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Development of a continuous-variable quantum key distribution system integrated in a telecommunication network

Final Activity Report Summary - CVQKD (Development of a continuous-variable quantum key distribution system integrated in a telecommunication network)

Finding ways to exchange messages secretly has been the subject of human efforts for centuries. It is particularly relevant today in a globalised world, where significant quantities of sensible information change hands every day. The cryptographic systems used in current communication networks ensure a so-called computational security, i.e. a security that is based on the assumed limitations in time and computational resources of a potential eavesdropper. Quantum cryptography, on the other hand, offers the prospect of unconditionally secure communications, in which the security of the transmitted message is guaranteed by the laws of quantum mechanics. More precisely, based on these laws, the two parties wishing to exchange a secret message firstly establish a secret key, that is a random binary string of bits, and then use this key to appropriately encrypt and decrypt their secret message. This leads to the main challenge in the field, which is how to efficiently achieve quantum key distribution (QKD).

During the last years, many QKD protocols exploiting various properties of quantum objects, and in particular photons, have been proposed. In this project, we were interested in a continuous-variable QKD protocol, which used properties of coherent states of light that took continuous values, such as the quadratures of the electromagnetic field, to encode key information. This protocol offered the potential of very high rate secret key distribution and could be implemented using standard components of the telecommunication industry. When the project started, the system implementing the protocol was a laboratory setup unsuitable for use in real communication networks. With quantum cryptography rapidly evolving into a very advanced field with promising real-life applications however, it became clear that it was necessary and important to develop a practical QKD prototype and test it in a field implementation of quantum cryptography network. These were indeed the objectives of this project, which were successfully met.

Several scientific and technological advancements had to be made to overcome all the obstacles in the way of such an achievement. In particular, we implemented time and polarisation multiplexing of the signal and phase reference used in our system, thus minimising the effects of polarisation and phase drifts in the transmission optical fibre. To achieve full automation of the prototype, essential for any practical system, we introduced appropriate feedforward loops and dynamic control procedures. We also extensively studied and optimised the complex algorithms required to extract a final perfectly secret key from the continuous data that the two parties possessed after their communication over the fibre link. Finally, in view of the integration of the prototype in a telecommunication network, we constructed a robust, compact and easy to transport packaged device on the hardware side, and we developed interfaces that allowed the use of the generated keys in practical applications provided by the network on the software side. The stability of the prototype operation was also thoroughly tested. These tests helped to identify the two major factors that limited the system performance, namely perturbations due to the device environment and insufficient computer processing power.

In the final field test, the continuous variable QKD prototype generated secret keys at an average rate of eight kbit per second over 57 hours of continuous, fully automatic operation on a 3-dB loss optical link, corresponding to a 15 km standard fibre. During the entire period of operation, the prototype was providing the network with keys that were used for practical applications, such as secure videoconferencing and phone communications. Overcoming the limitations that were previously discussed could enhance the system performance in terms of maximal communication rate and distance; however the obtained results already demonstrated the potential of this prototype for use in metropolitan-size networks with high speed and high security level requirements. Therefore, achieving the objectives set out for this project opened the door to truly secure everyday communication.