Coherent phase transfer for real-world twin-field quantum key distribution

Abstract

Quantum mechanics allows the distribution of intrinsically secure encryption keys by optical means. Twin-field quantum key distribution is the most promising technique for its implementation on long-distance fibers, but requires stabilizing the optical length of the communication channels between parties. In proof-of-principle experiments based on spooled fibers, this was achieved by interleaving the quantum communication with periodical adjustment frames. In this approach, longer duty cycles for the key streaming come at the cost of a looser control of channel length, and a successful key-transfer using this technique in a real world remains a significant challenge. Using interferometry techniques derived from frequency metrology, we developed a solution for the simultaneous key streaming and channel length control, and demonstrate it on a 206 km field-deployed fiber with 65 dB loss. Our technique reduces the quantum-bit-error-rate contributed by channel length variations to <1%, representing an effective solution for real-world quantum communications.Quantum key distribution (QKD) enables to share secret cryptographic keys between distant parties, whose intrinsic security is guaranteed by the laws of quantum mechanics [1][2][3]. Besides pioneering experiments involving satellite transmission [4,5], the challenge is now to integrate this technology on the long-distance fiber networks already used for telecommunications [6][7][8][9][10][11][12][13][14][15]. The maximum secure key rate for QKD decreases exponentially with the channel losses. Although the reach could be extended using quantum repeaters, the related research is still at a rudimentary level and these devices are far from operational [16][17][18]. Nowadays, intercity distances could only be covered using trusted nodes [13], whose security represents however a significant technical issue. A fundamental resource for next-generation long-distance secure communications is represented by the recently proposed twin-field QKD (TF-QKD) protocol [19], because of its weaker dependence on channel loss. In TF-QKD, the information is encoded as discrete phase states on dim laser pulses generated at distant Alice and Bob terminals and sent through optical fiber to a central node, Charlie, where they interfere. This idea, sketched in Fig. 1a, was proved secure against general attacks [20][21][22][23][24] also in the finite-size scenario [25][26][27] and with the aid of two-way communication [28], but it is based on the critical assumptions that the optical pulses are phase-coherent in Alice and Bob and preserve coherence throughout the path to Charlie. While the first requirement can be fulfilled by phase-locking the two QKD lasers in Alice and Bob to a common reference laser transmitted through a service channel, the uncorrelated fluctuations of the length and refractive index of the connecting paths (i.e. the optical length) introduce phase noise to the system and reduce the visibility of the interference measurement. In proof-of-principle experiments

Results of the NATO project: "analysis, design, and implementation of an end-to-end QKD link"

“…By exploiting frequency metrology stabilization techniques, it was demonstrated [1] the feasibility of the TF-QKD scheme on such a distance: when stabilization was activated [1].…”

Breaking the Rate-Loss Bound of Quantum Key Distribution with Asynchronous Two-Photon Interference

“…For example, remarkable progress has been made in the theory of finite key analysis [41][42][43][44]. Additionally, some notable experimental implementations have been reported [45][46][47][48][49][50][51][52][53][54]. The longest transmission distance of more than 600 km was recently achieved in the laboratory through optical fibers [55,56].…”

Quantum key distribution surpassing the repeaterless rate-transmittance bound without global phase locking

“…In a typical 200-km fibre with a telecommunication frequency of 1550 nm, the phase of a coherent state is susceptible to small fluctuations in the optical transmission time (∼ 10 −15 s), optical length (∼ 200 nm) and light frequency ( ∼ 100 kHz). Recently, experimentalists have made great efforts to demonstrate high-performance in one-mode schemes, utilising high-end technologies to perform a precise control operation to stabilise the global phase by locking the frequency and phase of the coherent states [29][30][31][32][33][34][35][36]. However, this significantly increases the experimental difficulty and undermines the applicability of one-mode schemes in real life.…”