  We propose a scheme for continuously measuring the evolving quantum phase of
a collective spin composed of $N$ pseudospins. Quantum non-demolition
measurements of a lossy cavity mode interacting with an atomic ensemble are
used to directly probe the phase of the collective atomic spin without
converting it into a population difference. Unlike traditional Ramsey
measurement sequences, our scheme allows for real-time tracking of time-varying
signals. As a bonus, spin-squeezed states develop naturally, providing
real-time phase estimation significantly more precise than the standard quantum
limit of $\Delta \phi_\text{SQL} = 1/\sqrt{N}$ radians.
