Matthew S. Winnel

Quantum Communications (QC) harness quantum mechanical phenomena such as superposition and entanglement to enhance information transfer between remote nodes. Coherent quantum communications refer to QC schemes relying on maintaining optical coherence between nodes for successful execution. These schemes typically involve single photon interference between optical fields generated by distant parties and represent a cornerstone of a promising architecture of the quantum internet. Despite their significant potential, scientific and technical hurdles - including optical coherence maintenance, integrating high-performance single-photon detectors, and precise stabilisation and synchronisation - have prevented the implementation of coherent QC over existing telecommunication infrastructure. Here we present the first realisation of a coherent QC fully integrated into standard telecommunication infrastructure over a link connecting the German cities of Frankfurt and Kehl. The implemented scheme is the Twin Field Quantum Key Distribution (QKD) protocol, enabling the distribution of a shared secret key for encryption at a rate of 110 bit/s over a highly asymmetric 254 km link. This result, obtained with a system featuring measurement-device-independent properties, marks the longest installed QKD implementation utilising non-cryogenic cooled detectors and was enabled by the QC system architecture we developed and by our approach to phase stabilisation, which involves active out-of-band phase stabilisation and avalanche photodiodes for single photon detection. This achievement, not only represents a milestone for practical quantum communications but also validates the compatibility of coherent QC with current telecommunication infrastructure, supporting the feasibility of a phase-based architecture for the quantum internet.

In conventional laser communications, classical information is transmitted by modulating the laser beam's amplitude and is measured via direct detection. Our research introduces a layer of quantum security to this standard process, specifically designed for free-space channels. We explore a hybrid communication method that simultaneously handles classical information in the traditional manner and quantum information through fluctuations in a sub-Poissonian noise-floor. Our approach utilizes a continuous-variable quantum key distribution (CV QKD) protocol, employing a Gaussian ensemble of squeezed states combined with direct detection. This allows for the generation of secure keys while maintaining classical communication, under passive attacks. This quantum-enhanced method offers simplicity, robustness, and efficiency without compromising classical data transfer rates. We have conducted detailed simulations to evaluate the protocol's performance in a free-space atmospheric channel and assessed the security of the CV QKD protocol in the composable finite-size scenario. This added quantum layer provides a practical, secure enhancement to standard laser communication systems.