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.
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.
Quantum key distribution (QKD) is one of the most prominent methods for secure exchange of cryptographic keys between two users. The laws of physics provide it with an immense tool \ntowards secure communications. Although QKD has been proven to reach distances on the order of a few hundreds of kilometers, the transmission rate of the key significantly drops when we go \nto further distances. One possible solution to this is to build a network of trusted nodes. The trust requirement will however narrow its scope of deployability. In this thesis, we focus on improving the key rate performance of secure communications by introducing imperfect quantum memories (QMs) in a measurement-device-independent (MDI) QKD system. \n \nIn this thesis, a protocol with the potential of beating the existing distance records for conventional QKD systems is proposed. It borrows ideas from quantum repeaters by using memories in the middle of the link, and that of MDI-QKD, which only requires optical source equipment at the user’s end. For certain fast memories, our scheme allows a higher repetition rate than that of quantum repeaters, thereby requiring lower coherence times. By accounting for various sources of nonideality, such as memory decoherence, dark counts, misalignment errors, and background noise, as well as timing issues with memories, we develop a mathematical framework within which we can compare QKD systems with and without memories. In particular, we show that with the state-of-the-art technology for quantum memories, it is possible to devise memory-assisted QKD systems that, at certain distances of practical interest, outperform current QKD implementations. \n \nTo extend this work, we consider a suitable candidate that fullfils the requirements we have set for the QMs, i.e., the ensemble-based QMs. This type of memories, nevertheless, suffers from multiple-excitation effects, which can deteriorate the performance of the memory-assisted MDI-QKD system. As a solution we propose an alternative approach to the memory-assisted MDI-QKD by employing entangled-photon sources. We fully analyse this system by including modulation errors during the state-preparation at a single-photon source. We identify under which regimes of operation this system outperforms present QKD implementations. Overall we obtain a realistic account of what can be done with current technologies in order to improve the performance, in terms of rate versus distance, of QKD systems. Our findings can guide us toward implementing larger quantum networks.
Abstract Measurement-device-independent quantum key distribution (MDI QKD) provides inherent immunity against attacks targeting practical measurement devices. Existing MDI QKD protocols all rely on Bell-state measurements (BSM) to establish correlations between the users. However, the success probability of linear-optics BSM is limited to 50%, which severely restricts the achievable key rate of MDI QKD. We propose a high-capacity MDI QKD protocol with entanglement-assisted linear-optical BSM. This protocol has three advantages. First, compared with the original MDI QKD, at least a 25% increase in the key rate can be achieved. Second, simulation results show that this protocol can effectively increase the maximum photon transmission distance of MDI QKD from 262 km to 272 km. Third, this protocol is feasible in a fully linear-optical system under current experimental conditions. Our protocol provides a potential approach for improving the performance of future quantum communication networks.
The measurement-device-independent quantum key distribution (MDI QKD) can resist all attacks against measurement devices. However, its practical performance is still limited due to very low coincidence counting rates in Bell-state measurements. In this paper, we propose a practical memory-assisted MDI QKD protocol by cooperating the heralded single-photon sources and an all-optical quantum memory. By making use of the heralding characteristic of heralded single-photon sources, the earlier photon will be stored in the Sagnac optical quantum memory, and then released till another photon's arrival. Moreover, by combining with the decoy-state method, the present work exhibits significantly improved key rates and transmission distances compared with all existing MDI QKD protocols without using quantum memories. Therefore, this work can provide valuable references for practical implementation of long-distance quantum communications.
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 Measurement-device-independent quantum key distribution (MDI-QKD), a key protocol in quantum communication, can eliminates measurement-device vulnerabilities. In practical implementations, however, imperfect encoding modulation may introduce side-channel information leakage, while channel loss significantly reduces the coincidence count rate in Bell-state measurements, thereby limiting both the achievable secret key rate and transmission distance. To address these issues, we propose a memory-assisted MDI-QKD scheme with Hong-Ou-Mandel (HOM)-based source monitoring. The HOM-based monitoring mechanism enables source-side characterization of state-preparation imperfections and provides rigorous security constraints for secret key rate estimation, while the quantum memory enhances the effective two-photon interference probability and improves the success rate of Bell-state measurements. Numerical simulations show that the proposed scheme outperforms conventional source-monitoring-assisted MDI-QKD schemes without quantum memories in both secret key rate and transmission distance, highlighting its potential for practical long-distance quantum communication.