Dynamical decoupling of laser phase noise in compound atomic clocks

The frequency stability of many optical atomic clocks is limited by the coherence of their local oscillator. Here, we present a measurement protocol that overcomes the laser coherence limit. It relies on engineered dynamical decoupling of laser phase noise and near-synchronous interrogation of two clocks. One clock coarsely tracks the laser phase using dynamical decoupling; the other refines this estimate using a high-resolution phase measurement. While the former needs to have a high signal-to-noise ratio, the latter clock may operate with any number of particles. The protocol effectively enables minute-long Ramsey interrogation for coherence times of few seconds as provided by the current best ultrastable laser systems. We demonstrate implementation of the protocol in a realistic proof-of-principle experiment, where we interrogate for 0.5 s at a laser coherence time of 77 ms. Here, a single lattice clock is used to emulate synchronous interrogation of two separate clocks in the presence of artificial laser frequency noise. We discuss the frequency instability of a single-ion clock that would result from using the protocol for stabilisation, under these conditions and for minute-long interrogation, and find expected instabilities of sigma(y)(tau) = 8 x 10(-16)(tau/s)(-1/2)and sigma(y)(tau) = 5 x 10(-17)(tau/s)(-1/2), respectively. Optical clocks have many applications, from improved GNSS measurements to fundamental tests of general relativity, but their frequency stability is limited by quantum noise and the Dick effect. The authors present and demonstrate a method to estimate the phase of an optical clock laser beyond the laser coherence time that can be used to improve the stability of these devices for applications in metrology and the search for new physics.