Decoy-state Scheme Research Articles

Improved decoy-state quantum key distribution with uncharacterized heralded single-photon sources

Encoding system plays a significant role in quantum key distribution (QKD). However, the security and performance of QKD systems can be compromised by encoding misalignment due to the inevitable defects in realistic devices. To alleviate the influence of misalignments, a method exploiting statistics from mismatched basis is proposed to enable uncharacterized sources to generate secure keys in QKD. In this work, we propose a scheme on four-intensity decoy-state quantum key distribution with uncharacterized heralded single-photon sources. It only requires the source states are prepared in a two-dimensional Hilbert space, and can thus reduce the complexity of practical realizations. Moreover, we carry out corresponding numerical simulations and demonstrate that our present four-intensity decoy-state scheme can achieve a much higher key rate compared than a three-intensity decoy-state method, and meantime it can obtain a longer transmission distance compared than the one using weak coherent sources.

Generalized Decoy-State Scheme for Rigorous Characterization of Single-Photon Detectors

Generalized Decoy-State Scheme for Rigorous Characterization of Single-Photon Detectors

Asymmetric channel phase matching quantum key distribution

The phase-matching protocol is a practical and promising protocol that can surpass the linear key generation rate boundary. However, classical phase-matching quantum key distribution requires the channel attenuation between communicating parties to be symmetric. In practice, channels used are often asymmetric, owing to geographical reasons in a quantum key distribution network. To enhance the practicality of phase-matching, this paper proposes an asymmetric phase-matching protocol based on the classical framework and establishes a relevant mathematical simulation model to study the influence of channel asymmetry on its performance. The simulation results show that channel asymmetry significantly affects the count rate, error rate, gain, and quantum bit error rate (QBER), ultimately, system performance. As the channel attenuation difference increases, the system performance decreases and the rate of decrease accelerates. Key generation becomes impossible when the channel attenuation difference exceeds 4 dB. Although the decoy-state scheme cannot change the system's tolerance to channel attenuation difference, when the channel attenuation difference is large, the increasing of the number of decoy states significantly can improve system performance, with a three-decoy-state phase-matching protocol outperforming a two-decoy-state protocol. Considering the limited data length, the system performance is improved as the data length increases, and the tolerance to channel attenuation differences gradually increases. When the data length exceeds 10<sup>12</sup>, this improvement does not continue any more. The system cannot break through the boundary of linear key generation rate when the channel attenuation difference is 2 dB and the data length is less than 10<sup>12</sup>. Comparing with symmetric channels, the system performance improvement is very significant under asymmetric channel conditions as the data length increases.

Study of practical state-preparation error tolerant reference-frame-independent quantum key distribution protocol

Quantum key distribution (QKD) enables the establishment of shared keys between two distant users, Alice and Bob, based on the fundamental principles of quantum mechanics, and it has proven to possess information-theoretic security. In most of QKD systems, Alice and Bob require a shared reference frame, and real-time calibration of the reference frame increases system costs and reduces its performance. Fortunately, the reference-frame-independent QKD protocol has been proposed, overcoming reference-frame drift issues and receiving widespread attention. However, in practical QKD systems, the non-ideal characteristics of realistic devices introduce certain inconsistency between the theory and the practice. In real-world quantum key distribution systems, device imperfections can lead to security vulnerabilities, thereby reducing system security. For example, imperfections in the encoding apparatus at the source end may result in errors in the quantum states. The inherent defects in the detection part may cause after-pulse effects and dead-time effects, thus reducing the key rate. Therefore, in this work, we propose a practical state-preparation error tolerant reference-frame-independent quantum key distribution protocol by taking imperfections in both the source and the detectors into account. Moreover, a three-intensity decoy-state scheme for modeling analysis and numerical simulations is employed. In this protocol, we reduce the influence of state-preparation errors on the key rate by utilizing virtual state methods to precisely estimate the phase-error rate. Furthermore, by characterizing the effects of after-pulses and dead-time on the key rate, our protocol exhibits higher robustness and can effectively address issues related to detector imperfections. This approach can also be extended to other quantum key distribution protocols with higher security levels, such as measurement-device-independent quantum key distribution protocol and twin-field quantum key distribution, further mitigating the influence of device imperfections on practical implementation of QKD system. Therefore, our present work provide important reference value for putting the quantum key distributions into practical application.

Improved statistical fluctuation analysis for twin-field quantum key distribution

The decoy state scheme is the most widely studied quantum key distribution protocol to detect existence of eavesdropping. Twin-field quantum key distribution (TF-QKD) is a promising protocol for realizing secret key sharing over long distances. However, the secret key rate becomes relatively low considering the finite-size effect. In this study, the statistical fluctuation analysis of the four-intensity decoy-state TF-QKD system proposed in a recent study (Zhang et al. in Phys Rev A 95:012333, 2017) is considered, and its performance is compared with the results of the Gaussian approximation and Chernoff bound methods. Numerical simulations show that the suggested method shows a considerable improvement in both the key generation rate and transmission distance over the Chernoff bound method under actual experimental environment. We find that the scheme increases secret key rate 1.04–4.06 times and transmission distances 10–18 km farther. We also present optimized parameters for Gaussian, Chernoff, and our scheme.

