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Quantum non-demolition (QND) measurement is a remarkable tool for themanipulation of quantum systems. It allows specific information to be extractedwhile still preserving fragile quantum observables of the system. Here we applycavity-based QND measurement to an optical lattice clock---a type of atomicclock with unrivalled frequency precision---preserving the quantum coherence ofthe atoms after readout with 80\% fidelity. We apply this technique tostabilise the phase of an ultrastable laser to a coherent atomic state via aseries of repeated QND measurements. We exploit the improved phase-coherence ofthe ultrastable laser to interrogate a separate optical lattice clock, using aRamsey spectroscopy time extended from 300~ms to 2~s. With this technique wemaintain 95\% contrast and observe a seven-fold increase in the clock's\emph{Q} factor to $1.7\times10^{15}$.

Improving the Q factor of an optical atomic clock using quantum non-demolition measurement