Perturbative and nonperturbative master equations for open quantum systems

This paper develops perturbative and nonperturbative master equations for open quantum systems based on time-dependent variational functionals. The perturbative equations are more concise and suitable for dealing with cases of weak system-environment coupling for short evolution time scales. The nonperturbative equations are valid for all time and appropriate to treat cases of strong system-environment coupling. When a system contains an external control field, both the perturbative and nonperturbative master equations reveal the embedded control field dependence upon the system decoherence, which provides a basis for decoherence management.