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Quantum cryptography optimised with faster single-photon detector

Researchers have developed single-photon detectors with what they say is ‘unprecedented’ performance, which could open up new possibilities in quantum cryptography.

This is where quantum physics is used to encode information (in the form of qubits) in single particles of light (photons) and circulate them within an optical fibre in a highly secure way.

The technique poses an excellent solution to combating data theft, an ongoing issue in society. However, the deployment of this technique is still in part hampered by the current performance of single-photon detectors.

The scientists, from the University of Geneva (UNIGE) and Swiss quantum cybersecurity firm ID Quantique (a UNIGE spin-off), have therefore been working to develop a new single-photon detector with speeds twenty-times that of current technologies. The work, reported in Nature Photonics, could lead to ‘unprecedented’ performances in quantum key distribution, according to the scientists.

Why is quantum cryptography needed?

Modern payment systems involve an exchange of secret information between a customer and a bank. To do this, the bank generates a public key, which is transmitted to the customer, and a private key, which it keeps secret. With the public key, the customer creates unreadable payment information and sends it to the bank, which can then decipher it using the private key..

This current system however is now threatened by the power of quantum computers. And so, fighting fire with fire, quantum cryptography – or quantum key distribution – is being explored as an alternative. This allows two parties to generate shared secret keys and transmit them via optical fibres in a highly secure way. This is because the laws of quantum mechanics state that the act of measuring a quantum system can affect its state. Thus, if an outsider tries to measure the photons to steal the key, the information will be instantly altered and the interception revealed.

Overcoming limitations

One roadblock hampering ongoing quantum cryptography efforts is the speed of the single-photon detectors used to receive the information. In fact, after each detection, current detectors must often recover for around 30 nanoseconds, which limits the throughput of the secret keys to about 10 megabits per second. 

‘‘Currently, the fastest detectors for our application are superconducting nanowire single-photon detectors,’’ explains Fadri Grünenfelder, first author of the study. ‘‘These devices contain a tiny superconducting wire cooled to -272°C. If a single photon hits it, it heats up and ceases to be superconducting for a short time, thus generating a detectable electrical signal. When the wire becomes cold again, another photon can be detected.’’

The research collaboration has now overcome this issue with the development of their new, highly-improved detector. 

In the new device, by integrating not one, but fourteen nanowires, the researchers were able to achieve record detection rates. ‘‘Our detectors can count twenty-times faster than a single-wire device,’’ explains Associate Professor Hugo Zbinden, who led the team. ‘‘If two photons arrive within a short time in these new detectors, they can touch different wires and both be detected. With a single wire this is impossible’’. The nanowires used are also shorter, which helps to decrease their recovery time.

Using the new detector, the scientists were able to generate a secret key at a rate of 64 megabits per second over 10km of fibre optic cable. This rate, which the team says is five-times the performance of current technology over this distance, would be high enough to secure a videoconference with several participants. As a bonus, these new detectors are no more complex to produce than the current devices available on the market.

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

Lead image: M. Perrenoud - G. Resta / UNIGE


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Quantum photonics, Research