  The ability to manipulate and measure the time-frequency structure of quantum
light is useful for information processing and metrology. Measuring this
structure is also important when developing quantum light sources with high
modal purity that can interfere with other independent sources. Here, we
present and experimentally demonstrate a scheme based on intensity
interferometry to measure the joint spectral mode of photon pairs produced by
spontaneous parametric down-conversion. We observe correlations in the spectral
phase of the photons due to chirp in the pump. We show that our scheme can be
combined with stimulated emission tomography to quickly measure their mode
using bright classical light. Our scheme does not require phase stability,
nonlinearities, or spectral shaping, and thus is an experimentally simple way
of measuring the modal structure of quantum light.
