Improving the Q Factor of an Optical Atomic Clock Using Quantum Nondemolition Measurement

Quantum nondemolition (QND) measurement is a remarkable tool for the manipulation of quantum systems. It allows specific information to be extracted while still preserving fragile quantum observables of the system. Here we apply cavity-based QND measurement to an optical lattice clock-a type of atomic clock with unrivaled frequency precision-preserving the quantum coherence of the atoms after readout with 80% fidelity. We apply this technique to stabilize the phase of an ultrastable laser to a coherent atomic state via a series of repeated QND measurements. We exploit the improved phase coherence of the ultrastable laser to interrogate a separate optical lattice clock, using a Ramsey spectroscopy time extended from 300 ms to 2 s. With this technique we maintain 95% contrast and observe a sevenfold increase in the clock's Q factor to 1.7 x 10(15).