Dynamics of reduced density matrices: Classical memory versus quantum nonlocality

The nonlocality in the path integral description of the reduced density matrix for a system interacting with a classical harmonic bath can be eliminated through the introduction of auxiliary variables, leading to Markovian equations. By contrast, the presence of an imaginary part in quantum mechanical response functions appears to prohibit similar approaches, necessitating explicit treatment of quantum nonlocality. (C) 1998 American Institute of Physics. [S0021-9606(98)00630-8].