Twin-Field Quantum Key Distribution without Phase Postselection

Abstract

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(η) with η 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( √ η). 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 collective attack and can also counter coherent attack in asymptotical case.

Practical issues of twin-field quantum key distribution

“…Surprisingly, a revolutionary protocol called twin-field quantum key distribution(TF-QKD) [8] was recently proposed to beat this bound. Inspired by the novel idea of TF-QKD, researchers proposed some variants and completed the corresponding security proofs [9][10][11][12][13][14]. From the view of experiments, these variants, i.e.…”

“…From the view of experiments, these variants, i.e. phase-matching QKD [10], sending-or-not-sending QKD [11] and no phase post-selection TF-QKD(NPP-TFQKD) [12][13][14], are simpler. Indeed, both the sendingor-not-sending QKD and NPP-TFQKD have been scuccessfully demonstrated [15][16][17][18].…”

“…However, there are still some gaps between theory and implementation of TF-QKD. In [12][13][14], asymptotic SKR of NPP-TFQKD is proposed, but the SKR in finite-key region is not given. On the other hand, the key-size in a practical implementation is always finite, thus a framework to deal with the finite-key size effect in TF-QKD is indispensable.…”

“…Another problem we will discuss is a potential security loophole of TF-QKD and its variants. Although the [8][9][10][11][12][13][14] have proved the TF-QKD and its variants are information-theoretically secure even with unstrusted measurement device just like the original measurement-device-independent protocol [19][20][21][22], the imperfections of laser source may spoil the security. One of the intractable loopholes of source is the intensity fluctuation [23][24][25].…”

Conference key agreement with single-photon interference

“…reducing the assumption on the devices) [9-14], but at the same time are also implementable with today's technology [15][16][17][18]. In this context, a protocol which recently received great attention is the Twin-Field (TF) QKD protocol originally proposed by Lucamarini et al [19], further developed to prove its security [20][21][22][23][24][25][26][27] and experimentally implemented [28][29][30][31]. Indeed, the TF-QKD protocol relies only on single-photon interference occurring in an untrusted node, making it a measurement-device-independent (MDI) QKD protocol capable of overcoming the repeaterless bounds [32,33].In a scenario where several users are required to share a common secret key, one can for instance perform bipartite QKD protocols between pairs of users and then use the secret keys established in this way to encode the final common secret key.…”