Observation of quantum stochastic synchronization in a dissipative quantum system

We report the observation of quantum stochastic synchronization (QSS) in a dissipative quantum system-a moderately damped current-biased Josephson junction. We show that when the junction is biased subcritically by a combination of a constant current and a small sinusoidal modulation signal, forced phase synchronization between the otherwise random, incoherent quantum tunneling and the signal may arise as a consequence of the interplay between quantum fluctuation and the weak periodic drive. It is found that optimal synchronization occurs at a signal frequency f(QSS) that is comparable to the quantum tunneling rate. We also find a phenomenon called "modulation induced suppression of quantum noise": for signal frequency well below fQSS, the power spectral density of escape time distribution of the junction could be reduced substantially from the floor level of the unmodulated system. Furthermore, we demonstrate that the application of a small signal with proper phase can suppress the average tunneling rate and enhance the stability of the metastable system. Our experimental results agree well with the numerical calculations without the use of adjustable parameters.