Phase Noise in Real‐World Twin‐Field Quantum Key Distribution

Abstract The impact of noise sources in real‐world implementations of twin‐field quantum key distribution (TF‐QKD) protocols is investigated, focusing on phase noise from photon sources and connecting fibers. This work emphasizes the role of laser quality, network topology, fiber length, arm balance, and detector performance in determining key rates. Remarkably, it reveals that the leading TF‐QKD protocols are similarly affected by phase noise despite different mechanisms. This study demonstrates duty cycle improvements of over a factor of two through narrow‐linewidth lasers and phase‐control techniques, highlighting the potential synergy with high‐precision time and frequency distribution services. Ultrastable lasers, evolving toward integration and miniaturization, offer promising solutions for agile TF‐QKD implementations on existing networks. Properly addressing phase noise and practical constraints allows for consistent key rate predictions, protocol selection, and layout design, crucial for establishing secure long‐haul links for the quantum communication infrastructures under development in several countries.