Superconducting nanowire single-photon detectors (SNSPDs) became the golden standard in single-photon counting, enabling key photonic quantum applications. Recent advances further expanded their capabilities, demonstrating that, beyond their exceptional performance in single-photon detection, SNSPDs also posses intrinsic photon-number resolution (PNR) up to a few photon numbers [1]. Mastering this would allow high-fidelity quantum light detection without requiring expensive detector arrays or complex multiplexing schemes.

Intrinsic PNR in SNSPDs is governed by the dynamics of resistive domains, which manifest in the timing of voltage response pulse following photon absorption. When multiple photons are absorbed, they may generate non-overlapping resitive domains, with their combined dynamics encoding photon number information. High-frequency readout electronics can decode this information from the voltage pulse. However, domain overlap, as well as timing jitter caused by fluctuations [2], distort photon number discrimination. Here, we present an intrinsic PNR model validated against experimental results from a commercial SNSPD. We discuss the fundamental constraints on the resolvable optical pulse duration and photon number imposed by overlapping effects and thermal fluctuations[2]. Our findings provide a framework for optimizing SNSPDs for photon-number-resolving applications.

[1] JW Los et al., APL Photonics 9, 066101 (2024)
[2] AD Semenov et al., Phys. Rev. B 102 (2024)