  The evolution of a quantum state undergoing radiative decay depends on how
the emission is detected. We employ phase-sensitive amplification to perform
homodyne detection of the spontaneous emission from a superconducting
artificial atom. Using quantum state tomography, we characterize the
correlation between the detected homodyne signal and the emitter's state, and
map out the conditional back-action of homodyne measurement. By tracking the
diffusive quantum trajectories of the state as it decays, we characterize
selective stochastic excitation induced by the choice of measurement basis. Our
results demonstrate dramatic differences from the quantum jump evolution that
is associated with photodetection and highlight how continuous field detection
can be harnessed to control quantum evolution.
