Atom-based coherent quantum-noise cancellation in optomechanics

We analyze a quantum force sensor that uses coherent quantum-noise cancellation (CQNC) to beat the standard quantum limit. This sensor, which allows for the continuous, broadband detection of feeble forces, is a hybrid dual-cavity system composed of a mesoscopic mechanical resonator optically coupled to an ensemble of ultracold atoms. In contrast to the stringent constraints on dissipation typically associated with purely optical schemes of CQNC, the dissipation rate of the mechanical resonator only needs to be matched to the decoherence rate of the atomic ensemble - a condition that is experimentally achievable even for the technologically relevant regime of low-frequency mechanical resonators with large quality factors. The modular nature of the system further allows the atomic ensemble to aid in the cooling of the mechanical resonator, thereby combining atom-mediated state preparation with sensing deep in the quantum regime.