  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.
