  Quantum communications harness quantum phenomena like superposition and
entanglement to enhance information transfer between remote nodes. Coherent
quantum communications, essential for phase-based quantum internet
architecture, require optical coherence among nodes and typically involve
single-photon interference. Challenges like preserving optical coherence and
integrating advanced single-photon detectors have impeded their deployment in
existing telecommunication networks. This study introduces innovative
approaches to the architecture and techniques supporting coherent quantum
communications, marking their first successful integration within a commercial
telecom infrastructure between Frankfurt and Kehl, Germany. Employing the Twin
Field Quantum Key Distribution protocol, we achieved encryption key
distribution at 110 bit/s over 254 km. This system features
measurement-device-independent properties and non-cryogenically cooled
detectors, and represents the first effective quantum repeater implementation
on telecom infrastructure, the longest practical quantum key distribution
deployment to date, and validates the feasibility of a phase-based quantum
internet architecture.
