Broadband polarization insensitivity and high detection efficiency in high-fill-factor superconducting microwire single-photon detectors

Single-photon detection via absorption in current-biased nanoscale superconducting structures has become a preferred technology in quantum optics and related fields. Single-mode fiber packaged devices have seen new records set in detection efficiency, timing jitter, recovery times, and largest sustainable count rates. The popular approaches to decreasing polarization sensitivity have thus far been limited to introduction of geometrically symmetric nanowire meanders, such as spirals and fractals, in the active area. The constraints on bending radii, and by extension, fill factors, in such designs limits their maximum efficiency. The discovery of single-photon sensitivity in micrometer-scale superconducting wires enables novel meander patterns with no effective upper limit on fill factor. This work demonstrates simultaneous low-polarization sensitivity ($1.02\pm 0.008$) and high detection efficiency ($> 91.8\%$ with $67\%$ confidence at $2\times10^5$ counts per second) across a $40$ nm bandwidth centered at 1550 nm in 0.51 $\mu\text{m}$ wide microwire devices made of silicon-rich tungsten silicide, with a $0.91$ fill factor in the active area. These devices boasted efficiencies of $96.5-96.9\% \pm 0.5\%$ at $1\times10^5$ counts per second for 1550 nm light.