National Institute of Standards and Technology (NIST) researchers announced a major hardware breakthrough in quantum technology, scaling up superconducting nanowire single-photon detectors (SNSPDs) to a tenth of a millimeter—more than 100 times wider than typical designs—to capture faint photons more efficiently for biomedical imaging, astronomy, and deep-space communications.
The Physics Bottleneck of Nanoscale Quantum Wires
Our everyday life is flooded with photons, the quantum building blocks of light. For cutting-edge technology, from quantum computing to deep-tissue imaging, detecting every single photon counts. “Photons carry information,” said Kristen Parzuchowski, a postdoctoral researcher at the National Institute of Standards and Technology. “Whenever a photon comes into your measurement system, you need to be able to detect it.”
Photons can transmit data in quantum networks or across deep space communication links. Catching and analyzing photons lets scientists build biomedical images and search the universe for dark matter. Superconducting nanowire single-photon detectors represent the best way to capture photons for all these applications. As their name implies, they use the phenomenon of superconductivity, in which electricity flows without resistance, to detect individual photons. Single particles of light create tiny splashes in the electric current, which disrupt the superconductivity and trigger a measurable electrical signal. NIST has drastically improved these devices over the years to the point at which they detect 98% of the photons that come in.
SNSPDs still have some drawbacks, however. They typically require highly specialized nanometer-scale fabrication techniques. Most importantly, the superconducting detector’s edges limit detector performance. Detectors that can carry more current are known to perform better, but fabrication defects cap the maximum flow.
Supersizing the Superconducting Architecture
NIST researchers decided to think bigger. In a paper published in Optica, NIST researchers found that they could size up the superconducting wires to a tenth of a millimeter—more than 100 times wider than typical SNSPDs—simplifying the photon detector’s design and its fabrication using a method to unlock the material’s true performance potential. “Typically, everyone has worked to make smaller and smaller wires, which makes fabrication increasingly challenging,” Parzuchowski said.
SNSPDs typically use a 100-nanometer-wide wire made of superconducting material connected to a readout circuit. Electric current flows through the wire like water through a river. When a photon hits the wire, like a rock hitting the river, it creates a hot spot disrupting the current. That hot spot creates a voltage pulse, which is picked up by the readout electronics. Scientists believed that superconducting wires in SNSPDs needed to be nanoscale to maximize the photon’s minuscule splash across the wire, points out NIST postdoctoral researcher Eli Mueller.
“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.”
Overcoming Edge Defects and False Signals
Due to that nanoscale size, SNSPDs have to operate at lower currents, which means that lower-energy photons don’t make as much of a splash and are harder to detect. Readouts are fainter blips. The electric current doesn’t behave perfectly, either: It doesn’t flow evenly across the wire. Defects can also cause current to build up along the edges, like whirlpools or eddies along a river, leading to false signals called dark counts, Parzuchowski added.
“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 explained. “If you’re operating even closer to that transition between the superconducting state and the normal state, then you’re still sensitive to the very low energy photons.”
