Quantum optics of frequency comb metrology

Frequency combs enable precision measurements across timekeeping, spectroscopy, ranging and astronomy, and are now extending to integrated and field-deployable platforms. Realizing their full performance demands a comprehensive account of the quantum noise that arises when broadband optical fields are converted into finite-bandwidth electrical signals. Here we present a rigorous first-principles quantum-mechanical framework for optical frequency-comb metrology based on continuous-mode field quantization and a comb-line-resolved description of quantum fluctuations. The theory describes how quantum fluctuations of the comb field are transduced into electrical measurement noise. We apply the framework to two canonical settings, optical frequency division (OFD) and dual-comb spectroscopy (DCS), where it reveals two effects beyond semiclassical reach: a dependence of the OFD standard quantum limit on the comb spectral envelope, and a cyclostationary noise penalty that obstructs straightforward squeezing in DCS. These insights identify practical, resource-efficient routes to quantum enhancement through engineered comb states, laying a foundation for the design of next-generation frequency combs operating at and beyond standard quantum limits.