Superconducting Nanowire Single-Photon Detector (SNSPD) Lab

An SNSPD consists of an ultrathin superconducting nanowire (such as NbN or WSi with a 5 nm thickness and 70 nm width) cooled to 0.8 to 2.1 Kelvin and biased with a DC current just below its critical depairing threshold. When a single photon hits the wire, its energy is absorbed by electrons, breaking Cooper pairs and creating a localized cloud of excited quasiparticles known as a hotspot. The diverted current density in the remaining superconducting sidewalls exceeds the critical current density, driving a normal-state resistive barrier across the wire width and diverting the current into an external 50-ohm RF transmission line to produce a sharp sub-nanosecond voltage pulse.

Conventional semiconductor InGaAs avalanche photodiodes (APDs) suffer from bandgap trap states that cause high dark counts (exceeding 1,000 to 10,000 counts per second) and significant afterpulsing that requires deadtime gating. Furthermore, their detection efficiency at telecom wavelengths (1550 nm) rarely exceeds 20 to 30 percent. In contrast, SNSPDs integrated with optical micro-cavities achieve system detection efficiencies exceeding 98 percent at 1550 nm, while cryogenic operation suppresses thermal dark count rates below 1 count per second with zero afterpulsing.

Timing jitter in an SNSPD is governed by intrinsic hotspot formation fluctuations, electronic amplifier noise, and geometric propagation delays along the meander. Because the phase transition occurs on sub-picosecond timescales when Cooper pairs break and current diverts, intrinsic jitter is exceptionally small. By optimizing nanowire uniformity with high-resolution electron-beam lithography and reading out differential RF pulses with cryogenic low-noise amplifiers, total system timing jitter is kept below 10 to 15 picoseconds FWHM, outperforming APDs by more than an order of magnitude.

Superconducting thin films exhibit a large kinetic inductance arising from the inertia of Cooper pairs carrying current without scattering. For an SNSPD meander containing thousands of squares, total kinetic inductance ranges from 150 to 600 nanohenries. Once the hotspot cools and superconductivity is restored, the bias current refills the nanowire exponentially with a relaxation time constant tau = L_k / R_load (where R_load is typically 50 ohms). This yields pulse fall times between 5 and 15 nanoseconds, setting the maximum count rate to roughly 50 to 100 MHz.