A cavity-QED protocol for precise field sensing in the optical domain*

In the context of quantum metrology, optical cavity-QED platforms have primarily been focused on the generation of entangled atomic spin states useful for next-generation frequency and time standards. We report a complementary application: The use of optical cavities to generate non-classical atom-light cat-states for quantum-enhanced sensing of small field displacements. We show that even in the presence of intrinsic photon loss from the cavity, the collective enhancement of atom-light interactions allows for potential metrological gains of 10-20 dB below the standard quantum limit in state-of-the-art cavity-QED systems operating with long-lived alkaline-earth atoms. Our protocol opens a path for sub-SQL sensing of electromagnetic fields in the optical domain, and could have applications in circumventing shot-noise limitations in optical interferometry, as well as in other frequency regimes since it is applicable to a broad range of platforms featuring similar types of spin-boson couplings, such as trapped ion arrays and opto-mechanical systems.