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Physical Review Applied

Polarization alignment in measurement-device-independent quantum key distribution with intrinsic events

Jia-Xuan Li, Guan‐Jie Fan‐Yuan, Shuang Wang, Ze-Hao Wang, Feng-Yu Lu, Zhen−Qiang Yin, Wei Chen, De‐Yong He, Guang‐Can Guo, Zheng‐Fu Han

Published Nov 20, 2023• 1 citation
Publisher

Abstract

Measurement-device-independent quantum key distribution (MDI QKD) is a crucial step for the quantum Internet, as it removes all detector side channels and enables secure QKD with untrusted nodes. However, MDI QKD faces a significant challenge from polarization alignment, since drift of the state of polarization affects the efficiency of key generation, especially when the quantum states are sent from remote locations. Conventional methods for polarization alignment either require additional resources, such as classical lasers or reference pulses, or interrupt the QKD process, which reduces the efficiency of the system. In this paper, an efficient polarization-alignment method in MDI QKD is proposed that leverages intrinsic events from MDI to directly calculate how to compensate for polarization drift. The proposed method does not require additional resources such as reference pulses, thereby eliminating bandwidth limitations, and it is fast, as it can directly calculate how to correct polarization with a small criterion fluctuation. We also introduce the polarization dimension in addition to the encoding dimension and establish a response-rate model for MDI QKD with additional polarization dimensions. Overall, the proposed method is a resource-efficient solution for polarization drift correction in MDI QKD, which can enhance the practicality and scalability of QKD networks.

