Experimental implementation of Twin-Field Quantum Key Distribution Protocols

Quantum Key Distribution allows two distant users to establish a common secret string of bits by sending photons across a communication line, often an optical fibre. The photons, however, are scattered by the propagation medium and have only a small probability of reaching the end of the line, which limits the QKD key rate and its transmission range. A rigorous theorem limits the number of secure bits delivered by a point-to-point QKD link to 1.44η, with η being the channel transmission probability. This is known as the ‘repeaterless secret key capacity’, or the PLOB bound. The key question at the core of this thesis is to design and implement QKD systems that surpass the PLOB bound.
Until very recently, this task was believed to be impossible with today’s technology. This changed with the introduction of the ‘Twin-Field’ (TF) QKD protocol, which features a key rate that scales proportionally to the square root of η and therefore offers a way to extend the current range of QKD.
This work provides a contextualisation and description of the TF-QKD protocol and its variants. The experimental challenges for its implementation are considered and followed by the development of experimental techniques, setups and analysis frameworks necessary to implement the protocol.
As a result, the first proof-of-principle demonstration of the protocol over highly attenuated channels is obtained and described. In this experiment, a secure key could be distributed in excess of 90 dB channel loss and the PLOB bound could be exceeded for the first time. The setup implemented for this experiment is currently considered the first realisation of an effective quantum repeater.
A second experiment, which exploits a novel dual-band phase stabilisation technique, is also developed. In this experiment, TF-QKD is performed over long communication channels that reached over 600 km of fibre length and 100 dB of channel loss. This experiment represents today’s longest fibre-based quantum communication system.