  We have observed spontaneous symmetry breaking of atomic populations in the
dynamic phase-space double-potential system, which is produced in the
parametrically driven magneto-optical trap of atoms. We find that the system
exhibits similar characteristics of the Ising-class phase transition and the
critical value of the control parameter, which is the total atomic number, can
be calculated. In particular, the collective effect of the laser shadow becomes
dominant at large atomic number, which is responsible for the population
asymmetry of the dynamic two-state system. This study may be useful for
investigation of dynamic phase transition and temporal behaviour of critical
phenomena.
