This Quantum Sensor Can Detect 98% Of The Photons That Hit It

Researchers at the U.S. National Institute of Standards and Technology (NIST) have demonstrated superconducting sensors capable of detecting 98% of incoming photons, a development that could improve technologies that depend on measuring extremely weak light signals.

The researchers used superconducting nanowire single-photon detectors, or SNSPDs, which are designed to register individual particles of light. Such detectors have applications in quantum computing and communications, biomedical imaging, astronomy and experiments searching for phenomena such as dark matter.

The result challenges a long-standing assumption that SNSPDs need extremely narrow wires to achieve their highest sensitivity.

“There are many applications where you’re working with handfuls of photons. Ideally, you need to detect all of them,” said Kristen Parzuchowski, a postdoctoral researcher at NIST.

How the detector works

An SNSPD is essentially an ultracold electrical circuit designed to detect individual photons. The detector contains a superconducting material through which an electrical current flows with essentially no resistance when the material is cooled to very low temperatures.

When a photon strikes the superconducting wire, it deposits a small amount of energy. That energy temporarily disrupts superconductivity in a tiny region, creating what researchers call a “hot spot.”

Because the electrical current can no longer pass through that region in the same way, the current is diverted and produces a measurable electrical pulse. That pulse tells the detection system that a photon has arrived.

The challenge is that a photon carries an extremely small amount of energy. Conventional SNSPDs therefore use nanowires only a few hundred nanometers wide so that a single photon’s energy can disrupt superconductivity across enough of the wire to produce a detectable signal.

Eli Mueller, a NIST postdoctoral researcher, described the conventional approach: “Your photon energy needs to break superconductivity over the entire width of the wire,” he said. “It’s very difficult to have your device in a regime where the photon could break superconductivity over 100 microns wide, so devices needed to be on the order of hundreds of nanometers wide.”

Making the wires extremely narrow, however, creates other problems. The small cross-section limits how much electrical current the detector can carry, reducing the strength of the signal generated when a photon is detected. It can also increase unwanted detections, known as dark counts, in which the system registers a signal even though no photon of interest has arrived.

NIST researchers took a different approach by making the detector’s central superconducting wire much wider while adding superconducting rails alongside it. The rails help redistribute the electrical current and prevent it from accumulating excessively along the edges of the wider wire.

The researchers expanded the detector wires to about one-tenth of a millimeter, more than 100 times wider than conventional SNSPD designs.

That allows substantially more current to flow through the detector. When a photon creates a hot spot, the resulting change in current can therefore produce a stronger electrical signal that is easier to measure.

“If we can get the middle of our device to flow more current than what was previously accessible, then we can generate a hot spot over an arbitrarily wide wire. And that hot spot is what’s giving you the pulse out,” Mueller said.

The architecture also makes the detectors less sensitive to photon polarization, meaning they can detect light without requiring the electric field of the incoming photon to have a particular orientation.

Reducing false signals

The researchers said the detectors reached an intrinsic detection efficiency of about 98% while also achieving a billion-fold reduction in dark counts.

That combination is important for applications in which the signal may consist of only a small number of photons. In astronomy, for example, detectors may need to distinguish extremely faint light from distant objects from background noise. Similar requirements apply to some biomedical imaging systems and quantum communications, where losing even a small fraction of photons can affect the usefulness of the information being transmitted.

The high efficiency also could benefit quantum technologies. Quantum communication systems often encode information in individual photons, while quantum computers and quantum sensors can require highly sensitive measurements of light and other physical signals.

The NIST result does not mean that all photon detection systems will immediately adopt the wider design. The sensors must operate at extremely low temperatures, and practical systems require specialized cooling, electronics and optical components.

But the work suggests that increasing the physical size of a superconducting detector does not necessarily require sacrificing its ability to detect individual photons, potentially simplifying fabrication while improving performance.

The researchers said the findings demonstrate that the fundamental performance limit of SNSPDs can be approached without relying on the extremely narrow nanowires traditionally used in the technology.

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