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Advanced Quantum Technologies

Practical Long‐Distance Measurement‐Device‐Independent Quantum Key Distribution By Four‐Intensity Protocol

Xiao‐Long Hu, Cong Jiang, Zong‐Wen Yu, Xiang‐Bin Wang

Published Oct 28, 2021• 12 citations
Publisher

Abstract

Abstract Quantum key distribution (QKD) can provide unconditionally secure private communication between two remote parties. The measurement‐device‐independent (MDI) QKD can remove all security loopholes in measurement devices. However, the large value of the observed error rate in X basis has severely limited its key rate. The four‐intensity protocol with fully optimized post data processing can improve the key rate and distance of MDI‐QKD effectively. In this paper, the four‐intensity MDI‐QKD protocol is reviewed. In the four‐intensity protocol, each of Alice and Bob uses one intensity in Z basis and three intensities in X basis (normally including one vacuum). This improves the number of effective bits in Z basis. The joint mathematical treatment for statistical fluctuation and the parameter scan pointing directly to the final key rate are applied to reduce the effect of statistical fluctuation. The double scanning method further improves the key rate and the secure distance of the four‐intensity protocol. These have greatly improved the performance of the protocol with finite‐key effects. So far, many MDI‐QKD experiments are based on the four‐intensity protocol. The successful results of these experiments have clearly shown the superiority of the four‐intensity protocol in practical long‐distance MDI‐QKD.

