Long-lived quasistationary coherences in a 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, we obtain analytic expressions for the noise-induced coherences and show that they exhibit an oscillating behavior in the limit of large excited level spacing Δ (Δ/γ≫1, where γ is the radiative decay width). Most remarkably, we find that in the opposite limit of small level spacing Δ/γ≪1, appropriate for large molecules, (a) the coherences can survive for an extremely long time τ=(2/γ)(Δ/γ)^{-2} before eventually decaying to zero, and (b) coherences at short times can be substantial. We further show that the long-lived coherences can survive environmental relaxation and decoherence, suggesting implications to the design of quantum heat engines and to incoherent light excitation of biological systems.