The rapid development of superconducting nanowire single-photon detectors (SNSPDs) over the past decade has led to numerous advances in quantum information technology. The record for the best system detection efficiency (SDE) at an incident photon wavelength of 1550 nm is 93%. This performance was attained from an SNSPD made of amorphous WSi; such SNSPDs are usually operated at sub-Kelvin temperatures. In this study, we fabricated an SNSPD using polycrystalline NbN. Its SDE is 90.2% at 2.1 K for incident photons with a 1550-nm wavelength, and this temperature is accessible with a compact cryocooler. The SDE saturated at 92.1% when the temperature was lowered to 1.8 K. We expect the results lighten the practical and high performance SNSPD to quantum information and other high-end applications. Main textA single-photon detector with high detection efficiency is the key enabling technology for quantum information and various applications, including the test of loophole-free Bell inequality violation 1 , quantum teleportation 2 , measurement-device-independent quantum key distribution 3 , and linear optical quantum computation 4 . Superconducting single-photon detectors outperform their semiconducting counterparts in terms of not only detection efficiency but also dark count rate, timing jitter, and counting rate 5 . In the case of the telecommunication wavelength (1550 nm), the highest system detection efficiency (SDE) greater than 90% has been reported for two types of detectors. One is a transition edge sensor (TES) made of tungsten (W), with an SDE of 95% 6 ; the other is a superconducting nanowire single-photon detector (SNSPD) made of amorphous WSi, with an SDE of 93% 7 . However, because of the low superconducting transition temperature of W and WSi, the requirement of sub-Kelvin cryogenics represents a burden for practical applications. Many studies focused on SNSPDs fabricated using different materials and aiming to obtain a high SDE at higher operating temperatures have been reported [8][9][10][11] ; however, none of these attempts has been successful. Regarding another important parameter, timing jitter, a WSi SNSPD and a W TES have values of approximately 150 ps and 50-100 ns, respectively, which limits their use in

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