Superconducting nanowire single-photon detector (SNSPD) with near-unity system efficiency is a key enabling, but still elusive technology for numerous quantum fundamental theory verifications and quantum information applications. The key challenge is to have both a near-unity photon-response probability and absorption efficiency simultaneously for the meandered nanowire with a finite filling ratio, which is more crucial for NbN than other superconducting materials (e.g., WSi) with lower transition temperatures. Here, we overcome the above challenge and produce NbN SNSPDs with a record system efficiency by replacing a single-layer nanowire with twin-layer nanowires on a dielectric mirror. The detector at 0.8 K shows a maximal system detection efficiency (SDE) of 98% at 1590 nm and a system efficiency of over 95% in the wavelength range of 1530-1630 nm. Moreover, the detector at 2.1K demonstrates a maximal SDE of 95% at 1550 nm using a compacted two-stage cryocooler. This type of detector also shows the robustness against various parameters, such as the geometrical size of the nanowire, and the spectral bandwidth, enabling a high yield of 73% (36%) with an SDE of >80% (90%) at 2.1K for 45 detectors fabricated in the same run. These SNSPDs made of twin-layer nanowires are of important practical significance for batch production.

Single-photon spectrometers utilizing optical spectral components and single-photon detectors have shown exceptional spectral resolving capability with single-photon sensitivity, enabling numerous spectroscopy applications in photon-scarce environments. Here, we report a compact computational spectrometer combining a superconducting single-photon detector array and 3D-printed photonic-crystal filters. The unknown light source is encoded by passing through the 3D-printed filters with various spectral transmissions, detected by a broadband superconducting single-photon detector array, and finally computationally reconstructed using the known spectral features of each array element. The fabricated spectrometer shows system sensitivity down to powers of −108.2 dBm at 1310 nm and achieves a resolution of 5 nm ranging from 1200 to 1700 nm.