Lifetime-Limited Interrogation of Two Independent 27Al+ Clocks Using Correlation Spectroscopy

Laser decoherence limits the stability of optical clocks by broadening the observable resonance linewidths and adding noise during the dead time between clock probes. Correlation spectroscopy avoids these limitations by measuring correlated atomic transitions between two ensembles, which provides a frequency difference measurement independent of laser noise. Here, we apply this technique to perform stability measurements between two independent clocks based on the 1S0 ↔ 3P0 transition in 27Al+. By stabilizing the dominant sources of differential phase noise between the two clocks, we observe coherence between them during synchronous Ramsey interrogations as long as 8 s at a frequency of 1.12×1015 Hz. The observed contrast in the correlation spectroscopy signal is consistent with the 20.6 s 3P0 state lifetime and represents a measurement instability of (1.8±0.5)×10−16/√τ/s for averaging periods longer than the probe duration when dead time is negligible.