All-Fibre Frequency Reference for Twin-Field Quantum Key Distribution

Twin-field (TF) quantum key distribution (QKD) has emerged as a promising method for repeater-free long-distance secure communication, with the longest demonstration covering 1000 km [1]. It is a measurement-device-independent QKD protocol, where the two users send weak coherent states, to a central node for single-photon interference followed by single-photon detection. Due to this, TF-QKD has a square root dependence of the key rate on channel transmission, making it possible to cover longer distances than conventional or commercial devices[2]. In previous implementations, a narrow linewidth laser has been used as the master or reference laser, frequency stabilised by locking the laser to a frequency reference, such as the atomic transition of acetylene or an ultra-low expansion (ULE) cavity using RF-locking [1], to achieve high coherence throughout the entire system. The RF-locking method requires a large and complex infrastructure that is sensitive to misalignment and is relatively expensive, bulky, and fragile [3]. Here, we demonstrate an all-fibre frequency reference as an alternative to ULE cavities for TF-QKD, offering lower complexity and intrinsic alignment, a simplified setup built with telecom compatible components, and a smaller footprint, which makes it easier to deploy in existing fibre telecom infrastructures.