We investigate the evolution of purity for dissipative symmetric two-level systems using numerically exact real-time path integral methods.  As commonly assumed, the purity decays monotonically to its fully mixed value at high temperatures and strong system-bath coupling, with a short-time Gaussian behavior which is subsequently followed by exponential evolution. However, under low-temperature and weak coupling conditions, a substantial recovery of purity is observed.  A simple theoretical analysis reveals three contributions that correspond to completely incoherent, eigenstate population difference and rate terms.  Two of these terms can counter the early drop of purity and are responsible for its rebound.  These analytical and numerical results show clearly that the loss of quantum coherence during the evolution toward equilibrium does not necessarily imply the decay of purity, and that the time scales relevant to these two processes may be entirely different.

Recovery of purity in dissipative tunneling dynamics

ACS Spring Meeting 2021

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