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arXiv:2409.05802 · IEEE Transactions on Quantum Engineering

Differential Phase Encoded Plug-and-Play Measurement-Device-Independent Quantum Key Distribution

Nilesh Sharma, Shashank Kumar Ranu, Prabha Mandayam, Anil Prabhakar

Published Jan 1, 2026• 1 citation
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Abstract

Measurement-Device-Independent Quantum Key Distribution (MDI-QKD) enhances security by removing vulnerabilities associated with detector side channels. Real-world implementations of measurement device independent (MDI) QKD face practical challenges such as channel asymmetry and physical imperfections, which degrade the visibility of Hong-Ou-Mandel (HOM) interference, an essential factor in determining the secure key rate. In this work, we evaluate the performance of differential phase encoded (DPE) MDI-QKD under realistic, asymmetric conditions. We analyse the effects of polarisation and pulse-width mismatches caused by channel asymmetry and identify tolerable limits for secure key generation. We introduce a plug-and-play DPE-MDI-QKD architecture that mitigates polarisation fluctuations using Faraday mirrors and also compensates for pulse-width mismatches. Furthermore, we propose an improved sifting scheme that raises the sifting efficiency from$\frac{4}{9}$to$\frac{2}{3}$, improving the secure key generation rates by 50% and making it especially suitable for plug-and-play systems. These results offer practical insights into enhancing the robustness and efficiency of DPE-MDI-QKD, enabling more secure quantum communication over imperfect and asymmetric quantum channels.

