Generalized master equations provide a theoretically rigorous framework to capture the dynamics of processes ranging from energy harvesting in plants and photovoltaic devices to qubit decoherence in quantum technologies and even protein folding. At their center is the concept of memory. The explicit time-nonlocal description of memory is both protracted and elaborate. When physical intuition is at a premium, one would desire a more compact, yet complete, description. Here, we demonstrate how and when the time-convolutionless formalism constitutes such a description. In particular, by focusing on the dissipative dynamics of the spin-boson and Frenkel exciton models, we show how to: easily construct the time-local generator from reference reduced dynamics, elucidate the dependence of its existence on the system parameters and the choice of reduced observables, identify the physical origin of its apparent divergences, and offer analysis tools to diagnose their severity and circumvent their deleterious effects. We demonstrate that, when applicable, the time-local approach requires as little information as the more commonly used time-nonlocal scheme, with the important advantages of providing a more compact description, greater algorithmic simplicity, and physical interpretability. We conclude by introducing the discrete-time analog and a straightforward protocol to employ it in cases where the reference dynamics have limited resolution. The insights we present here offer the potential for extending the reach of dynamical methods, reducing both their cost and conceptual complexity.
REFERENCES
One must make a distinction between traceless versus properly normalized initial conditions. While physically allowed density matrices in quantum mechanics are normalized, one can construct dynamical quantities that appear to arise from traceless initial density matrices. For example, in the Argyres-Kelley projector, which recovers the entire density matrix subject to all uncorrelated initial condition, traceless initial densities correspond to cases where the spin starts in a coherence, |j⟩⟨k|, while the bath is originally in thermal equilibrium, . Physically, such a situation arises from, say, the measurement of a transition dipole operator after an impulsive initial condition. In contrast, normalized initial densities in the Argyres-Kelley projector arise from the population-based initial conditions where the spin starts from a normalized superposition of states.
By “weak coupling” we mean that the reorganization energy, λ, is smaller than any other parameter in the Hamiltonian. However, for the low-order perturbation theory, namely TCL2 Redfield, to agree with the HEOM, the requirement is that the unitless parameter max[2λ/βωc2, 2λ/πωc] < 1, as is shown in Ref. 125 and analyzed in greater depth in Ref. 126.
We note that in our analysis surrounding Figs. 4(a) and 4(b) are the same.