Secure Quantum Key Distribution over 421 km of Optical Fiber
Alberto Boaron, Gianluca Boso, Davide Rusca, Cédric Vulliez, Claire Autebert, Misael Caloz, Matthieu Perrenoud, Gaëtan Gras, Félix Bussières, Ming-Jun Li, Daniel Nolan, Anthony Martin, and Hugo Zbinden
Physical Review Letters 121, 190502 (2018)
A collaboration between: The Quantum Technologies group at the University of Geneva, ID Quantique SA and Corning Incorporated.
You can read the full article here.
Our recent demonstration of a secret key exchange over a record distance of 421 km of optical fibre has been published in PRL as an Editor's suggestion as well as being featured in Physics Viewpoint. We push the limits to new heights by optimising all parts of the QKD system including increasing the generation rate of the quantum states to reach 2.5 GHz and high performance single-photon detectors developed in-house, which have very high efficiency and low noise. Finally, a new encoding method of the quantum signals has been developed in order to simplify the experimental setup while keeping it very efficient.
Experimental schematic for the long-distance QKD demonstration
Quantum key distribution (QKD) allows two remote users to exchange encryption keys securely. The security of such an exchange is based on the laws of quantum physics, which state that it is impossible to copy a quantum object (state) without disturbing it. The users exchange quantum states via photons (the indivisible light particles) in which information is encoded. If an adversary tries to intercept this communication, it introduces necessarily errors, which allow the users to detect the attack.
Since the first experimental realisation in 1989, QKD systems have been much improved such that commercial systems are available since the 2000s. However, increasing the transmission distance is a challenge, because the quantum signals transmitted through optical fibres suffer loss that increases with the distance.
In this article, we demonstrate a secret key exchange over a record distance of 421 km of optical fibre. We push the limits to new heights by optimising all parts of the QKD system. Firstly, this has been made possible by increasing the generation rate of the quantum states to reach 2.5 GHz (which means that 2.5 billion signals per second are sent). New generation optical fibres transmitting the light more efficiently are used. Moreover, in-house-made single-photon detectors detect the received photons very efficiently and with very low noise. Finally, a new encoding method of the quantum signals has been developed in order to simplify the experimental setup while keeping it very efficient.
Reaching such transmission distances is an important step towards establishing quantum communication networks between cities, in view for instance of building a Pan-European network.
Physics Viewpoint: Record Distance for Quantum Cryptography