Quantifying Early Time Quantum Decoherence Dynamics through Fluctuations

We introduce a general but simple relation between the timescale for quantum coherence loss and the initial fluctuations of operators that couple a quantum system with a surrounding bath. The relation allows the prediction and measurement of early time decoherence dynamics for open quantum systems, through purity, without reconstructing the system’s many-body density matrix. It is applied to predict the decoherence time for the Holstein chain, spin-boson and Caldeira-Legget models. Such development also offers a practical platform to test the ability of approximate quantum dynamics methods to capture decoherence. In particular, a class of mixed quantum-classical schemes for molecular dynamics where the bath is treated classically, such as Ehrenfest dynamics, are shown to correctly capture short-time decoherence when the initial conditions are sampled from the Wigner distribution. Further, this relation is used to develop a general theory of electronic decoherence in molecules, a basic molecular process that is essential to the development of approximation schemes to excited states and non-adiabatic dynamics. These advances provide a useful platform to develop decoherence times for molecular processes and to test approximate molecular dynamics methods.