Recent experimental advances in ultrafast phenomena have triggered renewed interest in the dynamics of correlated quantum systems away from equilibrium. We review nonequilibrium dynamical mean-field theory studies of both the transient and the steady states of a DC field-driven correlated quantum system. In particular, we focus on the nonequilibrium behavior and how it relates to the fluctuation-dissipation theorem. The fluctuation-dissipation theorem emerges as an indicator for how the system thermalizes and for how it reaches a steady state. When the system thermalizes in an infinite temperature steady state it can pass through a succession of quasi-thermal states that approximately obey the fluctuation-dissipation theorem. We also discuss the Wigner distribution and what its evolution tells us about the nonequilibrium many-body problem.
Quantum coherent dynamics of a
Λ-system driven by a thermal environment: Enhancing coherence lifetimes via incoherent driving
We present a theoretical study of quantum coherent dynamics in a three-level
Λ system driven by a thermal environment. By solving the nonsecular Bloch-Redfield master equations, we obtain analytical results for the ground-state population and coherence dynamics and classify the dynamical regimes of the incoherently driven
Λ-system as underdamped and overdamped depending on whether the ratio
Δ/[rf(p)] is greater or less than one, where
Δ is the ground-state energy splitting,
r is the incoherent pumping rate, and
f(p) is a dimensionless function of the transition dipole alignment parameter
p. In the underdamped regime, we observe long-lived coherent dynamics that lasts for
τc≃1/r, even though the initial state of the
Λ-system contains no coherences in the energy basis. In the overdampled regime for
p=1, we observe the emergence of coherent quasi-steady states with the lifetime
τc=1.34(r/Δ2), which have low von Neumann entropy compared to the conventional thermal states. Our results suggest that thermal excitations can enhance the lifetimes of initially created coherent superpositions, making thermal driving a potentially useful coherence-enhancing tool for quantum information science.