Simple security proof of twin-field type quantum key distribution protocol

Abstract

Twin-field (TF) quantum key distribution (QKD) was conjectured to beat the private capacity of a point-to-point QKD link by using single-photon interference in a central measuring station. This remarkable conjecture has recently triggered an intense research activity to prove its security. Here, we introduce a TF-type QKD protocol which is conceptually simpler than the original proposal. It relies on the pre-selection of a global phase, instead of the post-selection of a global phase, which significantly simplifies its security analysis and is arguably less demanding experimentally. We demonstrate that the secure key rate of our protocol has a square-root improvement over the point-to-point private capacity, as conjectured by the original TF QKD.

Simple method for asymmetric twin-field quantum key distribution

“…Here we consider a similar TF-QKD setup as in [9] 'Protocol 3'. Alice and Bob choose two bases X and Z randomly.…”

“…The papers [3,9] consider only the case where the channels between Alice (Bob) and Charles have equal transmittances. In reality, it is possible that the channels might have different levels of loss, due to e.g.…”

“…We will use our method from [15] and apply it to Protocol 3 in [9], to make an 'asymmetric-intensity' TF-QKD protocol that works well even when channels are highly asymmetric. Similar to MDI-QKD, the protocol in [9] has decoupled X and Z bases.…”

“…We will use our method from [15] and apply it to Protocol 3 in [9], to make an 'asymmetric-intensity' TF-QKD protocol that works well even when channels are highly asymmetric. Similar to MDI-QKD, the protocol in [9] has decoupled X and Z bases. Here we allow Alice and Bob to have different intensities in the X and the Z bases respectively, such that in X basis Alice (Bob) now send states a ñ A a { }, and for the three-decoy case, the sets of intensities can be specifically written as m n w , ,…”

“…Because of these advantages, TF-QKD has attracted much attention worldwide since its proposal. Since a rigorous security proof is not provided in the original proposal, several papers have improved the protocol and provided security proof [6][7][8][9][10]. Also, recently there have been multiple reports of TF-QKD demonstrated experimentally [11][12][13][14].…”