Dual-Trigger of Series Nanowire Detector for Event-Based Photon Number Assignment

Photon-number-resolving (PNR) detectors are essential components of photonic quantum technologies. However, conventional single-channel edge-triggered readout struggles to resolve the photon number $n$ in real time or to characterize how timing jitter depends on $n$. In this work, we use a dual-trigger method on a three-pixel series-connected superconducting nanowire single-photon detector (SNSPD) that triggers on both the rising and falling edges of the detection pulse. By doing so, we preserve the precise arrival time of the detection event while mapping the photon number onto the time interval between the rising and falling edges, allowing clear separation of the events. Using this technique, we assign each detection event to $n = 1, 2,$ or 3 photons with $99\,\%$ posterior confidence across all three classes. The timing jitter decreases as $n$ increases, reaching values below $41\, \text{ps}$ for $n \geq 2$. Comparing edge-triggering with constant-fraction discrimination (CFD) for arrival-time extraction, we find that CFD yields lower jitter for single-photon events and a nearly constant mean arrival time. Altogether, our results establish dual-triggering as a robust, low-latency readout scheme for PNR detectors, while revealing a photon-number dependence of the timing jitter relevant to timing precision achievable in heralded photonic quantum applications.