Noise-induced quantum coherence and persistent Rabi oscillations in a Josephson flux qubit

We predict theoretically the enhancement of quantum coherence in a superconducting flux qubit by a classical external noise. First, the off-diagonal components of the qubit density matrix are increased. Second, in the presence of both ac drive and noise, the resulting Rabi oscillations survive “in perpetuity,” i.e., for times greatly exceeding the Rabi decay time in a noiseless system. The coherence-enhancing effects of the classical noise can be considered as a manifestation of quantum stochastic resonance and are relevant to experimental techniques, such as Rabi spectroscopy.