Mechanical squeezing in an active-passive-coupled double-cavity optomechanical system via pump modulation.

We focus on the generation of mechanical squeezing by using periodically amplitude-modulated laser to drive an active-passive-coupled double-cavity optomechanical system, where the coupled gain cavity and loss cavity can form into a parity-time ( )-symmetry system. The numerical analysis of the system stability shows that the system is more likely to be stable in the unbroken- -symmetry regime than in the broken- -symmetry regime. The mechanical squeezing in the active-passive system exhibits stronger robustness against the thermal noise than that in the passive-passive system, and the so-called 3 dB limit can be broken in the resolved-sideband regime. Furthermore, it is also found that the mechanical squeezing obtained in the unbroken- -symmetry region is stronger than that in the broken- -symmetry region. This work may be meaningful for the quantum state engineering in the gain-loss quantum system that contributes to the study of -symmetric physics in the quantum regime.