Out-of-equilibrium quantum magnetism in a spin-3 many-body dipolar system

We explore out-of-equilibrium spin dynamics in a S=3 chromium Bose-Einstein condensate, after tilting the atomic spins by an angle θ with respect to the magnetic field. We monitor the populations of the seven spin components... [ view full abstract ]

We explore out-of-equilibrium spin dynamics in a S=3 chromium Bose-Einstein condensate, after tilting the atomic spins by an angle θ with respect to the magnetic field. We monitor the populations of the seven spin components after a collective rotation of an initially polarized ensemble, as a function of the angle θ.

Without an optical lattice, the dynamics is driven by an interplay between spin-dependent contact interactions and dipole-dipole interactions. We find that the initial ferromagnetic character of the gas is locally preserved, and that the system is well described by the hydrodynamic equations of a ferrofluid. A collective spin mode is observed, which corresponds to a trapped magnon mode.

In presence of a 3D optical lattice, the experiment consists of a unit-filled array of 10⁴ chromium atoms, and dynamics is entirely driven by dipole-dipole interactions. This realizes the spin-3 XXZ Heisenberg model. We find that spin dynamics, and a corresponding approach to equilibrium, are increasingly driven by quantum correlations as the angle θ approaches π/2. We derive a simple analytical thermodynamic formula, which compares well to our data at long times, i.e. after equilibrium is reached. This agreement indicates a scenario of quantum thermalization in which entanglement build-up of an isolated many-body system drives the approach towards a local thermal equilibrium.  

These results demonstrate the potential of magnetic atoms to study truly macroscopic and well-ordered arrays of spins, investigate strongly correlated quantum systems, and address fundamental questions on how unitary dynamics connects to statistical physics.