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arXiv:2103.08452 · Physical Review Applied

Round-Robin Differential Phase-Time-Shifting Protocol for Quantum Key Distribution: Theory and Experiment

Kai Wang, Ilaria Vagniluca, Jie Zhang, Søren O. Forchhammer, Alessandro Zavatta, Jesper B. Christensen, Davide Bacco

Published Apr 8, 2021• 4 citations
arXivHTMLPDF

Abstract

Quantum key distribution (QKD) allows the establishment of common cryptographic keys among distant parties. Many of the QKD protocols that were introduced in the past, involve the challenge of monitoring the signal disturbance over the communication line, in order to evaluate the information leakage to a potential eavesdropper. Recently, a QKD protocol that circumvents the need for monitoring signal disturbance, has been proposed and demonstrated in initial experiments. Here, we propose an improved version of this so-called round-robin differential phase-shifting (RRDPS) protocol, in which both time and phase degrees of freedom are utilized to enlarge the Hilbert-space dimensionality, without increasing experimental complexity or relaxing security assumptions. We derive the security proofs of the round-robin differential phase-time-shifting (RRDPTS) protocol in the collective attack scenario, and benchmark it against RRDPS for different experimental parameters. Furthermore, a proof-of-concept experiment of the RRDPTS protocol, using weak coherent pulses and decoy state method, is demonstrated over 80 km of fiber link. Our results show that the RRDPTS protocol can achieve higher secret key rate in comparison with the RRDPS, in the condition of high quantum bit error rate.

DOI
10.1103/physrevapplied.15.044017

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Quantum key distribution (QKD) allows the establishment of common cryptographic keys among distant parties. Many of the QKD protocols that were introduced in the past, involve the challenge of monitoring the signal disturbance over the communication line, in order to evaluate the information leakage to a potential eavesdropper. Recently, a QKD protocol that circumvents the need for monitoring signal disturbance, has been proposed and demonstrated in initial experiments. Here, we propose an improved version of this so-called round-robin differential phase-shifting (RRDPS) protocol, in which both time and phase degrees of freedom are utilized to enlarge the Hilbert-space dimensionality, without increasing experimental complexity or relaxing security assumptions. We derive the security proofs of the round-robin differential phase-time-shifting (RRDPTS) protocol in the collective attack scenario, and benchmark it against RRDPS for different experimental parameters. Furthermore, a proof-of-concept experiment of the RRDPTS protocol, using weak coherent pulses and decoy state method, is demonstrated over 80 km of fiber link. Our results show that the RRDPTS protocol can achieve higher secret key rate in comparison with the RRDPS, in the condition of high quantum bit error rate.
The round-robin-differential-phase-shift (RRDPS) quantum key distribution (QKD) protocol has attracted intensive study due to its distinct security characteristics; e.g., information leakage is bounded without learning the error rate of key bits. Nevertheless, its practicality and performance are still not satisfactory. Here, by observing the phase randomization of the encoding states and its connection with eavesdropper's attack, we develop an improved bound on information leakage. Interestingly, our theory is especially useful for implementations with short trains of pulses, and running without monitoring signal disturbance is still available. As a result, the practicality and performance of RRDPS are improved. Furthermore, we realize a proof-of-principle experiment with up to 140 km of fiber, which has been the longest achievable distance of RRDPS until now, whereas the original theory predicted that no secret key could be generated in our experiment. Our results will help in bringing practical RRDPS closer to practical implementations.
Quantum key distribution (QKD) offers the possibility for two individuals to communicate a securely encrypted message. A common technical requirement is the monitoring of signal disturbance in a QKD system to bound the information leakage toward an unwanted eavesdropper. Recently, the round-robin differential-phase-shift (RRDPS) protocol, which encodes bits of information in a high-dimensional state space, was proposed to introduce an upper bound on the amount of Eve's information based on dimensionality of prepared quantum states and randomized measurement settings. Since its introduction, many realizations of the RRDPS protocol were demonstrated using trains of coherent pulses (time bin encoding). Given that this realization of the protocol requires active and random choice of many delays in a delayed Mach-Zehnder interferometer, its implementation can be technically demanding. Here, we propose and experimentally demonstrate an implementation of the RRDPS protocol using the photonic orbital angular momentum degree of freedom in dimensions L=3L=3L=3--8, 16, 32, and 64. In particular, we show that Alice's generation stage and Bob's detection stage can each be reduced to a single phase element, greatly simplifying the implementation. Our scheme offers a practical path to using RRDPS protocol in free-space quantum channels.
In recent years, round-robin-differential-phase-shift (RRDPS) quantum key distribution (QKD) has attracted great attention for its unique characteristics, i.e., the information leakage can be bounded without learning bit error rate. Though the RRDPS QKD has made a breakthrough, it is still a question of how RRDPS will perform with monitoring signal disturbance, e.g., decoy-state and error rate statistics are both used. Here, we present simulations to study RRDPS protocol while monitoring signal disturbance. To our excitement, when using the infinite decoy-states method, RRDPS protocol can outperform the commonly used Bennett and Brassard 1984 (BB84) protocol in terms of channel length under typical experimental parameters. In the case of finite decoy states, we find that only two decoy-states and one signal state are sufficient to obtain performance very close to the infinite decoy-states case. Our simulations prove that RRDPS is a competitive protocol in real-life situations.
Round-robin-differential-phase (RRDPS) quantum key distribution (QKD) protocol has attracted intensive studies due to its distinct security characteristic, e.g., information leakage in RRDPS can be bounded without learning error rate of key bits. Nevertheless, its implementation is still far from practical due to the complication of its measurement device. Moreover, on the theoretical side, its security is still not clear in view of error rate. Here, by observing a potential phase randomization in the encoding states, we develop a theory to bound information leakage quite tightly and differently. Besides, the error rate is incorporated to improve the secret key rate, which is significant for the understanding of RRDPS. Based on our novel security proof, the practicality and performance of RRDPS can be both improved dramatically. Furthermore, we realize an experiment up to 140km fiber distance which is the longest achievable distance of RRDPS system until now, while the original security proof predicts no secret key can be generated in our experiment. Our results pave an avenue towards practical RRDPS.
Abstract The round-robin differential phase shift (RRDPS) quantum key distribution (QKD) protocol is a unique QKD protocol whose security has not been understood through an information-disturbance trade-off relation, and a sufficient amount of privacy amplification was given independently of signal disturbance. Here, we discuss the security of the RRDPS protocol in the asymptotic regime when a good estimate of the bit error rate is available as a measure of signal disturbance. The uniqueness of the RRDPS protocol shows up as a peculiar form of information-disturbance trade-off curve. When the length of a block of pulses used for encoding and the signal disturbance are both small, it provides a significantly better key rate than that from the original security proof. On the other hand, when the block length is large, the use of the signal disturbance makes little improvement in the key rate. Our analysis will bridge a gap between the RRDPS protocol and the conventional QKD protocols.
Recently, a novel protocol named round-robin differential phase-shift (RRDPS) quantum key distribution [Nature 509, 475(2014)] has been proposed. It can estimate information leakage without monitoring bit error rate. In this paper, we study the performance of RRDPS using heralded single photon source (HSPS) without and with decoy-state method, then compare it with the performance of weak coherent pulses (WCPs). From numerical simulation, we can see that HSPS performs better especially for shorter packet and higher bit error rate. Moreover, we propose a general theory of decoy-state method for RRDPS protocol based on only three decoy states and one signal state. Taking WCPs as an example, the three-intensity decoy-state protocol can distribute secret keys over a distance of 128 km when the length of pulses packet is 32, which confirms great practical interest of our method.