Universal dissipators for driven open quantum systems and the correction to linear response
We investigate in parallel two common pictures used to describe quantum systems interacting with their surrounding environment, i.e., the stochastic Hamiltonian description, where the environment is implicitly included in the fluctuating internal parameters of the system, and the explicit inclusion of the environment via the time-convolutionless projection operator method. Utilizing these two different frameworks, we show that the dissipator characterizing the dynamics of the reduced system, determined up to second order in the noise strength or bath-system coupling, is composed of two parts. One is universal, meaning that it keeps the same form regardless of the drive term. This form constitutes the relevant part of the dissipator only as long as the drive is weak. We thoroughly discuss the assumptions on which this treatment is based and its limitations. Then, by considering the first non-vanishing higher-order term in our expansion, we derive the other, drive-dependent, term completing the full dissipator. This part of the dissipator, originating from the third cumulant, is usually neglected when modeling the decoherent dynamics of controlled qubits. However, this further term constitutes the linear response correction due to memory-mediated environmental effects in driven-dissipative quantum systems. Also, it notably shows that the structure of our quantum master equation goes beyond the Lindblad form. The Lindblad form is recovered for memory-less baths. Finally, we demonstrate this technique to be highly accurate for the problems of dephasing in a driven qubit and for the theory of pseudo-modes for quantum environments.