No-interfering quantum key distribution

The well-known twin-field quantum key distribution (TF-QKD) protocol is the first one to overcome the fundamental rate-distance limit without quantum repeaters. It encodes the key information into phases of the light, and has a secret key rate scaling with the square root of the transmission transmittance by taking advantage of single-photon interference. Inspired by the idea in TF-QKD, we proposed a polarization encoding protocol to break the rate-distance limit by the property of the two states with orthogonal polarizations, named no-interfering QKD (NI-QKD). Two effective events are defined in which no interference happens. Simulation results show that the proposed protocol also holds the capacity of surpassing the Pirandola–Laurenza–Ottaviani–Banchi (PLOB) bound. Moreover, it has a better performance than no-phase-postselection TF-QKD (NPP-TF-QKD), one of TF-QKD’s variants, and its communication distance can reach at most 424 km. The relationships between the performance and the polarization misalignment, the phase mismatch are discussed. It turns out that one event is very robust against polarization misalignment while the other is not, and both events are sensitive to phase mismatch. The mutual information of the effective events under collective attack is also calculated, which is lower than that of NPP-TF-QKD. This new protocol provides a new angle of exploring QKD and improving the secret rate.