  We propose a generalisation of dispersive qubit readout which provides the
time evolution of a flux qubit observable. Our proposal relies on the
non-linear coupling of the qubit to a harmonic oscillator with high frequency,
representing a dc-SQUID. Information about the qubit dynamics is obtained by
recording the oscillator response to resonant driving and subsequent lock-in
detection. The measurement process is simulated for the example of coherent
qubit oscillations. This corroborates the underlying measurement relation and
also reveals that the measurement scheme possesses low backaction and high
fidelity.
