Photon number distribution of squeezed light from a silicon nitride microresonator measured without photon number resolving detectors

The measurement of the photon number distribution (PND) allows one to extract metrics of non-classicality of fundamental and technological relevance, but in principle it requires the use of detectors with photon number resolving (PNR) capabilities. In this work we reconstruct the PND of two-mode pulsed squeezed light generated from a silicon nitride microresonator using threshold detectors and variable optical attenuations. The PNDs are characterized up to ∼1.2 photons per pulse, through which we extracted an on-chip squeezing level of 6.2(2) dB and a noise reduction factor of −3.8(2) dB. The PNDs are successfully reconstructed up to a Hilbert space dimension of 6 × 6. The analysis performed on the photon number basis allows us to characterize the influence of a spurious thermal background field that spoils the photon number correlations. We evaluate the impact of self- and cross-phase modulation on the generation efficiency in case of a pulsed pump, and validate the results through numerical simulations of the master equation of the system.