20 times higher detection rate! Photonic Sensors for Quantum Communication

How can we combat data theft, which is a real social problem? Quantum physics has the solution. Its theory makes it possible to encode information (quantum bits) in a single particle of light (photon) and circulate it in a highly secure manner in optical fibers. However, the widespread use of this telecommunication technology is particularly hampered by the performance of single-photon detectors.

Recently, a team from the University of Geneva (UNIGE), together with ID Quantique, has succeeded in improving its performance by a factor of 20. This innovation, published in the journal Nature Photonics, makes it possible to achieve unprecedented performance in quantum key distribution.

Buying train tickets, booking cabs, delivering food: these are the transactions that take place every day through mobile applications. These are based on payment systems that involve the exchange of secret information between the user and the bank. For this purpose, banks generate a public key, which is transmitted to their customers, and a private key, which is kept secret. With the public key, the user can modify the message, making it unreadable, and send it to the bank; with the private key, the bank can decipher it.

However, this system is now threatened by the power of quantum computers. To solve this problem, quantum cryptography, or quantum key distribution (QKD), is the best option. It allows both parties to generate a shared secret key and transmit it over optical fiber in a highly secure manner. This is because the laws of quantum mechanics state that measurements affect the state of the system being measured. Therefore, if a spy tries to measure a photon in order to steal the key, the information will be immediately altered and the intercepted information will be discovered.

Using these sensors, the scientists were able to generate a secret key at 64 megabits per second over a 10-kilometer fiber optic cable.

One limitation of the system's application is the speed of the single-photon detectors used to receive the information. In fact, after each detection, the detector must recover for about 30 nanoseconds, which limits the throughput of the secret key to about 10 megabits per second. A UNIGE team, led by Hugo Zbinden, associate professor in the Department of Applied Physics in the UNIGE College of Science, has managed to overcome this limitation by developing a detector with better performance. The work was carried out in collaboration with the team of Félix Bussières from ID Quantique, a spin-off company of the university.

"Currently, the fastest detectors we apply are superconducting nanowire single photon detectors," explains Fadri Grünenfelder, a former PhD student in the Department of Applied Physics at UNIGE's Faculty of Science and first author of the study: "These devices contain a tiny superconducting wire. If a photon hits it, it heats up and stops superconducting for a short time, resulting in a detectable electrical signal. When this wire becomes cold again, another photon can be detected."

By integrating fourteen nanowires into their detector, the researchers achieved a record detection rate. "Our detectors count 20 times faster than single-wire devices," explains Hugo Zbinden, "and if two photons arrive at these new detectors in a very short time, they can touch different wires and be detected at the same time. And with a single wire, this is not possible. The nanowires used are also much shorter, which helps reduce their recovery time."

A schematic of the QKD device with all the key components in the experiment.

Using these sensors, the scientists were able to generate a secret key at a rate of 64 megabits per second over 10 kilometers of fiber optic cable. This rate is sufficient to secure a video conference with several participants: it is five times the performance of existing technology at this distance. At the same time, these new detectors are no more complex to produce than devices currently on the market.

These results open up new prospects for ultra-secure data transmission, which is crucial for banks, healthcare systems, governments and the military. They can also be applied to many other fields where light detection is a key element, such as astronomy and medical imaging.