Coherence resonance in open quantum systems*

In this study, we demonstrate that coherence resonance, in which regularity of the oscillatory response of a nonlinear system is maximized at a certain optimal noise intensity, occurs in open quantum systems. We numerically demonstrate that quantum coherence resonance occurs in a quantum van der Pol system subjected to squeezing. We first demonstrate that quantum coherence resonance occurs in the semiclassical regime, namely, the regularity of the system's oscillatory response is maximized at an optimal intensity of quantum fluctuations, and interpret this phenomenon by analogy with classical noisy excitable systems using semiclassical stochastic differential equations. We further investigate the stronger quantum regimes and demonstrate that the regularity of the system's response can exhibit the second peak as the intensity of the quantum fluctuations is further increased. We show that this second peak of resonance is a strong quantum effect that cannot be interpreted by a semiclassical picture, in which only a few energy states participate in the system dynamics.