Correlation spectroscopy as a tool for comparing optical clocks*

Highly accurate optical clocks are quantum sensors with fractional frequency uncertainty below one part in 10^18 [1]. Comparisons between such clocks can lead to a better understanding of fundamental physics and potentially replace cesium microwave clocks to redefine the SI second. However, noise from the local oscillator often limits the measurement stability of these comparisons, requiring long averaging times to reduce statistical uncertainty. One way to overcome this limitation is by performing correlation spectroscopy in which a Ramsey pulse sequence derived from the same probe laser is applied to the clocks synchronously. The coherent differential frequency measurements between atomic systems permit interrogation times beyond the laser coherence time, which leads to a reduction in the quantum projection noise limit. As a result, the frequency comparison instability is significantly lower than is possible for incoherent comparisons using the same local oscillator. We demonstrate this technique using two Al+ quantum-logic clocks separated in space by a few meters.

[1] Brewer, et al. Phys. Rev. Lett. 123 (2019): 033201.