Determining the modal structure of quantum light is important for photonic information processing, sensing, and imaging. A full modal characterization requires determining both the amplitude and phase of light’s spatial and spectral distributions, as well as its polarization. We propose using Hanbury Brown and Twiss “intensity interferometry” with a classical reference field to perform this full characterization without the need for phase stability or nonlinearities. The technique can be applied to any quantum signal, such as single photons and entangled photon pairs.

Firstly, we present an experimental demonstration of the technique in the spectral domain [1]. We combine a weak coherent state (i.e. the classical reference beam) with heralded photons produced by spontaneous parametric down-conversion (SPDC) on a beam splitter. All three modes are detected with time-of-flight single photon spectrometers. The measured coincidences reveal a fringe pattern which can be used to determine the joint spectral intensity and phase of the photon pairs if the spectral mode of the reference is known. Since the reference is simply an attenuated pulsed laser, its mode can be independently determined using conventional self-referencing techniques such as SPIDER and FROG.

Secondly, we present progress on a similar experiment in the spatial domain. Spatially-resolved single photon detection is achieved using a TimePix “3D” camera which time-tags detection events with nanosecond resolution, thus allowing us to image coincidences. As before, fringes in the coincidence image are used to determine the intensity and phase of the signal’s spatial mode. Since this technique does not require phase stability between the reference and signal beams, it can be used for remote wavefront sensing of faint light sources.

This is a joint work with Duncan England and Ben Sussman of the National Research Council of Canada, as well as Bryn Bell, Raj Patel, Ian Walmsley, and Myungshik Kim of Imperial College London.

[1] G.S. Thekkadath et al. “Measuring the joint spectral mode of photon pairs using intensity interferometry”, Physical Review Letters 128 023601 (2022)