Proof-of-principle demonstration of decoy-state quantum key distribution with biased basis choices

Decoy-state method has been widely employed in the quantum key distribution (QKD), since it can not only solve photon-number-splitting attacks but also substantially improve the QKD performance. In conventional three-intensity decoy-state proposal, one not only needs to randomly modulate light sources to different intensities, but also has to prepare them into different bases, which may cost a lot of random numbers in practical applications. Here, we propose a simple decoy-state scheme with biased basis choices where the decoy pulses are only prepared in X basis. Through this way, it can save cost of random numbers and further simplify the electronic control system. Moreover, we carry out corresponding proof-of-principle demonstration. By incorporating with lower-loss asymmetric Mach–Zehnder interferometers and superconducting single-photon detectors, we can obtain a secret key rate of 1.65 kbps at 201 km and 19.5 bps at 280 km coiled optical fibers, respectively, showing very promising applications in future quantum communications.

Improving the performance of four-intensity decoy-state measurement-device-independent quantum key distribution via heralded pair-coherent sources

Measurement-device-independent quantum key distribution (MDI-QKD) can remove all the side-channel attacks and significantly improve the secure transmission distance. However, the key generation rate is relatively low when taking finite-key-size effect into consideration. In this manuscript, we adopt the latest four-intensity decoy-state scheme combining the heralded pair-coherent sources to study the performance of the MDI-QKD. Moreover, through utilizing joint constraints and collective error estimation techniques, we can obviously improve the performance of practical MDI-QKD systems compared with those using weak coherent sources or heralded single-photon sources.

Proof-of-Principle Demonstration of Passive Decoy-State Quantum Digital Signatures Over 200 km

The authors demonstrate that passive decoy-state quantum digital signature (QDS) using parametric down-conversion sources can be used effectively for encrypted communication over a distance of 200 km. With this method, the probability of leaking information to the eavesdropper is avoided, which improves security. The superiority of this system is due to its efficient passive decoy-state scheme and low-loss experimental system. The present work can also be extended to the recently proposed measurement-device-independent quantum digital signatures, and thus represents a significant step for QDSs on the path from the laboratory to practical applications.

A Simple Scheme for Realizing the Passive Decoy-State Quantum Key Distribution

In this paper, we present a scheme on realizing the passive decoy-state quantum key distribution. In this scheme, two weak coherent pulses interfere at a beam-splitter, and pulses in one output port are split by one beam-splitter into two parts which are sent into two local detectors individually. Then, all the clicking and nonclicking events from the local detectors are used to herald the arriving and nonarriving of the signal pulse in the other output port. Accordingly, we can obtain more decoy states in the photon-number space and can thus, achieve more precise estimations for single-photon contributions. In this scheme, we use only one intensity at each input of the beam-splitter to avoid intensity modulation fluctuations existing in other active decoy-state methods. We compare the present scheme with Curty et al. 's passive scheme and the conventional three-intensity decoy-state scheme with the weak coherent sources. Through numerical simulations, we demonstrate that this scheme is better than the other two methods.

Biased three-intensity decoy-state scheme on the measurement-device-independent quantum key distribution using heralded single-photon sources

At present, most of the measurement-device-independent quantum key distributions (MDI-QKD) are based on weak coherent sources and limited in the transmission distance under realistic experimental conditions, e.g., considering the finite-size-key effects. Hence in this paper, we propose a new biased decoy-state scheme using heralded single-photon sources for the three-intensity MDI-QKD, where we prepare the decoy pulses only in X basis and adopt both the collective constraints and joint parameter estimation techniques. Compared with former schemes with WCS or HSPS, after implementing full parameter optimizations, our scheme gives distinct reduced quantum bit error rate in the X basis and thus show excellent performance, especially when the data size is relatively small.

Obtaining better performance in the measurement-device-independent quantum key distribution with heralded single-photon sources

Measurement-device-independent quantum key distribution (MDI-QKD) has been widely investigated due to its remarkable advantages on the achievable transmission distance and practical security. However, the relative low key generation rate limits its real-life implementations. In this work, we adopt the newly proposed four-intensity decoy-state scheme [Phys. Rev. A 93, 042324 (2016)] to study the performance of MDI-QKD with heralded single-photon sources (HSPS). Corresponding simulation results demonstrate that the four-intensity decoy-state scheme combining HSPS can drastically improve both the key generation rate and transmission distance in MDI-QKD, which may be very promising in future MDI-QKD systems.

