Superconducting Nanowire Single-Photon Detector
A wire so thin and cold that one photon can break it open — and then heal in nanoseconds.
Counts individual photons arriving from a fiber, with timing precision of picoseconds and almost no false clicks. State-of-the-art SNSPDs reach >98% detection efficiency at telecom wavelengths — by far the most efficient single-photon detector ever made.
A superconducting wire — typically NbN or WSi, 5–10 nm thick — is biased near its critical current. When a single photon strikes, it deposits enough energy locally to break Cooper pairs and form a 'hotspot' that briefly drives that segment of wire into the normal (resistive) state. The bias current is forced through a parallel external resistor, producing a sharp voltage pulse. Within nanoseconds the heat dissipates, the wire becomes superconducting again, and the detector is ready for the next photon.
In plain terms
Picture a tightrope made of ice, frozen so cold the rope feels nothing. When a single grain of warm sand touches it, the ice melts in one tiny spot, the rope bends, and a sensor catches the bend. Then the spot refreezes, ready for the next grain.
Also called: SNSPD, single-photon detector.
Why it matters
- Detection efficiency >98% at 1550 nm — competitive with no other single-photon technology.
- Timing jitter as low as 3 picoseconds — fast enough to resolve photon arrival across a meter of free space.
- Dark count rate below 1 Hz — almost never fires without a real photon.
- Backbone of every modern quantum-communications experiment, including Micius and the new fiber QKD networks.
Where it lives
- Micius satellite ground stations
- NIST quantum-communications testbeds
- JPL Deep Space Optical Communications (DSOC) demo
- Most active fiber QKD systems
Key numbers
- Detection efficiency: ≥98 % (At 1550 nm telecom wavelength)
- Wire thickness: 5–10 nm (About 20 atoms)
- Timing jitter: 3–20 ps
- Dark counts: <1 Hz
- Operating temp: 0.8–4 K
Surprising facts
- A photon arriving from Mars is about as bright as a candle ten thousand kilometers away. SNSPDs at JPL caught photons from the Psyche spacecraft at exactly that level during the 2024 DSOC demo.
- The wire is so thin — about 6 nanometers — that it's only twenty atoms thick.
- From the moment a photon hits to the moment the detector recovers is about 50 nanoseconds. Some setups sustain over 100 million photon counts per second.
- The detector has to be cooled to about 1 Kelvin or below — meaning each one needs its own little dilution-fridge stage.
Open questions
- Vortex-triggered dark counts: SNSPD dark counts — clicks without any real photon — can be produced by quantum vortices spontaneously crossing the nanowire driven by thermal fluctuations. The same vortex dynamics responsible for photon detection also generate false positives at rates that depend on temperature, bias current, and edge defects in a manner that early hotspot models failed to predict quantitatively. A unified vortex-crossing model that correctly predicts both dark count rate and detection efficiency across material systems was only validated in 2025.
- Detection efficiency plateau anomaly: At bias currents just below the switching current, SNSPD detection efficiency saturates to a plateau — a regime where the detector responds to nearly every photon. Adding more bias current beyond this plateau does not improve efficiency but dramatically increases dark counts. The precise width and flatness of this plateau varies between devices made from nominally identical material runs in ways that materials models do not fully explain, suggesting unknown nanoscale disorder plays a role.
A short history
First demonstrated by Gol'tsman, Sobolewski and colleagues in 2001 using NbN nanowires patterned on sapphire substrates. Efficiency climbed steadily through the 2010s, crossing 90% with WSi designs at NIST and reaching >98% in tungsten-silicide stacks at JPL by 2020.
Sources
- Gol'tsman et al., Appl. Phys. Lett. (2001) — first SNSPD (https://doi.org/10.1063/1.1388868)
- Reddy et al., Optica (2020) — 98% detection efficiency (https://opg.optica.org/optica/abstract.cfm?uri=optica-7-12-1649)
- JPL DSOC milestone announcement (2024) (https://www.jpl.nasa.gov/news/nasas-tech-demo-streams-first-video-from-deep-space-via-laser)