Exploiting classical decoherence in dissipative quantum dynamics: Memory, phonon emission, and the blip sum

The mechanisms that lead to the quenching of the coherence of a quantum system interacting with a dissipative environment are analyzed within the path integral/influence functional framework. Classical and quantum contributions are identified and linked to stimulated and spontaneous phonon emission processes. Three ways of exploiting decoherence in numerical calculations are discussed. The first is based on the decay of memory effects with time separation and leads to well-known iterative quasi-adiabatic propagator path integral (i-QuAPI) schemes. The other two mechanisms exploit the prominent role of classical decoherence. Removal of classical memory is possible through the introduction of auxiliary variables, leading to efficient and accurate quantum-classical path integral (QCPI) methods. A third way is proposed, which involves systematic corrections to the fully incoherent limit. An efficient procedure for summing all paths associated with 'blips' (short off-diagonal path pair segments) is described. The ideas are illustrated with numerical calculations of tunneling dynamics.