Abstract As the vision of global-scale unconditional information security becomes gradually realized, the importance of inter-satellite quantum communications has been rapidly increasing. The recently proposed round-robin differential-phase-shift (RRDPS) quantum key distribution (QKD) protocol has attracted much attention not only due to its potential high error tolerance, but also due to its distinct feature that the information leakage can be bounded without monitoring signal disturbances. Despite many existing implementations over fiber-optic channels, the feasibility of RRDPS QKD over an inter-satellite channel is still unknown. Moreover, despite the current advances in orbital angular momentum (OAM) encoding and temporal mode (TM) encoding, most of the existing studies on RRDPS QKD are restricted to time-bin encoding. In this work, we remedy this situation by exploring the feasibility of performing RRDPS QKD using OAM encoding and TM encoding over an inter-satellite channel. Our results indicate that OAM encoding is preferable to time-bin encoding only under the circumstances where a low dimension and a large receiver aperture are used. However, we find that TM encoding is the best encoding scheme in RRDPS QKD over an inter-satellite channel. In particular, we show that TM encoding not only leads to the best performance and the largest feasible parameter range, but also, for the first time, enables all the theoretically available advantages of an increased dimension to be realized in the context of RRDPS QKD.
Abstract As the vision of global-scale unconditional information security becomes gradually realized, the importance of inter-satellite quantum communications has been rapidly increasing. The recently proposed round-robin differential-phase-shift (RRDPS) quantum key distribution (QKD) protocol has attracted much attention not only due to its potential high error tolerance, but also due to its distinct feature that the information leakage can be bounded without monitoring signal disturbances. Despite many existing implementations over fiber-optic channels, the feasibility of RRDPS QKD over an inter-satellite channel is still unknown. Moreover, despite the current advances in orbital angular momentum (OAM) encoding and temporal mode (TM) encoding, most of the existing studies on RRDPS QKD are restricted to time-bin encoding. In this work, we remedy this situation by exploring the feasibility of performing RRDPS QKD using OAM encoding and TM encoding over an inter-satellite channel. Our results indicate that OAM encoding is preferable to time-bin encoding only under the circumstances where a low dimension and a large receiver aperture are used. However, we find that TM encoding is the best encoding scheme in RRDPS QKD over an inter-satellite channel. In particular, we show that TM encoding not only leads to the best performance and the largest feasible parameter range, but also, for the first time, enables all the theoretically available advantages of an increased dimension to be realized in the context of RRDPS QKD.
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=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.
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.
Realizing long-distance quantum key distribution (QKD) in fiber channel where classical optical communications and quantum signals are multiplexed by their different wavelengths has attracted considerable attentions. The achievable secure distance of commonly-used Bennet-Brassard 1984 (BB84) protocol is lowered severely due to inevitable crosstalk from classical optical pulses. Unlike conventional quantum key distribution (QKD) protocols, round-robin differential-phase-shift (RRDPS) QKD protocol has a high tolerance for noise, since the potential information leakage in this protocol can be bounded without monitoring signal disturbance. Thus, it may be a promising protocol under noisy channel. In this work, we investigate the performance, e.g., achievable secure distance of RRPDS protocol, when crosstalk from classical communication is considered. Surprisingly, we find that RRPDS only has quite limited advantage over BB84 protocol when optical misalignment of QKD system is serious. If misalignment is trivial, BB84 can even outperform RRDPS protocol.
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.
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.
Since the round-robin-differential-phase-shift (RRDPS) quantum key distribution (QKD) protocol was proposed, it has attracted much attention due to its unique characteristic i.e., it can bind the amount of information leakage without monitoring signal disturbance. Recently, Yin et al. have developed a novel theory to estimate its information leakage tightly. However, the finite-sized key effects are not taken into account. Here, we fill this gap and extend the security proof of the RRDPS protocol to the finite-sized regime using post-selection technique. As a consequence, it's predicted that the key rate of RRDPS in a finite-sized key scenario can be comparable to the asymptotic one, which is meaningful for the real-life applications.