systems ranging from electrons in solids to cold atomic gases to black holes.  I will describe some highlights of an active research program to advance many-body theory beyond the regime of near-equilibrium time-independent Hamiltonians, with a view towards uncovering novel emergent phenomena in the non-equilibrium dynamics of many-body systems. I will show that not only can non-equilibrium systems exhibit a sharp notion of phase structure, but that some of these phases are completely novel and unique to the out-of-equilibrium setting. For example, certain phases of matter that are forbidden in equilibrium, such as quantum time crystals, have found new life in the out-of-equilibrium setting.  I will describe the theoretical formulation of this phase, some of its many fascinating properties, and its recent experimental realization on Google's quantum processor.