Twin-Field Quantum Key Distribution without Phase Postselection

The twin-field quantum key distribution (TFQKD) protocol and its variants, e.g., phase-matching (PM) QKD and TFQKD based on sending or not sending, are highly attractive since they are able to overcome the well-known rate-loss limit for QKD protocols without a repeater: R = O(eta), with eta standing for the channel transmittance. However, all these protocols require active phase randomization and postselection that play an essential role together in their security proof. Counterintuitively, we find that in TFQKD, beating the rate-loss limit is still possible even if phase randomization and postselection in the coding mode are both removed, which means our final secure key rate R = O(root n). Furthermore, our protocol is more feasible in practice and more promising according to its higher final key rate in the valid distance. Our security proof counters a collective attack and can also counter a coherent attack in the asymptotical case.