  Quantum optics plays a central role in the study of fundamental concepts in
quantum mechanics, and in the development of new technological applications.
Typical experiments employ non-classical light, such as entangled photons,
generated by parametric processes. The standard characterization of the sources
by quantum tomography, which relies on detecting the pairs themselves and thus
requires single photon detectors, limits both measurement speed and accuracy.
Here we show that the spectral characterization of the quantum correlations
generated by two-photon sources can be directly performed classically with an
unprecedented spectral resolution. This streamlined technique has the potential
to speed up design and testing of massively parallel integrated sources by
providing a fast and reliable quality control procedure. Adapting our method to
explore other degrees of freedom would allow the complete characterization of
biphoton states generated by parametric processes.
