  Cavity optomechanical (COM) sensors, featuring efficient light-motion
couplings, have been widely used for ultra sensitive measurements of various
physical quantities ranging from displacements to accelerations or weak forces.
Previous works, however, have mainly focused on reciprocal COM systems. Here,
we propose how to further improve the performance of quantum COM sensors by
breaking reciprocal symmetry in purely quantum regime. Specifically, we
consider a spinning COM resonator and show that by selectively driving it in
opposite directions, highly nonreciprocal optical squeezing can emerge, which
in turn provides an efficient way to surpass the standard quantum limit that
otherwise exists in conventional reciprocal devices. Our work confirms that
breaking reciprocal symmetry, already achieved in diverse systems well beyond
spinning systems, can serve as a new strategy to further enhance the abilities
of advanced quantum sensors, for applications ranging from testing fundamental
physical laws to practical quantum metrology.
