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arXiv:2204.08011 · Physical Review A

Dynamic attenuation scheme in measurement-device-independent quantum key distribution over turbulent channels

Brian J. Rollick, George Siopsis, Bing Qi

Published Sep 2, 2022• 5 citations
arXivHTMLPDF

Abstract

Measurement-device-independent quantum key distribution (MDI QKD) offers great security in practice because it removes all detector side channels. However, conducting MDI QKD over free-space channels is challenging. One of the largest culprits is the mismatched transmittance of the two independent turbulent channels causing a reduced Hong-Ou-Mandel visibility and thus a lower secret key rate. Here we introduce a dynamic attenuation scheme, where the transmittance of each of the two channels is monitored in real time by transmitting bright light pulses from each users to the measurement device. Based on the measured channel transmittance, a suitable amount of attenuation is introduced to the low-loss channel at the measurement device. Our simulation results show a significant improvement of QKD performance, especially when using short raw keys.

DOI
10.1103/physreva.106.032405

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Measurement-device-independent quantum key distribution (MDI QKD) offers great security in practice because it removes all detector side channels. However, conducting MDI QKD over free-space channels is challenging. One of the largest culprits is the mismatched transmittance of the two independent turbulent channels causing a reduced Hong-Ou-Mandel visibility and thus a lower secret key rate. Here we introduce a dynamic attenuation scheme, where the transmittance of each of the two channels is monitored in real time by transmitting bright light pulses from each users to the measurement device. Based on the measured channel transmittance, a suitable amount of attenuation is introduced to the low-loss channel at the measurement device. Our simulation results show a significant improvement of QKD performance, especially when using short raw keys.
Real-world Bennett-Brassard 1984 quantum key distribution (QKD) systems utilize imperfect devices that introduce vulnerabilities to their security, known as side-channel attacks. Measurement-device-independent (MDI) QKD authorizes an untrusted third party to make measurements and removes all side-channel attacks. The typical implementations of MDI QKD employ nearly symmetric channels which are difficult to realize physically in many practical scenarios such as when asymmetric channel losses are present, normally a consequence of the communication environment. Maritime and satellite-based communications are two such instances in which the channels are characterized by continuously changing losses in different channels. In this work we perform asymmetric MDI QKD in a laboratory environment with simulated turbulence using an acousto-optic modulator to interrogate the performance of free-space quantum communication. Under turbulent conditions, scattering and beam wandering cause intensity fluctuations which decrease the detected signal-to-noise ratio. Using the seven-intensity optimization method proposed by Wang et al. [Phys. Rev. X 9, 041012 (2019)], coupled with prefixed-threshold real-time selection (PRTS), we demonstrate enhancement in the secure key rate under turbulent conditions for finite-size decoy-state MDI QKD. Furthermore, we show that PRTS can yield higher secure key rates, particularly in the high-loss regime.
Measurement-Device-Independent (MDI) QKD eliminates detector side channels in QKD and allows an untrusted relay between two users. A desirable yet highly challenging application is to implement MDI-QKD through free-space channels. One of the major factors that affect the secure key rate in free-space MDI-QKD is atmospheric turbulence. In this work we show two important results: First, the independent fluctuations of transmittances in the two channels can significantly reduce MDI-QKD performance due to turbulence-induced channel asymmetry. Second, we consider the Prefixed Real-Time Selection (P-RTS) method we formerly introduced to BB84 and extend it to MDI-QKD. Users can monitor classical transmittances in their channels and improve performance by post-selecting signals in real-time based on pre-calculated thresholds. We show that we can establish a 2-dimensional threshold between Alice and Bob to post-select signals with both high signal-to-noise ratio and low channel asymmetry in real time, and greatly extend the maximum range of MDI-QKD in the presence of turbulence, which can be an important step towards future free-space MDI-QKD experiments.
Free-space quantum key distribution (QKD) allows two distant parties to share secret keys with information-theoretic security, which can pave the way for satellite-ground quantum communication to set up a global network for sharing secret message. However, free-space channels in the presence of atmospheric turbulence are affected by losses and fluctuating transmissivity which further affect the quantum bit error rate and the secure key rate. To implement free-space QKD, it is indispensable to study the effect of atmospheric turbulence. Different models have been used to describe the probability distribution for channel transmission coefficient under atmospheric turbulence, including the log-normal distribution and K distribution. In this paper, we focus on free space measurement-device-independent quantum key distribution (MDI-QKD) under K-distributed strong atmospheric turbulence. The MDI-QKD can close all loopholes on detection and achieve a similar performance to QKD, relying on time-reversed version of entanglement-based QKD protocol. Threshold post-selection method is adopted to restrain detrimental effects of the atmospheric turbulence, which is based on the selection of the intervals with higher channel transmissivity. By combining the general MDI-QKD system model with this method, we present a framework for the optimal choice of threshold. Our simulation result shows that the optimal threshold is dependent on the turbulence intensity and expected channel loss. Furthermore, compared with the original MDI-QKD protocols, the proposed protocol with threshold post-selection method can acquire a considerable better performance in key rate, especially in regions of high turbulence and high loss. What is more, this is instructive to the building of a practical free-space MDI-QKD system with better performance.
Measurement-device-independent quantum key distribution (MDI-QKD) can eliminate all detector side channels and it is practical with current technology. Previous implementations of MDI-QKD all used two symmetric channels with similar losses. However, the secret key rate is severely limited when different channels have different losses. Here we report the results of the first high-rate MDI-QKD experiment over asymmetric channels. By using the recent 7-intensity optimization approach, we demonstrate>10×higher key rate than the previous best-known protocols for MDI-QKD in the situation of large channel asymmetry, and extend the secure transmission distance by more than 20-50 km in standard telecom fiber. The results have moved MDI-QKD towards widespread applications in practical network settings, where the channel losses are asymmetric and user nodes could be dynamically added or deleted.
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.