  We present a method of tomography that measures the joint spectral phase
(JSP) of spontaneously emitted photon pairs originating from a largely
uncharacterized ``target" source. We use quantum interference between our
target source and a reference source to extract the JSP with four spectrally
resolved measurements, in a process that we call quantum-referenced spontaneous
emission tomography (Q-SpET). We have demonstrated this method on a photonic
integrated circuit for a target micro-ring resonator photon-pair source. Our
results show that spontaneously emitted photon pairs from a micro-ring
resonator are distinctively different from that of stimulated emission, and
thus cannot in general be fully characterized using classical stimulated
emission tomography without detailed knowledge of the source.
