Superconducting Nanowire Single Photon Detectors

Short answer: A superconducting nanowire single-photon detector (SNSPD) detects individual photons when a photon breaks Cooper pairs in a thin superconducting wire. Reviews describe it as combining high detection efficiency, low dark counts, short dead times and picosecond timing, which is why photonic quantum technologies and quantum key distribution use it. The trade-off is that it must be cooled, so refrigeration and optical coupling are practical considerations. The claims below are the reviewers' own descriptions.

What an SNSPD is

A 2020 review in Nanophotonics describes the SNSPD as a quantum-limit superconducting optical detector based on the Cooper-pair breaking effect by a single photon. It says SNSPDs show higher detection efficiency, lower dark count rate, higher counting rate and lower timing jitter than their counterparts, and have been extensively applied in quantum information processing, including quantum key distribution and optical quantum computation. The review covers the requirements of single-photon detectors for quantum information, the operating principle, key metrics and performance issues.

How it compares with other detectors

The 2020 review also describes other superconducting detectors. A transition-edge sensor, it says, shows high detection efficiency, low speed, high timing jitter and photon-number resolvability, and usually needs a sub-Kelvin operating temperature. It also describes superconducting tunnel junctions and microwave kinetic inductance detectors, which respond to broken Cooper pairs.

A 2021 perspective in Applied Physics Letters notes that single-photon avalanche diodes and photomultiplier tubes were used first, but combining high detection efficiency with high time resolution and low noise remains a challenge for them, and silicon devices have a limited spectral response and suffer from afterpulsing. The perspective says SNSPDs offer an unrivalled combination of metrics, and that state-of-the-art devices combine near-unity detection efficiency over a wide spectral range, low dark counts, short dead times and picosecond time resolution. It says SNSPDs have become indispensable tools for quantum photonics two decades after their demonstration and have triggered industrial efforts. It also outlines future challenges, including photon-counting, fast time-tagging imaging and multi-pixel technology compatible with quantum photonic integrated circuits.

Efficiency in practice

A 2012 review in Superconductor Science and Technology explains that an ideal detector produces a signal only on absorbing a photon, while real detectors have non-ideal characteristics. Detection efficiency, the probability that an output signal is registered when a photon arrives, is lower than 100% and depends strongly on wavelength. Photons can also be lost before reaching the detector through absorption, scattering or reflection, which the review describes as coupling efficiency. That review evaluates device limitations and noise mechanisms, surveys refrigeration technologies and optical coupling schemes, and summarizes applications from quantum cryptography to remote sensing.

What this does not tell you

These reviews describe the best reported device performance and do not say what a given commercial detector achieves. Efficiency figures depend on wavelength, coupling and operating conditions, and detectors need cryogenic operation. Detector hardware is also a target in quantum hacking research on QKD systems, a separate topic.

FAQ

How does an SNSPD detect a photon?

A photon breaks Cooper pairs in a superconducting nanowire, producing a measurable signal.

Why use it for quantum information?

Reviews describe high efficiency, low dark counts, fast counting and low timing jitter.

Does it need cooling?

Yes. The 2012 review surveys practical refrigeration technologies for these detectors.

How does it compare with a SPAD?

The 2021 perspective says SPADs struggle to combine high efficiency, time resolution and low noise, and silicon is limited spectrally.

What limits real efficiency?

Wavelength dependence and coupling losses, per the 2012 review.