Superconductive Nanowire Single-Photon Detectors

The Technology

One of the most useful sources of information about our world is light. While light-gathering technologies such as telescopes and microscopes date back centuries, NIST is advancing the frontier of detecting and measuring light in its fundamental quantum form: the particle known as the photon.

Photons carry extremely small amounts of energy, so specialized technology is needed to detect them. The most sensitive single-photon detectors use superconductors: materials that, at low enough temperatures, conduct electricity without resistance. Specifically, scientists take advantage of the fact that these materials can flip from superconducting to nonsuperconducting states over a tiny temperature range when a photon strikes them and heats them up slightly.

In a superconducting nanowire single-photon detector, or SNSPD, a thin wire made of a superconducting material twists and turns over a surface. NIST researchers primarily use tungsten silicide wires deposited on a silicon substrate to make these devices. The sensor is roughly the width of a human hair.

To operate the sensor, scientists hold the wire just below its critical temperature, typically around 1 to 2 degrees above absolute zero, while applying an electrical current slightly below the maximum current at which the material can sustain superconductivity. When a photon hits the wire, the wire absorbs the photon’s energy. This heats up a small area of the superconducting material and temporarily disrupts the superconductivity, locally increasing the resistance and creating a measurable voltage pulse. The pulse indicates that a photon has been detected.

To achieve high detection efficiencies, scientists add an antireflective coating above the wire and a mirror below, to create an optical cavity that traps photons.

As sensors, SNSPDs stand out for several reasons. With well-engineered cavities, they can capture nearly every photon that hits them. They can also reset themselves quickly to be ready to detect the next photon.

In addition, scientists can tune SNSPDs to capture different wavelengths, or colors, of light by changing the dimensions of the wire. To detect longer wavelengths — i.e., lower-energy photons — scientists make the wire thinner, enabling the photon to heat the entire width of the wire and disrupt its superconductivity. SNSPDs have been engineered to detect ultraviolet, visible and infrared photons.

Beyond the engineering of the wires themselves, SNSPDs need a cooling system to reach the ultralow temperatures necessary for superconductivity. The NIST team has worked to make cryogenics smaller and more efficient, to help make these sensors more accessible and useful. Current cryogenic systems are roughly the size of a standard server rack, or smaller than a home refrigerator.

Advantages Over Previous Methods

Most commercial single-photon detectors are based on semiconductors. These off-the-shelf detectors have become very cheap, but they are much less effective than superconducting sensors at capturing and counting every photon that hits them, with typical efficiencies of around 50%. They also suffer from “dark counts” — false photon detections.

NIST’s superconducting sensors can detect up to 98% of the photons that hit them, with very low dark count rates. While SNSPDs are slightly less efficient than a different kind of sensor known as a transition-edge sensor at capturing every photon that hits them, they can operate at higher temperatures, which reduces the size and complexity of the cryogenics that are needed to cool them. They also reset themselves more quickly after a detection, making them ideal for applications requiring high speeds.

Applications

SNSPDs have been used in clinical settings to measure blood flow in the brain. To do this, a device shines infrared laser light at a patient’s skull. The amount of blood flow in different regions of the brain affects how the light scatters. Optical fibers collect scattered photons and carry them to a refrigerator that houses the superconducting detectors, which read out the scattering pattern. This technology can help doctors investigate possible brain damage without drilling into a patient’s skull.

SNSPDs are also boosting quantum technologies and basic science. For example, some designs for light-based quantum computers incorporate large numbers of detectors that don’t need the photon-counting resolution provided by transition-edge sensors. In these designs, where most detectors see no photons and a few see one photon, SNSPDs can provide an ideal balance of efficiency and simplicity.

NIST scientists have used SNSPDs to carry out “Bell tests” that prove the quantum mechanical prediction of entanglement — which a skeptical Einstein once called “spooky action at a distance.” More recently, NIST researchers used the detectors to create the world’s first random number generator that uses entangled photons.

To become useful, emerging quantum networks will need detectors that can capture photons with near-perfect efficiency, high speed and photon number resolution. No current technology provides this combination, but the NIST team is working to develop SNSPDs that will.

SNSPDs also have potential for astronomy. NIST researchers have arranged hundreds of thousands of these detectors in grids to make cameras. Such cameras could help astronomers directly detect exoplanets — planets outside our solar system. These cameras will detect light emitted by the exoplanet’s host star that hits the planet and is reflected toward Earth, similar to how we see the Moon via reflected sunlight.

Because other planetary systems are many light-years away, Earth- or space-based telescopes pointed toward these distant exoplanets would see only around one photon per second. To capture such an extraordinarily faint signal, telescope detectors would need a dark count rate of essentially zero, well beyond the reach of today’s commercially available technology.

SNSPDs have also been used in dark matter searches, which, like exoplanet hunts, require sensors with extremely low false event rates because scientists are looking for signals from exceptionally rare events.

Key Papers

B.G. Oripov, D.S. Rampini, J. Allmaras, M.D. Shaw, S.W. Nam, A. Beyer, R. Briggs, B. Bumble, H.-Z. Chen, A. Kozorezov, A. Lita, M. Verma, Y. Zhai and A.N. McCaughan. A superconducting nanowire single-photon camera with 400,000 pixels. Nature. Published Oct. 26, 2023. DOI: 10.1038/s41586-023-06550-2

Jeff Chiles, Ioana Craiciu, Daniel Hanson, Boris Korzh, Alexander N. Kolli, Andrew Mueller, Daniel Oblak, Rahul Mani, Travis Autry, Boris Bumble, Peter Day, Adriana Lita, Sae Woo Nam, Jason Teufel, Matthew D. Shaw and Krister Shalm. New Constraints on Dark Photon Dark Matter with Superconducting Nanowire Detectors in an Optical Haloscope. Physical Review Letters. Published June 10, 2022. DOI: 10.1103/PhysRevLett.128.231802

Dileep V. Reddy, Robert R. Nerem, Sae Woo Nam, Richard P. Mirin and Varun B. Verma. Superconducting nanowire single-photon detectors with 98% system detection efficiency at 1550 nm. Optica. Published Nov. 23, 2020. DOI: 10.1364/OPTICA.400751