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Advanced Photon Counting Techniques

Optimizing SNSPD pulse processing and time tagging for reliable photon-number resolution

Jeremy S. Doredla, Matteo Moioli, Timothy M. Rambo

Published Jun 12, 2026• 0 citations
Publisher

Abstract

The ability to accurately determine the exact number of photons in an event, namely photon number resolution (PNR), is crucial for quantum networking, communication, and sensing. While pseudo-PNR approaches rely on multiplexing, true PNR depends on the ability of the detector and electronics to efficiently discriminate electrical signals under different count rates, optical, and electrical conditions. This work explores a fidelity and accuracy assessment and discrimination methods from peak separation PNR methods. The signals generated by Quantum Opus SNSPDs were acquired and analyzed with picosecond resolution via Swabian Instruments Time Taggers. Experimental data reveal the tradeoffs between photon-number distinguishability acquisition and processing methods. Both mathematical analysis methods (a Poisson-weighted or independent exponentially modified Gaussian) agree on the optimum threshold voltage, but vary in practical deployment, robustness against parameter degeneracy, and further capabilities. Both timing acquisition methods allow for PNR, but self-referenced rise-time measurement sacrifices some distinguishability for increased optical flexibility.

DOI
10.1117/12.3094309

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The ability to accurately determine the exact number of photons in an event, namely photon number resolution (PNR), is crucial for quantum networking, communication, and sensing. While pseudo-PNR approaches rely on multiplexing, true PNR depends on the ability of the detector and electronics to efficiently discriminate electrical signals under different count rates, optical, and electrical conditions. This work explores a fidelity and accuracy assessment and discrimination methods from peak separation PNR methods. The signals generated by Quantum Opus SNSPDs were acquired and analyzed with picosecond resolution via Swabian Instruments Time Taggers. Experimental data reveal the tradeoffs between photon-number distinguishability acquisition and processing methods. Both mathematical analysis methods (a Poisson-weighted or independent exponentially modified Gaussian) agree on the optimum threshold voltage, but vary in practical deployment, robustness against parameter degeneracy, and further capabilities. Both timing acquisition methods allow for PNR, but self-referenced rise-time measurement sacrifices some distinguishability for increased optical flexibility.
Superconducting nanowire single-photon detectors (SNSPDs) can enable photon-number resolution (PNR) based on accurate measurements of the detector’s response time to few-photon optical pulses. In this work, we investigate the impact of the optical pulse shape and duration on the accuracy of this method. We find that Gaussian temporal pulse shapes yield cleaner arrival-time histograms and, thus, more accurate PNR, compared to bandpass-filtered pulses of equal bandwidth. For low system jitter and an optical pulse duration comparable to the other jitter contributions, photon numbers can be discriminated in our system with a commercial SNSPD. At 60 ps optical pulse duration, photon-number discrimination is significantly reduced. Furthermore, we highlight the importance of using the correct arrival-time histogram model when analyzing photon-number assignment. Using exponentially modified Gaussian distributions, instead of the commonly used Gaussian distributions, we can more accurately determine photon-number misidentification probabilities. Finally, we reconstruct the positive operator-valued measures of the detector, revealing sharp features that indicate the intrinsic PNR capabilities.
Photon-number resolving (PNR) single-photon detectors are of interest for a wide range of applications in the emerging field of photon based quantum technologies. Especially photonic integrated circuits will pave the way for a high complexity and ease of use of quantum photonics. Superconducting nanowire single-photon detectors (SNSPDs) are of special interest since they combine a high detection efficiency and a high timing accuracy with a high count rate and they can be configured as PNR-SNSPDs. Here, we present a PNR-SNSPD with a four photon resolution suitable for waveguide integration. A high statistical accuracy for the photon number is achieved for a Poissonian light source at photon flux below \num{5} photons/pulse with a detection efficiency of 22.7$\pm$3.0 % at 900 nm and pulse rate frequency of 76 MHz. We demonstrate the ability of such a detector to discriminate a sub-Poissonian from a Poissonian light source. This demonstration shows the practical benefit of using a PNR detector in a quantum photonic circuit.
Photon-number-resolving (PNR) detectors are essential components of photonic quantum technologies, yet thus far, no practical metric exists to certify how many photons they can genuinely resolve in a single measurement. Here we introduce an operational framework for quantifying the capability of a PNR detector to distinguish between different numbers of photons, i.e. its genuine resolution. In turn, we develop a practical and scalable protocol for certifying the genuine resolution of a detector, which is based on coherent state probes. We apply the method to a 28-pixel photon-number-resolving superconducting nanowire single-photon detector (PNR-SNSPD) and certify genuine four-outcome resolution. Our work highlights the critical requirements in terms of detector efficiency towards achieving high genuine resolution. This approach provides an operational benchmark for PNR detectors and fills a crucial gap in the characterization of photonic quantum devices.
Since their first demonstration in 2001 [Gol’tsman et al., Appl. Phys. Lett. 79, 705–707 (2001)], superconducting-nanowire single-photon detectors (SNSPDs) have witnessed two decades of great developments. SNSPDs are the detector of choice in most modern quantum optics experiments and are slowly finding their way into other photon-starved fields of optics. Until now, however, in nearly all experiments, SNSPDs were used as “binary” detectors, meaning that they could only distinguish between 0 and >=1 photons, and photon number information was lost. Recent research has demonstrated proof-of-principle photon-number resolution (PNR) SNSPDs counting 2–5 photons. The photon-number-resolving capability is highly demanded in various quantum-optics experiments, including Hong–Ou–Mandel interference, photonic quantum computing, quantum communication, and non-Gaussian quantum state preparation. In particular, PNR detectors at the wavelength range of 850–950 nm are of great interest due to the availability of high-quality semiconductor quantum dots (QDs) [Heindel et al., Adv. Opt. Photonics 15, 613–738 (2023)] and high-performance cesium-based quantum memories [Ma et al., J. Opt. 19, 043001 (2017)]. In this paper, we demonstrate NbTiN-based SNSPDs with >94% system detection efficiency, sub-11 ps timing jitter for one photon, and sub-7 ps for 2 photons. More importantly, our detectors resolve up to 7 photons using conventional cryogenic electric readout circuitry. Through theoretical analysis, we show that the PNR performance of demonstrated detectors can be further improved by enhancing the signal-to-noise ratio and bandwidth of our readout circuitry. Our results are promising for the future of optical quantum computing and quantum communication.
We apply principal component analysis (PCA) to a set of electrical output signals from a commercially available superconducting nanowire single-photon detector (SNSPD) to investigate their photon-number-resolving capability. We find that the rising edge as well as the amplitude of the electrical signal have the most dependence on photon number. Accurately measuring the rising edge while simultaneously measuring the voltage of the pulse amplitude maximizes the photon-number resolution of SNSPDs. Using an optimal basis of principal components, we show unambiguous discrimination between one- and two-photon events, as well as partial resolution up to five photons. This expands the use case of SNSPDs to photon-counting experiments, without the need of detector multiplexing architectures. Published by the American Physical Society 2024