Phase Locking an Atom Interferometer

In atom interferometry the phase evolution of a quantum superposition state is measured with respect to a reference signal. The measurement has a limited unambiguous interval, since not the phase but its projection is measured as a population unbalance on two energetic levels. Resolving phase wrapping brings to a longer interrogation interval and hence instrument sensitivity. We extended the unambiguous probe interval using coherence preserving measurements and phase corrections, and demonstrated the phase lock of the clock oscillator to an atomic superposition state. On this basis we implemented a protocol to improve atomic clocks limited by local oscillator noise, which is the case of optical atomic clocks, nowadays the best frequency standard in terms of both accuracy and precision. The technique could be adopted to enhance the sensitivity of atomic gravimeters.