  We propose and analyze a method to detect and characterize the drift of a
nonstationary quantum source. It generalizes a standard measurement for
detecting phase diffusion of laser fields to quantum systems of arbitrary
Hilbert space dimension, qubits in particular. We distinguish diffusive and
systematic drifts, and examine how quickly one can determine that a source is
drifting. We show that for single-photon wavepackets our measurement is
implemented by the Hong-Ou-Mandel effect.
