Picosecond-Resolution Photon Number Resolution with Superconducting Nanowire Detectors and Precision Timing Electronics

Photon number resolution (PNR) enables the precise determination of the number of photons in a pulse detection event, a key capability for advancing quantum communication, computing, and precision photonics. Accurate PNR requires ultralow-jitter detection and precise time-correlated photon counting. We demonstrate coincidence and time-of-arrival measurements with superconducting nanowire single-photon detectors (SNSPDs) from Single Quantum (15 ps FWHM jitter) and the Swabian Instruments Time Tagger X operated in High-Resolution mode (3.5 ps FWHM jitter). The detectors' integrated gating and fast recovery allow clean timing discrimination and minimal afterpulsing, while the Time Tagger's multichannel streaming architecture supports real-time histogramming and sub-nanosecond coincidence windows. The combined system achieves a ~15.4 ps FWHM single-pixel response function, enabling clear multiphoton separation and high-fidelity photon-statistics reconstruction. These results establish the lowest experimentally demonstrated jitter threshold in a commercial setup for successful PNR detection with single-pixel SNSPDs, highlighting the suitability of integrated time-tagging architectures for scalable quantum photonics and single-photon metrology.