2019 Colloquium: Lincoln Carr on Quantum Many-Body Systems

Although the quantum-classical correspondence is well laid out for a single particle in a closed quantum system, it is not at all clear that such a correspondence holds generically in many-body systems. At the same time our experimental capacities in high-precision control of quantum many-body systems has continued to expand rapidly, spurred in good part by developments at the many institutions engaged in the creative development of ultracold quantum gas architectures for quantum information processing. Thus the search for macroscopic quantum effects, or even beyond-quantum effects, is presently a rapidly growing field. In this talk, I will discuss a key emergent feature of attractive Bose-Einstein condensates. This well-established system presents many puzzles, amongst them the lack of an established semiclassical limit for far-from-equilibrium dynamics of emergent features like bright solitons. Such localized "lumps" self-cool at the semiclassical level and are incredibly robust against perturbations. Yet a close examination of their entangled dynamics reveals an increasingly non-classical set of dynamics, from semiclassical particle-like excitations of a wave one might find in laser pulses in fiber optics or the ocean, to quantum-fluctuation induced crystals of solitons, to solitons as the classical bones in the density of states of a fully quantum system, to quantum entangled breathers that rely on entanglement for their very existence. At this latter end of the soliton quantumness scale is predicted a truly macroscopic quantum state against which the predictions of many-body quantum mechanics can be tested at a whole new level.