DOI
10.1109/tqe.2026.3665005

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Measurement-Device-Independent Quantum Key Distribution (MDI-QKD) enhances security by removing vulnerabilities associated with detector side channels. Real-world implementations of measurement device independent (MDI) QKD face practical challenges such as channel asymmetry and physical imperfections, which degrade the visibility of Hong-Ou-Mandel (HOM) interference, an essential factor in determining the secure key rate. In this work, we evaluate the performance of differential phase encoded (DPE) MDI-QKD under realistic, asymmetric conditions. We analyse the effects of polarisation and pulse-width mismatches caused by channel asymmetry and identify tolerable limits for secure key generation. We introduce a plug-and-play DPE-MDI-QKD architecture that mitigates polarisation fluctuations using Faraday mirrors and also compensates for pulse-width mismatches. Furthermore, we propose an improved sifting scheme that raises the sifting efficiency from49\frac{4}{9}94​to23\frac{2}{3}32​, improving the secure key generation rates by 50% and making it especially suitable for plug-and-play systems. These results offer practical insights into enhancing the robustness and efficiency of DPE-MDI-QKD, enabling more secure quantum communication over imperfect and asymmetric quantum channels.
We have implemented a practical plug-and-play measurement-device-independent quantum key distribution (MDI QKD) system with polarization division multiplexing (PDM). MDI QKD is known as a secure protocol that can inherently prevent detection loopholes. Significant efforts have been made toward implementing MDI QKD systems. Recently, a plug-and-play architecture has been proposed for implementing the MDI QKD system, and its feasibility has been experimentally verified in both free space and fiber channel. However, in order to apply it to the real world, it is necessary to develop a more practical architecture including multiplexing methods and self-compensation techniques. In this paper, we have proposed and implemented a practical plug-and-play MDI QKD architecture that can be implemented regardless of whether the quantum channel lengths of Alice and Bob are symmetric or asymmetric using PDM and can be operated even under ambient-temperature-changing environments through an optical path-length self-compensation technique. Experimentally, we have achieved6.25×10−66.25\times 10^{-6}6.25×10−6bits per pulse as the key rate and the quantum bit error rates under 3% on 25-km quantum channels.
This paper proposes a measurement-device-independent quantum key distribution (MDI-QKD) scheme based on differential phase encoding. The differential phase shift MDI-QKD (DPS-MDI-QKD) couples the advantages of DPS-QKD with that of MDI-QKD. The proposed scheme offers resistance against photon number splitting attack and phase fluctuations as well as immunity against detector side-channel vulnerabilities. The design proposed in this paper uses weak coherent pulses in a superposition of three orthogonal states, corresponding to one of three distinct paths in a delay-line interferometer. The classical bit information is encoded in the phase difference between pulses traversing successive paths. This 3-pulse superposition offers enhanced security compared to using a train of pulses by decreasing the learning rate of an eavesdropper and unmasking her presence with an increased error rate upon application of intercept and resend attack and beamsplitter attack. The proposed scheme employs phase locking of the sources of the two trusted parties so as to maintain the coherence between their optical signal, and uses a beamsplitter (BS) at the untrusted node (Charlie) to extract the key information from the phase encoded signals.
The ideal measurement-device-independent quantum key distribution (MDI-QKD), which is immune to all detector side-channel attacks, is built on the Bell state measurement of two single-photon states. However, in practical MDI-QKD where phase-randomized weak coherent pulses (PR-WCPs) are used, the mismatch between states preparation and measurement in X basis leads to the bit error rate of more than 25%, which poses a challenge to the security of QKD. In this paper, we provide a security analysis of practical MDI-QKD based on polarization coding. We analyze the Hong-Ou-Mandel interference of PR-WCPs and Poisson-distributed photon number states (PNs), and find that these two sources are equivalent in MDI-QKD. The security analysis based on entanglement distillation is carried out on PNs, and the measurement results in Z basis and X basis are averaged to overcome the noncorrelation of the error rates in these two bases. Compared with GLLP scheme, a tighter key rate is obtained in this work, and the key rate deviates from the linear key rate bound in a short distance due to the finite proportion of multiphoton terms.
Quantum key distribution (QKD) can serve large numbers of users with unconditional secure communication, in which measurement-device-independent (MDI) QKD protocol can conveniently enhance realistic security by eliminating all detector side channels. In multi-user scenarios, MDI-QKD networks should meet the requirements of reliability and robustness. Around the issues, we complete a series of work on performance improvement, low-cost deployment, and robust networking. First is to answer how to change the trusted node to the untrusted node with low cost, here we propose and demonstrate an MDIQKD which is compatible with phase encoding BB84 networks, allowing for economical deployment of MDI-QKD networks and reconciliation between high key rate and high realistic security in the same network. Then we make the MDI networks more robust. We design an MDI-QKD networking scheme for multi-user access where more than two users can generate keys synchronously regardless of aligning reference frames and compensating channel disturbance on polarization. Third, we find a general method to distil secure keys from quantum discord, which is different to the previous entanglement-based frameworks of MDI-QKD, allowing imperfect preparation and measurement of quantum states. The series of work can help the practical application of MDI-QKD networks, and promote the development of quantum networks.
Measurement-device-independent quantum key distribution (MDI-QKD) is immune to all security loopholes on detection. Previous experiments on MDI-QKD required spatially separated signal lasers and complicated stabilization systems. In this paper, we perform a proof-of-principle experimental demonstration of plug-and-play MDI-QKD over an asymmetric channel setting with a single signal laser in which the whole system is automatically stabilized in spectrum, polarization, arrival time, and phase reference. Both the signal laser and the single-photon detectors are in the possession of a common server. A passive timing-calibration technique is applied to ensure the precise and stable overlap of signal pulses. The results pave the way for the realization of a quantum network in which the users only need the encoding devices.
A recently proposed fully passive quantum key distribution (QKD) removes all source modulator side channels. In this work, we combine fully passive sources with measurement-device-independent (MDI) QKD to simultaneously remove side channels from source modulators and detectors. We show a numerical simulation of passive MDI QKD, and we obtain an acceptable key rate while achieving much better implementation security, as well as ease of implementation, compared with a recently proposed fully passive twin-field QKD, paving the way towards more secure and practical QKD systems. We prove that a fully passive protocol is compatible with MDI QKD and we propose a novel idea that can improve the sifting efficiency.