DOI
10.1002/qute.202100069

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Abstract Quantum key distribution (QKD) can provide unconditionally secure private communication between two remote parties. The measurement‐device‐independent (MDI) QKD can remove all security loopholes in measurement devices. However, the large value of the observed error rate in X basis has severely limited its key rate. The four‐intensity protocol with fully optimized post data processing can improve the key rate and distance of MDI‐QKD effectively. In this paper, the four‐intensity MDI‐QKD protocol is reviewed. In the four‐intensity protocol, each of Alice and Bob uses one intensity in Z basis and three intensities in X basis (normally including one vacuum). This improves the number of effective bits in Z basis. The joint mathematical treatment for statistical fluctuation and the parameter scan pointing directly to the final key rate are applied to reduce the effect of statistical fluctuation. The double scanning method further improves the key rate and the secure distance of the four‐intensity protocol. These have greatly improved the performance of the protocol with finite‐key effects. So far, many MDI‐QKD experiments are based on the four‐intensity protocol. The successful results of these experiments have clearly shown the superiority of the four‐intensity protocol in practical long‐distance MDI‐QKD.
Abstract The data post-processing scheme based on two-way classical communication(TWCC) can improve the tolerable bit error rate and expand the secure transmission distance when used in QKD system. In this paper, we demonstrate a significant improvement in key rate and secure transmission distance when two-way classical communication is applied in MDI-QKD protocol. Additionally, to further improve the performance of the MDI-QKD system, we extend the two-way classical communication to MDI-QKD considering statistical fluctuations. Our simulation results show that the two-way classical communication method has great advantages in improving the performance of the actual MDI-QKD system.
Measurement-device-independent quantum key distribution (MDI-QKD) protocol can remove all the loopholes of the detection devices and, thus, has attracted much attention. Based on the technique of single-photon interference, we propose a modified MDI-QKD protocol with phase post-selection. We prove the security of the announcement of the private phases in the X basis and show how to apply the phase post-selection method to the double-scanning four-intensity MDI-QKD protocol. The numerical results show that the phase post-selection method can significantly improve the key rates at all distances. In the double-scanning method, two parameters need to be scanned in the calculation of the final key rate, and the global parameter optimization is pretty time-consuming. We propose an accelerated method that can greatly reduce the running time of the global parameter optimization program. This makes the method practically useful in an unstable channel.
We propose a long distance measurement-device-independent (MDI) quantum-key-distribution (QKD) with quantum memory, and analyze the relationship between the key generation rate and the storage efficiency of quantum memory. Our protocol is considered and compared with MDI-QKD without quantum memory. We present general formulas for our protocol with three-intensity decoy states. The simulation results show that the maximum secure distance supported by MDI-QKD with quantum memory is about 500 km, while the maximum secure distance of MDI-QKD without quantum memory is only 216 km. With certain limits, prolonging the time of maintaining the necessary quantum fidelity can increase security key transmission distance. Furthermore, the protocol is robust against device imperfection such as quantum memory decoherence effects, which can be easily applied to practical QKD system.
Measurement-device-independent quantum key distribution (MDI-QKD) protocol can effectively resist all possible attacks targeting the measurement devices in a quantum key distribution (QKD) system, thus exhibiting high security. However, due to the protocol’s high sensitivity to channel attenuation, its key generation rate and transmission distance are significantly limited in practical applications. To improve the performance of MDI-QKD, researchers have proposed quantum-memory (QM)-assisted MDI-QKD protocol, which has enhanced the protocol's performance to a certain extent. Nevertheless, under finite-size conditions where the total number of transmitted pulses is limited, accurately estimating the relevant statistical parameters is still a challenge. As a result, existing QM-assisted MDI-QKD schemes still encounters issues such as low key rates and limited secure transmission distances. To solve these problems, this work proposes a novel improved finite-size QM-assisted MDI-QKD protocol. By utilizing quantum memories to temporarily store early-arriving pulses and release them synchronously, the protocol effectively reduces the influence caused by channel asymmetry. Additionally, the protocol introduces a four-intensity decoy-state method to improve the estimation accuracy of single-photon components. Meanwhile, to mitigate the influence of finite-length effects on QM schemes, the proposed protocol combines a collective constraint model and a double-scanning algorithm to jointly estimate scanning error counts and vacuum-related counts. This approach enhances the estimation accuracy of the single-photon detection rate and phase error rate under finite-size conditions, thereby significantly improving the secure key rate of the MDI-QKD system. Simulation results show that under the same experimental conditions, compared with the existing QM-assisted three-intensity decoy-state MDI-QKD protocol and the four-intensity decoy-state MDI-QKD protocol based on Heralded Single-photon Source (HSPS), the proposed protocol extends the secure transmission distance by more than 30 km and 100 km, respectively. This proves that under the same parameter settings, the proposed scheme exhibits significant advantages in both key rate and secure transmission distance. Therefore, this research provides important theoretical references and valuable benchmarks for developing long-distance, high-security quantum communication networks.
Abstract Statistical fluctuations are unavoidable in realistic quantum key distribution (QKD) due to finite-size effect. Based on the four-intensity proposal on measurement-device-independent QKD (MDI-QKD) in [Phys. Rev. A 93 (2016) 042324], we particularly analyze the scenario that only three intensities are used, namely a three-intensity decoy-state MDI-QKD with biased basis choice. After performing full parameter optimization method, simulations results demonstrate that this scenario can obtain distinct enhancement compared with the conventional unbiased three-intensity decoy-state method, e.g. Xu et al.’s [Phys. Rev. A 89 (2014) 052333]. Furthermore, results also show that it works more efficiently by using HSPS than using WCS at longer transmission distance.
The length of signal pulses is finite in practical quantum key distribution. The finite-key analysis of an unconditional quantum key distribution is a burning problem, and the efficient quantum key distribution protocol suitable for practical implementation, measurement-device-independent quantum key distribution (MDI QKD), was proposed very recently. We give the finite-key analysis of MDI QKD, which removes all detector side channels and generates many orders of key rate higher than that of full-device-independent quantum key distribution. The secure bound of the ultimate key rate is obtained under the statistical fluctuations of relative frequency, which can be applied directly to practical threshold detectors with low detection efficiency and highly lossy channels. The bound is evaluated for reasonable values of the observed parameters. The simulation shows that the secure distance is around 10 km when the number of sifted data is 1010{10}^{10}1010. Moreover the secure distance would be much longer in practice because of some simplified treatments used in our paper.