Robert Schittko

Quantum thermalization through entanglement in an isolated many-body system.

To thermalize, or not to thermalize? Intuition tells us that an isolated physical system subjected to a sudden change (i.e., quenching) will evolve in a way that maximizes its entropy. If the system is in a pure, zero-entropy quantum state, it is expected to remain so even after quenching. How do we then reconcile statistical mechanics with quantum laws? To address this question, Kaufman et al. used their quantum microscope to study strings of six rubidium atoms confined in the wells of an optical lattice (see the Perspective by Polkovnikov and Sels). When tunneling along the strings was suddenly switched on, the strings as a whole remained in a pure state, but smaller subsets of two or three atoms conformed to a thermal distribution. The force driving the thermalization was quantum entanglement. Science, this issue p. 794; see also p. 752 Single-site microscopy of strings of rubidium atoms in an optical lattice shows thermalization on a local scale. Statistical mechanics relies on the maximization of entropy in a system at thermal equilibrium. However, an isolated quantum many-body system initialized in a pure state remains pure during Schrödinger evolution, and in this sense it has static, zero entropy. We experimentally studied the emergence of statistical mechanics in a quantum state and observed the fundamental role of quantum entanglement in facilitating this emergence. Microscopy of an evolving quantum system indicates that the full quantum state remains pure, whereas thermalization occurs on a local scale. We directly measured entanglement entropy, which assumes the role of the thermal entropy in thermalization. The entanglement creates local entropy that validates the use of statistical physics for local observables. Our measurements are consistent with the eigenstate thermalization hypothesis.

Microscopy of the interacting Harper–Hofstadter model in the two-body limit

The interplay between magnetic fields and interacting particles can lead to exotic phases of matter that exhibit topological order and high degrees of spatial entanglement. Although these phases were discovered in a solid-state setting, recent innovations in systems of ultracold neutral atoms—uncharged atoms that do not naturally experience a Lorentz force—allow the synthesis of artificial magnetic, or gauge, fields. This experimental platform holds promise for exploring exotic physics in fractional quantum Hall systems, owing to the microscopic control and precision that is achievable in cold-atom systems. However, so far these experiments have mostly explored the regime of weak interactions, which precludes access to correlated many-body states. Here, through microscopic atomic control and detection, we demonstrate the controlled incorporation of strong interactions into a two-body system with a chiral band structure. We observe and explain the way in which interparticle interactions induce chirality in the propagation dynamics of particles in a ladder-like, real-space lattice governed by the interacting Harper–Hofstadter model, which describes lattice-confined, coherently mobile particles in the presence of a magnetic field. We use a bottom-up strategy to prepare interacting chiral quantum states, thus circumventing the challenges of a top-down approach that begins with a many-body system, the size of which can hinder the preparation of controlled states. Our experimental platform combines all of the necessary components for investigating highly entangled topological states, and our observations provide a benchmark for future experiments in the fractional quantum Hall regime.