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.
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.
In conventional quantum key distribution (QKD) protocols, security is guaranteed by estimating the amount of leaked information. Such estimation tends to overrate, leading to a fundamental threshold of the bit error rate, which becomes a bottleneck of practical QKD development. This bottleneck is broken through by the recent work of round-robin differential phase-shift (RRDPS) protocol, which eliminates the fundamental threshold of the bit error rate. The key challenge for the implementation of the RRDPS scheme lies in the realization of a variable-delay Mach--Zehnder interferometer, which requires active and random choice of many delays. By designing an optical system with multiple switches and employing an active phase stabilization technology, we successfully construct a variable-delay interferometer with 127 actively selectable delays. With this measurement, we experimentally demonstrate the RRDPS protocol and obtain a final key rate of 15.54 bps with a total loss of 18 dB and an error rate of 8.9%.
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.
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.
Recently, a round-robin differential phase-shift (RRDPS) protocol was proposed [ Nature 509 , 475 (2014)], in which the amount of leakage is bounded without monitoring the signal disturbance. Introducing states of the phase-encoded Bennett–Brassard 1984 protocol (PE-BB84) to the RRDPS, this paper presents another quantum key distribution protocol called round-robin differential quadrature phase-shift (RRDQPS) quantum key distribution. Regarding a train of many pulses as a single packet, the sender modulates the phase of each pulse by one of {0, π/2, π, 3π/2}, then the receiver measures each packet with a Mach–Zehnder interferometer having a phase basis of 0 or π/2. The RRDQPS protocol can be implemented with essential similar hardware to the PE-BB84, so it has great compatibility with the current quantum system. Here we analyze the security of the RRDQPS protocol against the intercept-resend attack and the beam-splitting attack. Results show that the proposed protocol inherits the advantages arising from the simplicity of the RRDPS protocol and is more robust against these attacks than the original protocol.
In quantum key distribution (QKD), protocols are tailored to adopt desirable experimental attributes, including high key rates, operation in high noise levels, and practical security considerations. The round-robin differential phase shift protocol (RRDPS), falling in the family of differential phase shift protocols, was introduced to remove restrictions on the security analysis, such as the requirement to monitor signal disturbances, improving its practicality in implementations. While the RRDPS protocol requires the encoding of single photons in high-dimensional quantum states, at most, only one bit of secret key is distributed per sifted photon. However, another family of protocols, namely high-dimensional (HD) QKD, enlarges the encoding alphabet, allowing single photons to carry more than one bit of secret key each. The high-dimensional BB84 protocol exemplifies the potential benefits of such an encoding scheme, such as larger key rates and higher noise tolerance. Here, we devise an approach to extend the RRDPS QKD to an arbitrarily large encoding alphabet and explore the security consequences. We demonstrate our new framework with a proof-of-concept experiment and show that it can adapt to various experimental conditions by optimizing the protocol parameters. Our approach offers insight into bridging the gap between seemingly incompatible quantum communication schemes by leveraging the unique approaches to information encoding of both HD and DPS QKD.