LASER & PHOTONICS REVIEWS

Abstract Quantum optics plays a central role in the study of fundamental concepts in quantum mechanics, and in the devel-opment of new technological applications. Typical experiments employ sources of photon pairs generated by parametric pro-cesses such as spontaneous parametric down-conversion and spontaneous four-wave-mixing. The standard characterization of these sources relies on detecting the pairs themselves and thus requires single photon detectors, which limit both mea-surement speed and accuracy. Here it is shown that the two-photon quantum state that would be generated by parametric fluorescence can be characterised with unprecedented spectral resolution by performing a classical experiment. This stream-lined technique gives access to hitherto unexplored features of two-photon states and has the potential to speed up design and testing of massively parallel integrated nonlinear sources by providing a fast and reliable quality control procedure. Ad-ditionally, it allows for the engineering of quantum light states at a significantly higher level of spectral detail, powering future quantum optical applications based on time-energy photon cor-relations. High-resolution spectral characterization of two photon states via classical measurement