High-Performance Sensors To Counter Spies

A team from UNIGE and ID Quantique has developed unique photon detectors with unprecedented performance, opening up new perspectives for quantum cryptography.

How to combat data theft, a real social issue? Quantum physics has the solution. His theories make it possible to encode information (a qubit) in particles of light (a photon) and to circulate them in an optical fiber in an ultra-secure manner. But the use of this large-scale telecommunications technology is hampered in particular by the performance of the single photon sensors used. A team from the University of Geneva (UNIGE), with the company ID Quantique, managed to multiply their speed by twenty. This innovation, to be discovered in the journal Nature Photonics , makes it possible to achieve performances never before obtained in the quantum distribution of keys.

Buying a train ticket, booking a taxi, having a meal delivered: so many transactions carried out daily via mobile applications. These are based on payment systems involving an exchange of secret information between the user and his/her bank. To do this, the bank generates a public key, which it sends to its client, and a private key, which it keeps secret. With the public key, the user can modify the information, make it unreadable and send it to the bank. Thanks to its private key, the bank can decrypt it.

This system is now threatened by the computing power of quantum computers. To remedy this, quantum cryptography – or “quantum key distribution” (QKD ) – is the best option. It allows two parties to produce shared secret keys and transmit them using photons, via optical fiber, in an ultra-secure way. Indeed, the laws of quantum mechanics stipulate that a measurement affects the state of the measured system. Thus, if a spy tries to measure the photons to steal the key, the information will be instantly altered and the interception revealed.

Current limits
The application of this system is notably limited by the speed of the single photon detectors, used to receive the information. After each detection, they must in fact recover for around thirty nanoseconds, which limits the rate of secret keys to around 10 megabits per second. A UNIGE team led by Hugo Zbinden, associate professor in the Department of Applied Physics of the UNIGE Faculty of Science, has succeeded in pushing back this limit by developing a detector with better performance. This work was carried out in collaboration with the team of Félix Bussières from the company ID Quantique , a spin-off from the university.

"Currently, the fastest detectors for this application are single photon detectors with superconducting nanowires", explains Fadri Grünenfelder, ex-doctoral student in the Department of Applied Physics of the UNIGE Faculty of Science and first author of the study. . “These devices feature a tiny superconducting wire cooled to -272°C. If a single photon strikes it, it heats up, ceases to be superconductive for a short time, which generates a detectable electrical signal. When the wire becomes cold again, another photon can be detected.”

Record performance
By integrating not one but fourteen nanowires into their sensors, the researchers managed to obtain record detection rates. “Our detectors can count twenty times faster than a single-wire device,” explains Hugo Zbinden. "If two photons arrive in a short time within these new detectors, they can hit different wires and both be detected, whereas with a single wire it is impossible." The nanowires used are also shorter, which contributes to reducing their recovery time.

Thanks to these sensors, the scientists managed to generate a secret key at a rate of 64 megabits per second over 10km of optical fibers. This speed is high enough to secure, for example, a videoconference with several participants. This is five times the performance of current technology over this distance. As a bonus, these new detectors are no more complex to produce than the current devices available on the market.

These results open up new perspectives for the ultra-secure transfer of data, which is crucial for banks, health systems, but also governments and the army. They can also be applied in many other fields where light detection is a key element, such as astronomy and medical imaging.