Long-lived quasi-stationary coherences in V-type system driven by   incoherent light
  We present a theoretical study of noise-induced quantum coherences in a model three-level V-type system interacting with incoherent radiation, an important prototype for a wide range of physical systems ranging from trapped ions to biomolecules and quantum dots. By solving the quantum optical equations of motion for the V-type system, we obtain analytical expressions for the noise-induced coherences and show that they exhibit an oscillating behavior in the limit of large excited level spacing $\Delta$ ($\Delta /\gamma \gg 1$, where $\gamma$ is the radiative decay rate). Most remarkably, we find that in the opposite limit of small level spacing $\Delta/\gamma \ll 1$, appropriate for large molecules, (a) the coherences can survive for an arbitrarily long time $\tau=(2/\gamma) (\Delta/\gamma)^{-2}$ before eventually decaying to zero, and (b) coherences at short times can be substantial. We further show that the long-lived coherences can be robust against environmental relaxation and decoherence, and discuss implications to the design of quantum heat engines and incoherent light excitation of biological systems. 
Timur V. Tscherbul and Paul Brumer
physics.chem-ph quant-ph