A team at Paderborn University has built a photon detector that counts pulses of more than 9,000 photons, the smallest units of light, and measured pulses of up to 276 photons with an uncertainty below one photon, according to a paper in Optica news release.Counting photons gets harder as their numbers grow. Many single-photon detectors, including semiconductor avalanche photodiodes, show only whether light arrived, not how many photons. Detectors built for brighter light add electrical noise that blurs the count, the paper said.The team paired superconducting nanowire single-photon detectors (SNSPDs), which can resolve a few photons on their own, with multiplexing, which splits a signal across many channels. The result is 1,024 detection bins. Timon Schapeler of Paderborn University said no earlier demonstration has used both approaches together at this scale, which he described as an order of magnitude larger than previous multiplexed single-photon counting systems.Splitting light into 1,024 binsThe system splits each laser pulse into 128 smaller pulses, spaced in time, then sends them to eight detectors, according to the release. That produces 1,024 small pulses that share the original photons. Tim Bartley of Paderborn University said low-loss optics spread the light evenly across the bins. A model the group built earlier with Mariia Sidorova of PTB, Germany’s national metrology institute, describes each detector element.On their own, SNSPDs resolve only a handful of photons, typically up to about five, according to the team’s preprint, a version posted on the arXiv server before publication. A Leiden University physicist recently reported a similar superconducting detector that separates one, two and three or more photons.To test the system, the team sent laser pulses averaging between one and 15,000 photons into it. The detector measured pulses of up to 276 photons with an uncertainty below one photon, the researchers said. They calculated that a conventional multiplexed system would need about 37,000 detector elements to match that precision.Limits and next stepsBeyond 276 photons, the uncertainty exceeds one photon. The preprint says it stayed below the Poisson noise limit, the built-in noise of laser light, where uncertainty equals the square root of the photon count, by at least 4.1 decibels, a logarithmic measure of the gap. That means the 9,000 figure refers to counts that beat this limit, not counts accurate to one photon. The authors also say accuracy is limited by detector imperfections, particularly temporary blinding of some bins, to which they attribute deviations above about 1,000 photons. Efficiency varies strongly with light level, and the test used only laser pulses.The group also reconstructed the full detector’s response with quantum detector tomography, a calibration method, building on earlier high-performance computing work. Other teams are scaling SNSPD systems too, including a 100-channel detector built in Shanghai.The researchers plan to apply the detectors to bright quantum states of light, which the release said would make it possible to uncover quantum effects at large scales. The paper appears in Optica, volume 13.