Development of a mode-pairing QKD system

The development of quantum computers raises security concerns because most cryptographic protocols rely on computational security, which quantum algorithms can break. Quantum Key Distribution (QKD) offers a solution by generating secret keys based on quantum mechanics, encoding information in photonic degrees of freedom, as in BB84. However, practical QKD faces limitations from hardware imperfections, especially in photodetectors, which can create side-channel vulnerabilities. Additionally, without quantum memories, channel losses set a fundamental limit on the secret-key rate R, given by the PLOB bound. Protocols like Measurement-Device-Independent (MDI) QKD and Twin-Field (TF) QKD address these challenges. MDI-QKD eliminates detector side-channel vulnerabilities, but channel loss still limits its key rate. TF-QKD surpasses the PLOB rate-distance limit but demands highly stable phase control between remote optical signals, making long-distance setups experimentally challenging. A possible solution is the Mode-Pairing protocol. Unlike Twin-Field QKD, it does not require active phase stabilization because it exploits second-order interference, simplifying the experimental setup. In this protocol, a bit is created by pairing detection events announced by the central node within a maximum temporal separation (L), called the pairing length. This parameter is limited by the laser linewidth and the relative frequency stability of the two sources. Various experimental implementations exist, but setups are complex, using multiple intensity modulators or Sagnac rings to achieve high extinction ratios and several phase modulators to encode, randomize and compensate for phase fluctuations. My master’s thesis focuses on simplifying this protocol's experimental setup, making it more compact, stable and suitable for future field deployment. This work addresses three key experimental requirements for Mode-Pairing: characterizing narrow linewidth sources, generating high extinction optical pulses and verifying indistinguishability between independent sources.