We explore the coherent dynamics of a three-level $\mathsf{V}$-type system interacting with a thermal bath in the regime where thermal excitation occurs much faster than spontaneous decay. We present analytic solutions of the Bloch-Redfield quantum master equations, which show that strong incoherent pumping can generate long-lived quantum coherences among the excited states of the $\mathsf{V}$-type system in the overdamped regime defined by the condition $\mathrm{\Delta }/\left(\overline{n}\gamma \right)<f\left(p\right)$, where $\mathrm{\Delta }$ is the excited-state level splitting, $\gamma$ is the spontaneous decay rate, $\overline{n}\gg 1$ is the effective photon occupation number proportional to the pumping intensity, and $f\left(p\right)$ is a universal function of the transition dipole alignment parameter $p$. In the limit of nearly parallel transition dipoles ($p\to 1$) the coherence lifetime ${\tau }_c=1.34(\overline{n}/\gamma ){(\mathrm{\Delta }/\gamma )}^{-2}$ scales linearly with $\overline{n}$ and is enhanced by the factor $0.67\overline{n}$ with respect to the weak-pumping limit [Phys. Rev. Lett. 113, 113601 (2014); J. Chem. Phys. 144, 244108 (2016)]. We also establish the existence of long-lived quasistationary states, which occur in the overdamped regime and affect the process of thermalization of the $\mathsf{V}$-type system with the bath, slowing down the approach to thermal equilibrium. In the case of nonparallel transition dipole moments ($p<1$), no quasistationary states are formed and the coherence lifetime decreases sharply. The sharp transition between the different regimes of coherent dynamics is due to an interplay between coherence-generating Fano interference and various coherence-destroying processes (such as stimulated decay). Using a newly developed effective decoherence rate model, we find that in the limit $p\to 1$ the rates of coherence generation and decay are almost exactly balanced and the effective decoherence rate is minimized, leading to long coherence lifetimes. Our results reveal new regimes of long-lived quantum coherent dynamics, which could be observed in thermally driven atomic and molecular systems.

• Received 1 December 2017
• Revised 27 May 2018

DOI:https://doi.org/10.1103/PhysRevA.98.023811