Assembling a complex quantum ensemble

Abstract

A cold-atom technique is used to identify hidden constraints in thermodynamic ensembles [Also see Report by Langen et al. ] Statistical mechanics provides a systematic approach for predicting the behavior of systems when only partial information is present. The key assumption underlying all of statistical mechanics is that every allowed configuration of the system, called a microstate, occurs with equal probability. This approach, valid for both classical and quantum systems, requires only minimal additional information for it to be astonishingly predictive. On page 207 of this issue, Langen et al. ( 1 ) describe a system in which this paradigm is violated and conventional statistical mechanics fails; almost all of the usually allowed microstates are inaccessible via the system's dynamics.