Superconducting nanowire single-photon detector (SNSPD)

An SNSPD is a nanowire of superconducting film, tens of nanometers wide, meandered over an optical spot and biased just below its critical current. One absorbed photon breaks enough Cooper pairs to nucleate a resistive hotspot, the bias current diverts into the readout, and a voltage pulse marks the arrival time; the wire then cools and resets in nanoseconds. The device is a threshold detector: no gain noise, no avalanche, just a click with superb timing.

Across every metric that matters for photon counting, SNSPDs hold the records simultaneously rather than one at a time: system detection efficiency above 90%, with 98% demonstrated at 1550 nm in cavity-integrated devices, timing jitter below 20 ps routinely and a few picoseconds in short-wire demonstrations, dark counts below 1 Hz with proper stray-light control, and count rates into the hundreds of megahertz. The price is operating temperature, 1–4 K from a closed-cycle cryocooler, which turned out to be an engineering cost rather than a research barrier: multi-channel commercial systems are standard lab equipment, and NASA's deep-space optical link used SNSPD arrays as its ground receiver.

They matter here for two reasons. They are the enabling detector for quantum photonic circuits, photon-starved lidar, and any coincidence measurement where jitter sets resolution. And their characterization is a clean exercise in this site's home territory: efficiency calibration against a power meter through calibrated attenuators, where the attenuator chain is the real uncertainty budget; dark-count separation from stray light by blanking the input; jitter measured against a mode-locked reference; and latching behavior at high rates, which sets the usable dynamic range.

References: C. M. Natarajan, M. G. Tanner, R. H. Hadfield, Supercond. Sci. Technol. 25, 063001 (2012); D. V. Reddy et al., Optica 7, 1649 (2020).