Wigner functions, squeezing properties, and slow decoherence of a mesoscopic superposition of two-level atoms

We consider a class of states in an ensemble of two-level atoms: a mesoscop ic superposition of two distinct atomic coherent states, which can be regar ded as atomic analogs of the states usually called Schrodinger-cat states i n quantum optics. According to the relation of the constituents, we define polar and nonpolar cat states. The properties of these are investigated by the aid of the spherical Wigner function. We show that nonpolar cat states generally exhibit squeezing, the measure of which depends on the separation of the components of the cat, and also on the number of the constituent at oms. By solving the master equation for the polar cat state embedded in an external environment, we determine the characteristic times of decoherence and dissipation, and also the characteristic time of a new; parameter, the nonclassicality of the state. This latter one is introduced by the help of the Wigner function, which is used also to visualize the process. The depen dence of the characteristic times on the number of atoms of the cat and on the temperature of the environment shows that the decoherence of polar cat states is surprisingly slow. [S1050-2947(99)01611-X].