  Quantum non-demolition (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 unrivalled frequency precision---preserving the quantum coherence of
the atoms after readout with 80\% fidelity. We apply this technique to
stabilise 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 seven-fold increase in the clock's
\emph{Q} factor to $1.7\times10^{15}$.
