Phase-resolved joint spectra tomography of a ring resonator photon pair source using a silicon photonic chip

The exponential growth of photonic quantum technologies is driving the demand for tools to measure the quality of their information carriers. One of the most prominent is stimulated emission tomography (SET), which uses classical coherent fields to measure the joint spectral amplitude (JSA) of photon pairs with high speed and resolution. While the modulus of the JSA can be directly addressed from a single intensity measurement, the retrieval of the joint spectral phase (JSP) is far more challenging and received minor attention. However, a wide class of spontaneous sources of technological relevance, as chip integrated micro-resonators, have a JSP with a rich structure that carries correlations hidden in the intensity domain. Here, using a compact and reconfigurable silicon photonic chip, the complex JSA of a micro-ring resonator photon pair source is measured for the first time. The photonic circuit coherently excites the ring and a reference waveguide, and the interferogram formed by their stimulated fields is used to map the ring JSP through a novel phase reconstruction technique. This tool complements the traditionally bulky and sophisticated methods implemented so far, simultaneously minimizing the set of required resources.