DOI
10.1103/physrevapplied.20.054041

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Measurement-device-independent quantum key distribution (MDI QKD) is a crucial step for the quantum Internet, as it removes all detector side channels and enables secure QKD with untrusted nodes. However, MDI QKD faces a significant challenge from polarization alignment, since drift of the state of polarization affects the efficiency of key generation, especially when the quantum states are sent from remote locations. Conventional methods for polarization alignment either require additional resources, such as classical lasers or reference pulses, or interrupt the QKD process, which reduces the efficiency of the system. In this paper, an efficient polarization-alignment method in MDI QKD is proposed that leverages intrinsic events from MDI to directly calculate how to compensate for polarization drift. The proposed method does not require additional resources such as reference pulses, thereby eliminating bandwidth limitations, and it is fast, as it can directly calculate how to correct polarization with a small criterion fluctuation. We also introduce the polarization dimension in addition to the encoding dimension and establish a response-rate model for MDI QKD with additional polarization dimensions. Overall, the proposed method is a resource-efficient solution for polarization drift correction in MDI QKD, which can enhance the practicality and scalability of QKD networks.
Measurement-device-independent quantum key distribution (MDI-QKD) closes all the security loopholes in the detection system and is a promising solution for secret key sharing. Polarization encoding is the most common QKD encoding scheme, as it is straightforward to prepare and measure. However, implementing polarization encoding in MDI QKD imposes extra challenges, as polarization alignment must be maintained over both mutually unbiased bases and be maintained for both paths (Alice-Charlie and Bob-Charlie). Polarization alignment is usually done by interrupting the QKD process (reducing overall key generation rates) or using additional classical laser sources multiplexed with quantum channels for polarization alignment. Since low key rates and cost are the two most pressing challenges preventing wide adoption of QKD systems, using additional resources or reducing key rates runs contrary to making QKD commercially viable. Therefore, we propose and implement a novel polarization compensation scheme in the MDI-QKD system that avoids the aforementioned drawbacks by recycling part of discarded detection events. Our scheme evaluates the polarization drift in real-time based on single measurements corresponding to decoy intensities. Our fully automated experimental demonstration maintains the average polarization drift below 0.13 rad over 40 km of spooled fibre (without an insulating jacket) for at least four hours. The average quantum bit error rate is 3.8 % , and we achieved an average key rate of 7.45×10−6 bits per pulse.
Quantum key distribution is information-theoretically secure and a promising candidate for security against quantum computers. However, experimental implementations might deviate from assumptions made in the security proofs and leave the system vulnerable. Measurement device-independent QKD (MDI-QKD) scheme closes all the loopholes on the detection side, which is the most susceptible part of QKD. Therefore, the state preparation part of QKD systems becomes the only source of imperfections that needs to be considered in the security proofs of MDI-QKD. We examine our polarization encoding module and how imperfection in its components can affect state preparation. Our experiment demonstrates the feasibility of generating a secure key with imperfect states prepared by off-the-shelf devices. Another practical challenge that we address in implementing fibre-based MDI-QKD is polarization compensation. Polarization variations due to temperature fluctuations and fibre birefringence are inevitable and require polarization stabilization. Before this work, polarization compensation schemes for QKD either reduced the key sharing cycle or required extra equipment, making them less scalable or commercially viable and preventing the wide adaptation of polarization encoding. We propose and implement a novel polarization compensation scheme in the MDI-QKD systems that avoids the abovementioned drawbacks by using part of discarded detections. Our scheme evaluates the polarization drift in real-time based on single measurements corresponding to decoy intensities. We successfully implemented this active polarization compensation for four hours over 40 km of buffered fibre spool without thermal insulation or vibrational isolation. This thesis is a decisive step toward making polarization encoding MDI-QKD practical and secure, paving the way to realizing a polarization encoding MDI-QKD network. Future networks could be hybrid (ground and satellite links), and polarization encoding is a suitable candidate if an efficient stabilization scheme for fibre links is present. To this aim, our novel compensation scheme solely relies on recycling detections and retains a simple structure for a user node so that expanding the network could be feasible in terms of cost and practicality. Also, it does not reduce the key-sharing cycle, which is essential in networks where the detection system is shared, and each user has limited access time.
Measurement-device-independent quantum key distribution (MDI QKD) is a promising protocol for removing all detector side channel attacks. However, the variation of reference frames, e.g., polarization and phase reference, would be an acute threat to the performance of MDI systems. Here, based on polarization scrambling units, we demonstrate a reference-frame-independent MDI QKD scheme that is inherently stable against volatile channel conditions; therefore, the final secure key rate will be insensitive to the random disturbances of polarization and the drifts of phase reference. Thus, calibrations of the primary reference frames are intrinsically removed in our scheme, which essentially reduces potential vulnerabilities as well as the resource consumption of the whole system. In addition, a proof-of-principle experiment with an improved fluctuation analysis method is demonstrated to verify the feasibility and advantages of the proposed scheme.
Measurement-device-independent quantum key distribution (MDI-QKD) can be immune to all detector side-channel attacks and guarantee the information-theoretical security even with uncharacterized single photon detectors. MDI-QKD has been demonstrated in both laboratories and field-tests by using attenuated lasers combined with the decoy-state technique. However, it is a critical assumption that the sources used by legitimate participants are trusted in MDI-QKD. Hence, it is possible that a potential security risk exists. Here we propose a new scheme of polarization-encoding-based MDI-QKD with a single untrusted source, by which the complexity of the synchronization system can be reduced and the success rate of the Bell-state measurement can be improved. Meanwhile, the decoy-state method is employed to avoid the security issues introduced by a non-ideal single photon source. We also derive a security analysis of the proposed system. In addition, it seems to be a promising candidate for the implementation for QKD network in the near future.
Quantum key distribution (QKD) networks are promising to serve large numbers of users with information-theoretic secure communication. In QKD networks, the detection-safe protocol, termed measurement-device-independent (MDI) QKD, can naturally enhance realistic security by supporting untrusted measurement nodes. However, the environmental disturbances to quantum states degrade the performance of multi-user communication. Here we propose an MDI-QKD networking scheme with robustness against environmental disturbance and adaptability to multi-user access, where more than two users can generate keys simultaneously regardless of aligning reference frames and compensating channel disturbance on polarization. To achieve this, we introduce the reference-frame-independent protocol as well as a polarization-compensation-free method, design a multi-user measurement unit, and combine it with original two-user units. The scheme is experimentally demonstrated for the improvement of network robustness and adaptability in multi-user scenarios, and the time and device costs of disturbance compensation can be saved from O ( N ) to O ( 1 ) for an N-user network.
Measurement-device-independent quantum key distribution (MDI QKD) is a substantial step toward practical information-theoretic security for key sharing between remote legitimate users (Alice and Bob). As with other standard device-dependent quantum key distribution protocols, such as BB84, MDI QKD assumes that the reference frames have been shared between Alice and Bob. In practice, a nontrivial alignment procedure is often necessary, which requires system resources and may significantly reduce the secure key generation rate. Here, we propose a phase-coding reference-frame-independent MDI QKD scheme that requires no phase alignment between the interferometers of two distant legitimate parties. As a demonstration, a proof-of-principle experiment using Faraday-Michelson interferometers is presented. The experimental system worked at 1 MHz, and an average secure key rate of 8.309 bps was obtained at a fiber length of 20 km between Alice and Bob. The system can maintain a positive key generation rate without phase compensation under normal conditions. The results exhibit the feasibility of our system for use in mature MDI QKD devices and its value for network scenarios.