Quantum key distribution is the most viable scheme to ensure information security in the presence of large-scale quantum computers. However, the secret key rates are limited to 10 Mbps due to bottlenecks on the receiver side. A new study describes a custom multipixel superconducting nanowire single-photon detector designed to ensure high count rates and accurate timing discrimination.

Quantum physics can thwart data theft by encoding information into individual light particles and circulating them through an optical fiber. In particular, the performance of single-photon detectors could be improved for the broad implementation of this telecommunication technology. A team from the University of Geneva (UNIGE) and the company ID Quantique have increased their speed by a factor of twenty. This innovation, to be discovered, makes it possible to achieve unprecedented performance in quantum key distribution.

Quantum Cryptography or Quantum Key Distribution allows two parties to generate shared secret keys and transmit them over optical fibers in a highly secure manner. This is because the laws of quantum mechanics state that measurement affects the state of the system being measured. So if a spy tries to measure the photons to steal the key, the information is immediately altered and the interception is revealed.

However, the speed of the single-photon detectors used to receive the information is limited to 30 nanoseconds, limiting the throughput of the secret keys to 10 megabits per second.
Scientists in this study overcome this limit by developing a higher performing detector. It can count twenty times faster than a single-wire device.

Fadri Grünenfelder, a former PhD candidate in the Department of Applied Physics of the UNIGE Faculty of Science and the first author of the study, explains: “Currently, the fastest detectors for our application are superconducting nanowire single-photon detectors. These devices contain a small superconducting wire cooled to -272°C. If a single photon hits it, it heats up. It is no longer superconducting for a short time and thus generates a detectable electrical signal. When the wire gets cold again, another photon can be detected.”

They achieved record detection rates by integrating 14 nanowires into their detector, which can count 20 times faster than a single-wire device. The nanowires used are also shorter, helping to shorten their recovery time. The researchers were able to generate a secret key at a speed of 64 megabits per second over 10 km of fiber optic cable, five times the performance of current technology over this distance.

Hugo Zbinden said, “Our detectors can count twenty times faster than a single-wire device. If two photons arrive at these new detectors within a short period of time, they can hit different wires and both will be detected. This is impossible with a single thread”.

These results open up possibilities for ultra-secure data transport, which can benefit banks, healthcare systems, governments and the military. They can also be used in other fields where the detection of light is essential, such as astronomy and medical imaging.

This result paves the way for applications that require secret keys, such as real-time, one-time-pad secure video encryption in a metropolitan area. These results open new perspectives for quantum cryptography.

Magazine reference:

  1. Grünenfelder, Boaron, et al. Fast single-photon detectors and real-time key distillation enable high key rate quantum key distribution systems. Nature photonics. DOI: 10.1038/s41566-023-01168-2