Continuous Measurement and Feedback Control for Enhancement of Quantum Synchronization

We analyze synchronization of a quantum van der Pol (vdP) oscillator driven by a harmonic signal and demonstrate that performing continuous homodyne measurement on an additional bath linearly coupled to the oscillator and applying a feedback control to the oscillator can enhance quantum synchronization. We show that the phase coherence of the oscillator is increased by the reduction of quantum fluctuations due to the continuous measurement, but also that the measurement backaction inevitably induces fluctuations around the phase-locking point. We propose a simple feedback policy that can suppress the measurement-induced fluctuations by adjusting the frequency detuning between the oscillator and the driving signal, which leads to enhancement of quantum synchronization. We further demonstrate that the maximum phase coherence can be achieved by performing the quantum measurement on the quadrature angle at which the phase diffusion of the oscillator takes the largest value and the maximum information of the oscillator phase is attained.