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arXiv:1208.0609 · New Journal of Physics

Studying free-space transmission statistics and improving free-space quantum key distribution in the turbulent atmosphere

Chris Erven, Bettina Heim, Evan Meyer-Scott, J P Bourgoin, Raymond Laflamme, Gregor Weihs, Thomas D. Jennewein

Published Dec 11, 2012• 86 citations
arXivHTMLPDF

Abstract

The statistical fluctuations in free-space links in the turbulent atmosphere are important for the distribution of quantum signals. To that end, we first study statistics generated by the turbulent atmosphere in an entanglement-based free-space quantum key distribution (QKD) system. Using the insights gained from this analysis, we study the effect of link fluctuations on the security and key generation rate of decoy state QKD concluding that it has minimal effect in the typical operating regimes. We then investigate the novel idea of using these turbulent fluctuations to our advantage in QKD experiments. We implement a signal-to-noise ratio filter (SNRF) in our QKD system which rejects measurements during periods of low transmission efficiency, where the measured quantum bit error rate is temporarily elevated. Using this, we increase the total secret key generated by the system from 78 009 bits to 97 678 bits, representing an increase of 25.2% in the final secure key rate, generated from the same raw signals. Lastly, we present simulations of a QKD exchange with an orbiting low earth orbit satellite and show that an SNRF will be extremely useful in such a situation, allowing many more passes to extract a secret key than would otherwise be possible.

DOI
10.1088/1367-2630/14/12/123018

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The statistical fluctuations in free-space links in the turbulent atmosphere are important for the distribution of quantum signals. To that end, we first study statistics generated by the turbulent atmosphere in an entanglement-based free-space quantum key distribution (QKD) system. Using the insights gained from this analysis, we study the effect of link fluctuations on the security and key generation rate of decoy state QKD concluding that it has minimal effect in the typical operating regimes. We then investigate the novel idea of using these turbulent fluctuations to our advantage in QKD experiments. We implement a signal-to-noise ratio filter (SNRF) in our QKD system which rejects measurements during periods of low transmission efficiency, where the measured quantum bit error rate is temporarily elevated. Using this, we increase the total secret key generated by the system from 78 009 bits to 97 678 bits, representing an increase of 25.2% in the final secure key rate, generated from the same raw signals. Lastly, we present simulations of a QKD exchange with an orbiting low earth orbit satellite and show that an SNRF will be extremely useful in such a situation, allowing many more passes to extract a secret key than would otherwise be possible.
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Free-space quantum cryptography plays an integral role in realizing a global-scale quantum internet system. Compared to fiber-based communication networks, free-space networks experience significantly less decoherence and photon loss due to the absence of birefringent effects in the atmosphere. However, the atmospheric turbulence contributes to deviation in transmittance distribution, which introduces noise and channel loss. Several methods have been proposed to overcome the low signal-to-noise ratio. Active research is currently focused on establishing secure and practical quantum communication in a high-loss channel, and enhancing the secure key rate by implementing bit rejection strategies when the channel transmittance drops below a certain threshold. By simulating the atmospheric turbulence using an acousto-optical-modulator (AOM) and implementing the prefixed-threshold real-time selection (P-RTS) method, our group performed finite-size decoy-state Bennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocol for 19 dB channel loss. With better optical calibration and efficient superconducting nano-wire single photon detector (SNSPD), we have extended our previous work to 40 dB channel loss characterizing the transmittance distribution of our system under upper moderate turbulence conditions.
There are substantial problems in sending quantum information safely over free-space channels because of air turbulence, diffraction, beam divergence, and noise. This study offers an extensive performance evaluation of quantum computing-based security methods utilised in free-space communication systems, specifically emphasising Quantum Secure Direct Communication (QSDC) and Quantum Key Distribution (QKD) protocols. The purpose of this work is to examine the attainable secret-key generation rates, assess the robustness of quantum security protocols in practical free-space environments, and contrast their effectiveness in preventing eavesdropping and signal degradation. This research employs quantum optics, turbulence theory, and quantum information theory to examine the performance constraints of continuous-variable (CV) coherent-state protocols amid fluctuating turbulence intensities. The suggested analytical approach simulates both fading and non-fading atmospheric channels to calculate essential rates and quantum bit error rates (QBER) for free-space quantum secure direct communication (QSDC) systems. Experimental findings and simulation outcomes indicate that free-space quantum channels can provide high-rate and long-distance secure communication when optimized using pilot-guided coherent-state protocols. The review shows how QSDC is better than regular QKD since it lets messages be sent directly without pre-shared keys. It also talks about how useful it may be for secure communication networks between satellites and between satellites and the earth. The paper closes by delineating the primary research deficiencies and suggesting avenues for improving protocol efficiency, error rectification, and turbulence mitigation in forthcoming global quantum communication systems.
We investigate the performance of relativistic quantum key distribution (RQKD) over turbulent free-space optical channels for both single-photon and coherent-state implementations. A theoretical model is developed to describe the effect of atmospheric turbulence on interferometric visibility and secret-key generation by incorporating turbulence-induced phase fluctuations into the key-rate analysis. Numerical simulations are performed under realistic channel conditions to evaluate the achievable communication distance and secret-key rate. The results show that atmospheric turbulence degrades the protocol performance by reducing interference visibility, with the severity of the degradation determined by the ratio between the interferometer-arm separation rrr (or temporal separation of signal and reference states ΔTΔTΔT) and Fried's parameter. The single-photon protocol remains operational over all investigated turbulence regimes, whereas the coherent-state implementation exhibits a critical dependence on the interferometer geometry (especially on ΔTΔTΔT), leading to a threshold beyond which secure key generation is no longer possible. These findings quantify the impact of atmospheric turbulence on relativistic quantum key distribution and provide practical guidelines for the design and optimization of free-space RQKD systems.
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.
Abstract Quantum Key Distribution (QKD) provides information-theoretic security for future communication networks by exploiting the fundamental principles of quantum mechanics. However, satellite-to-ground QKD links suffer significant performance degradation due to atmospheric turbulence, absorption, scattering, and satellite-induced pointing errors. This paper investigates the BB84 QKD protocol over a Free-Space Optical (FSO) channel modeled using the Gamma–Gamma distribution, which accurately captures both small-scale and large-scale atmospheric irradiance fluctuations. Closed-form and numerical results for outage probability and Quantum Bit Error Rate (QBER) are presented under weak, moderate, and strong turbulence regimes. The Secure Key Rate (SKR) is analyzed as a function of link distance under clear and foggy weather conditions. Additionally, the impact of satellite vibration is evaluated using a jitter-dependent pointing error model, demonstrating that misalignment significantly reduces link reliability. Simulation results show that strong turbulence conditions introduce more than 15 dB SNR penalty compared to clear-sky operation, while haze conditions limit QKD range to under 8–10 km. These findings provide practical performance benchmarks for Low-Earth Orbit (LEO) quantum downlinks in realistic atmospheric environments. The results provide practical insights for electronic receiver design and system implementation of satellite quantum communication links.