Intercity Quantum Key Distribution Using BB84 Protocol

Quantum key distribution (QKD) is a well-established quantum technology with the potential to address growing security concerns associated with advancements in quantum computation. While the theoretical foundation of QKD is sufficiently understood, challenges persist in establishing long-distance connections without the necessity of costly satellite missions or the complexities of deploying quantum repeaters.We present a demonstration of QKD over a fibre with 21 dB signal attenuation, utilising a link between St. Pölten and Vienna. Overall, we achieved a secret key rate of 4 kbps, matching our prior test measurements under laboratory conditions.The implemented protocol was a four-state, one-decoy BB84 at 1550 nm, using a desktop transmitter-receiver pair configured as a flexible setup. The quantum signal was prepared via time-bin encoding, a technique that has been demonstrated to be reliable and robust against depolarisation and polarisation mode dispersion. This work demonstrates the capability of quantum communication in conventional fibre networks by showing the functionality of performing QKD over an existing single-mode fibre (SMF) link.