Time-asymmetric quantum-state-exchange mechanism

We show here that due to nonadiabatic couplings in decaying systems, applying the same time-dependent protocol in the forward and reverse direction to the same mixed initial state leads to different final pure states. In particular, in laser-driven molecular systems, applying a specifically chosen positively chirped laser pulse or an equivalent negatively chirped laser pulse yields entirely different final vibrational states. This phenomenon occurs when the laser frequency and intensity are slowly varied around an exceptional point (EP) in the laser intensity and frequency parameter space where the non-Hermitian spectrum of the problem is degenerate. The protocol implies that a positively chirped laser pulse traces a loop in time in the laser parameters' space whereas a negatively chirped pulse follows the same loop in the opposite direction. According to this protocol one can choose the final pure state from any initial state. The obtained results imply the intrinsic nonadiabaticity of quantum transport around an EP, and offer a way to observe the EP experimentally in time-dependent quantum systems.