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Optical Fiber Communication Conference

MDI-QKD with resource-efficient polarization compensation

Olinka Bedroya, Chenyang Li, Wenyuan Wang, Jianyong Hu, Hoi‐Kwong Lo, Li Qian

Published Jan 1, 2023• 1 citation
Publisher

Abstract

We implemented MDI-QKD with a novel polarization compensation scheme using discarded bits without reducing the key-sharing cycle or demanding additional resources. Polarization drift was maintained below 0.13 rad over a 40 km unisolated fibre spool for four hours, and the average secret key rate generated was 7.45 ×10 − 6 bits per pulse.

DOI
10.1364/ofc.2023.th3j.3

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We implemented MDI-QKD with a novel polarization compensation scheme using discarded bits without reducing the key-sharing cycle or demanding additional resources. Polarization drift was maintained below 0.13 rad over a 40 km unisolated fibre spool for four hours, and the average secret key rate generated was 7.45 ×10 − 6 bits per pulse.
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 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.