  Measuring the phase spectrum at the single-photon level is essential for the
full characterization of the temporal-spectral mode of quantum sources. We
present a phase retrieval algorithm-based method to recover the phase spectrum
difference between two independent pulses from their Hong-Ou-Mandel
interference pattern and intensity spectra. Our confirmatory experiment with
coherent state pulses confirms the accuracy of the recovered phase spectrum
difference to within plus or minus 0.1 rad. The method we employ is readily
generalizable to the measurement of single-photon wave packets and even
correlated photon pairs.
