Nonclassical behavior of an intense cavity field revealed by quantum discord

We investigate the quantum-to-classical crossover of a dissipative cavity field by measuring the correlations between two noninteracting atoms coupled to the cavity mode. First, we note that there is a time window in which the mode shows a classical behavior, which depends on the cavity decay rate, the atom-field coupling strength, and the number of atoms. Then, considering the steady state of two atoms inside the cavity, we note that the entanglement between the atoms disappears while the mean number of photons of the cavity field (n) rises. However, the quantum discord reaches an asymptotic nonzero value even in the limit of n→∞, whether n is increased coherently or incoherently. Therefore, the cavity mode always preserves some quantum characteristics in the macroscopic limit, which is revealed by the quantum discord.