Dynamic decoupling of laser phase noise in compound atomic clocks

The frequency stability achieved by an optical atomic clock ultimately depends on the coherence of its local oscillator. Even the best ultrastable lasers only allow interrogation times of a few seconds, at present. Here we present a universal measurement protocol that overcomes this limitation. Engineered dynamic decoupling of laser phase noise allows any optical atomic clock with high signal-to-noise ratio in a single interrogation to reconstruct the laser's phase well beyond its coherence limit. A compound clock is then formed in combination with another optical clock of any type, allowing the latter to achieve significantly higher frequency stability than on its own. We demonstrate implementation of the protocol in a realistic proof-of-principle experiment with a phase reconstruction fidelity of 99 %. The protocol enables minute-long interrogation for the best ultrastable laser systems. Likewise, it can improve clock performance where less stable local oscillators are used, such as in transortable systems.