Observation of a persistent non-equilibrium state in cold atoms

Boltzmann noticed that his transport equation predicts special cases in which gases never reach thermal equilibrium. One example is the monopole breathe mode of atoms confined in a perfectly isotropic three-dimensional (3D) harmonic potential1. Such a complete absence of damping had not been observed in nature, and this anomaly weakened Boltzmann’s then-controversial claim to have established a microscopic, atomistic basis for thermodynamics. Only recently has nondamping of a monopole mode in lower-dimensional systems been reported in cold-atom experiments performed in highly elongated trap geometries2,3. The di culty in generating a su ciently spherical harmonic confinement for cold atoms has so far prevented the observation of Boltzmann’s fully 3D, isotropic case. Here, thanks to a new magnetic trap4 capable of producing near-spherical harmonic confinement for cold atoms,we report a long-delayed vindication for Boltzmann: the observation of a 3D monopole mode for which the collisional contribution to damping vanishes. The Boltzmann equation determines how the phase-space distribution of a gas, f (r,v, t), evolves as a function of time, t , with binary collisions between particles with mass,m, in the presence of an external force, F