Review of twin-field quantum key distribution: from theoretical architectural variants to practical implementations

Abstract

The development of advanced methods for secure key distribution and data encryption has become increasingly important in light of the rapid progress in practical quantum computing. Although post-quantum cryptographic algorithms are designed to withstand attacks by quantum computers, their security relies on computational assumptions, and there is no absolute guarantee that future quantum algorithms will not compromise them. Quantum key distribution (QKD), in contrast, derives its security from the fundamental laws of quantum physics and can therefore provide information-theoretic security. First-generation QKD systems generally involve two communicating parties, conventionally known as Alice and Bob, connected through an optical-fiber or free-space channel. However, in First-generation QKD, the secret-key rate decreases approximately linearly with channel transmittance because of propagation losses. This fundamental limitation is described by the repeaterless secret-key capacity, commonly represented by the PLOB bound, and severely restricts the achievable communication distance. Twin-field quantum key distribution (TF-QKD) has recently emerged as a promising approach for overcoming this limitation. In TF-QKD, Alice and Bob independently transmit optical signals to an intermediate measurement station, typically located between them. Since the secret-key rate scales approximately with the square root of the channel transmittance rather than linearly with it, TF-QKD enables secure key exchange over substantially longer distances than conventional QKD systems. This review examines the evolution of twin-field quantum key distribution, from its theoretical foundations and principal architectural variants to its practical implementation challenges and experimental demonstrations.