Twin-field Quantum Key Distribution without Phase Post-Selection

Understand

Twin-field quantum key distribution (TF-QKD) protocol and its variants, e.g.

• phase-matching (PM) QKD and TF-QKD 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 repeater: $R=O(\eta)$ with $\eta$ standing for the channel transmittance.
• However, all these protocols require active phase randomization and post-selection that play an essential role together in their security proof.
• Counterintuitively, we find that in TF-QKD, beating the rate-loss limit is still possible even if phase randomization and post-selection in the coding mode are both removed, which means our final secure key rate $R=O(\sqrt{\eta})$.