  We demonstrate dispersive readout of the spin of an ensemble of
Nitrogen-Vacancy centers in a high-quality dielectric microwave resonator at
room temperature. The spin state is inferred from the reflection phase of a
microwave signal probing the resonator. Time-dependent tracking of the spin
state is demonstrated, and is employed to measure the T1 relaxation time of the
spin ensemble. Dispersive readout provides a microwave interface to solid state
spins, translating a spin signal into a microwave phase shift. We estimate that
its sensitivity can outperform optical readout schemes, owing to the high
accuracy achievable in a measurement of phase. The scheme is moreover
applicable to optically inactive spin defects and it is non-destructive, which
renders it insensitive to several systematic errors of optical readout and
enables the use of quantum feedback.