Round-Robin Differential Phase Shift with Heralded Single-Photon Source☆☆Supported by the National Basic Research Program of China under Grant No 2013CB338002, and the National Natural Science Foundation of China under Grant Nos 11304397 and 61505261.

Round-robin differential phase shift (RRDPS) is a novel quantum key distribution protocol which can bound information leakage without monitoring signal disturbance. In this work, to decrease the effect of the vacuum component in a weak coherent pulses source, we employ a practical decoy-state scheme with heralded single-photon source for the RRDPS protocol and analyze the performance of this method. In this scheme, only two decoy states are needed and the yields of single-photon state and multi-photon states, as well as the bit error rates of each photon states, can be estimated. The final key rate of this scheme is bounded and simulated over transmission distance. The results show that the two-decoy-state method with heralded single-photon source performs better than the two-decoy-state method with weak coherent pulses.

Improved key-rate bounds for practical decoy-state quantum-key-distribution systems

The decoy-state scheme is the most widely implemented quantum key distribution protocol in practice. In order to account for the finite-size key effects on the achievable secret key generation rate, a rigorous statistical fluctuation analysis is required. Originally, a heuristic Gaussian-approximation technique was used for this purpose, which, despite of its analytical convenience, was not sufficiently rigorous. The fluctuation analysis has recently been made rigorous by using the Chernoff bound. There is a considerable gap, however, between the key rate bounds obtained from these new techniques and that obtained from the Gaussian assumption. Here, we develop a tighter bound for the decoy-state method, which yields a smaller failure probability. This improvement results in a higher key rate and increases the maximum distance over which secure key exchange is possible. By optimizing the system parameters, our simulation results show that our new method almost closes the gap between the two previously proposed techniques and achieves a similar performance to that of conventional Gaussian approximations.

An enhanced proposal on decoy-state measurement device-independent quantum key distribution

By employing pulses involving three-intensity, we propose a scheme for the measurement device-independent quantum key distribution with heralded single-photon sources. We make a comparative study of this scheme with the standard three-intensity decoy-state scheme using weak coherent sources or heralded single-photon sources. The advantage of this scheme is illustrated through numerical simulations: It can approach very closely the asymptotic case of using an infinite number of decoy-states and exhibits excellent behavior in both the secure transmission distance and the final key generation rate.

Heralded single-photon sources for quantum-key-distribution applications

Single photon sources (SPS) are a fundamental building block for optical implementations of quantum information protocols. Among SPSs, multiple crystal heralded single photon sources seem to give the best compromise between high pair production rate and low multiple photon events. In this work, we study their performance in a practical quantum key distribution experiment, by evaluating the achievable key rates. The analysis focuses on the two different schemes, symmetric and asymmetric, proposed for the practical implementation of heralded single photon sources, with attention on the performance of their composing elements. The analysis is based on the protocol proposed by Bennett and Brassard in 1984 and on its improvement exploiting decoy state technique. Finally, a simple way of exploiting the post-selection mechanism for a passive, one decoy state scheme is evaluated.

Decoy-state quantum key distribution with biased basis choice.

We propose a quantum key distribution scheme that combines a biased basis choice with the decoy-state method. In this scheme, Alice sends all signal states in the Z basis and decoy states in the X and Z basis with certain probabilities, and Bob measures received pulses with optimal basis choice. This scheme simplifies the system and reduces the random number consumption. From the simulation result taking into account of statistical fluctuations, we find that in a typical experimental setup, the proposed scheme can increase the key rate by at least 45% comparing to the standard decoy-state scheme. In the postprocessing, we also apply a rigorous method to upper bound the phase error rate of the single-photon components of signal states.

Metropolitan all-pass and inter-city quantum communication network

We have demonstrated a metropolitan all-pass quantum communication network in field fiber for four nodes. Any two nodes of them can be connected in the network to perform quantum key distribution (QKD). An optical switching module is presented that enables arbitrary 2-connectivity among output ports. Integrated QKD terminals are worked out, which can operate either as a transmitter, a receiver, or even both at the same time. Furthermore, an additional link in another city of 60 km fiber (up to 130 km) is seamless integrated into this network based on a trusted relay architecture. On all the links, we have implemented protocol of decoy state scheme. All of necessary electrical hardware, synchronization, feedback control, network software, execution of QKD protocols are made by tailored designing, which allow a completely automatical and stable running. Our system has been put into operation in Hefei in August 2009, and publicly demonstrated during an evaluation conference on quantum network organized by the Chinese Academy of Sciences on August 29, 2009. Real-time voice telephone with one-time pad encoding between any two of the five nodes (four all-pass nodes plus one additional node through relay) is successfully established in the network within 60 km.