  The fast development of superconducting nanowire single photon detector
(SNSPD) in the past decade has enabled many advances in quantum information
technology. The best system detection efficiency (SDE) record at 1550 nm
wavelength was 93% obtained from SNSPD made of amorphous WSi which usually
operated at sub-kelvin temperatures. We first demonstrate SNSPD made of
polycrystalline NbN with SDE of 90.2% for 1550 nm wavelength at 2.1K,
accessible with a compact cryocooler. The SDE saturated to 92.1% when the
temperature was lowered to 1.8K. The results lighten the practical and high
performance SNSPD to quantum information and other high-end applications.
