Imaging spatio-temporal Hong-Ou-Mandel interference of bi-photon states of extremely high Schmidt number.

We report the experimental observation of a spatio-temporal Hong-Ou-Mandel (HOM) interference of bi-photon states of extremely high Schmidt number. Two-photon interference of 1500 spatial modes and a total of more than 3 ? 106 spatio-temporal modes is evidenced by measuring momentum spatial coincidences between the pixels of the far-field images of two strongly multimode spontaneous parametric down conversion (SPDC) beams propagating through a HOM interferometer. The outgoing SPDC beams are recorded onto two separate detectors arrays operating in the photon-counting regime. The properties of HOM interference are investigated both in the time and space domains. We show that the two-photon interferences exhibit temporal and two-dimensional spatial HOM dips with visibilities of 30% and widths in good agreement with the spatio-temporal coherence properties of the bi-photon state. Moreover, we demonstrate that a peak of momentum spatial coincidences is evidenced inside each image, in correspondence with this dip.