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IEEE photonics journal

Secret Key Rates and Optimization of BB84 and Decoy State Protocols Over Time-Varying Free-Space Optical Channels

Xiaole Sun, Ivan B. Djordjević, Mark Allen Neifeld

Published May 17, 2016• 36 citations
Publisher

Abstract

We optimize secret key rates (SKRs) of weak coherent pulse (WCP)-based quantum key distribution (QKD) over time-varying free-space optical channels affected by atmospheric turbulence. The random irradiance fluctuation due to scintillation degrades the SKR performance of WCP-based QKD, and to improve the SKR performance, we propose an adaptive scheme in which transmit power is changed in accordance with the channel state information. We first optimize BB84 and decoy state-based QKD protocols for different channel transmittances. We then present our adaptation method, to overcome scintillation effects, of changing the source intensity based on channel state predictions from a linear autoregressive model while ensuring the security against the eavesdropper. By simulation, we demonstrate that by making the source adaptive to the time-varying channel conditions, SKRs of WCP-based QKD can be improved up to over 20%.

DOI
10.1109/jphot.2016.2570000

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We optimize secret key rates (SKRs) of weak coherent pulse (WCP)-based quantum key distribution (QKD) over time-varying free-space optical channels affected by atmospheric turbulence. The random irradiance fluctuation due to scintillation degrades the SKR performance of WCP-based QKD, and to improve the SKR performance, we propose an adaptive scheme in which transmit power is changed in accordance with the channel state information. We first optimize BB84 and decoy state-based QKD protocols for different channel transmittances. We then present our adaptation method, to overcome scintillation effects, of changing the source intensity based on channel state predictions from a linear autoregressive model while ensuring the security against the eavesdropper. By simulation, we demonstrate that by making the source adaptive to the time-varying channel conditions, SKRs of WCP-based QKD can be improved up to over 20%.
Free-space optical (FSO) channels can be characterized by random power fluctuations due to atmospheric turbulence, which is known as scintillation. Weak coherent source based FSO quantum key distribution (QKD) systems suffer from the scintillation effect because during the deep channel fading the expected detection rate drops, which then gives an eavesdropper opportunity to get additional information about protocol by performing photon number splitting (PNS) attack and blocking single-photon pulses without changing QBER. To overcome this problem, in this paper, we study a large-alphabet QKD protocol, which is achieved by using pulse-position modulation (PPM)-like approach that utilizes the time-frequency uncertainty relation of the weak coherent photon state, called here TF-PPM-QKD protocol. We first complete finite size analysis for TF-PPM-QKD protocol to give practical bounds against non-negligible statistical fluctuation due to finite resources in practical implementations. The impact of scintillation under strong atmospheric turbulence regime is studied then. To overcome the secure key rate performance degradation of TF-PPM-QKD caused by scintillation, we propose an adaptation method for compensating the scintillation impact. By changing source intensity according to the channel state information (CSI), obtained by classical channel, the adaptation method improves the performance of QKD system with respect to the secret key rate. The CSI of a time-varying channel can be predicted using stochastic models, such as autoregressive (AR) models. Based on the channel state predictions, we change the source intensity to the optimal value to achieve a higher secret key rate. We demonstrate that the improvement of the adaptation method is dependent on the prediction accuracy.
Abstract Free-space ground-to-ground links will be an integral part of future quantum communication networks. The implementation of free-space and fiber links in daylight inter-modal configurations is however still hard to achieve, due to the impact of atmospheric turbulence, which strongly decreases the coupling efficiency into the fiber. In this work, we present a comprehensive model of the performance of a free-space ground-to-ground quantum key distribution (QKD) system based on the efficient-BB84 protocol with active decoy states. Our model takes into account the atmospheric channel contribution, the transmitter and receiver telescope design constraints, the parameters of the quantum source and detectors, and the finite-key analysis to produce a set of requirements and optimal design choices for a QKD system operating under specific channel conditions. The channel attenuation is calculated considering all effects deriving from the atmospheric propagation (absorption, beam broadening, beam wandering, scintillation, and wavefront distortions), as well as the effect of fiber-coupling in the presence of a partial adaptive optics correction with finite control bandwidth. We find that the channel fluctuation statistics must be considered to correctly estimate the effect of the saturation rate of the single-photon detectors, which may otherwise lead to an overestimation of the secret key rate. We further present strategies to minimize the impact of diffuse atmospheric background in daylight operation by means of spectral and temporal filtering.
This paper presents a new adaptive QKD system with a Genetic Algorithm optimizer. The issue of security in modern wireless network is of alarming significance, especially to freespace optical connections in which the dynamics in the environment are hazardous with regard to confidentiality. Quantum Key Distribution (QKD) is now available over air channels and can be used to secure a key exchange but it is highly impaired by atmospheric turbulence, beam misalignment, background radiation, path loss, and scintillation and pointing errors. These weaknesses elevate Quantum Bit Error Rate (QBER) and slow down secret key rate, making it impractical to deploy. In this paper, we present a new adaptive QKD system with a Genetic Algorithm (GAD) optimizer adjusting decoy intensities, the probability of choice of a basis in selecting a detector, detector threshold, and privacy amplification rates dynamically. The active control of channels under different atmospheric conditions by continuous monitoring of channels with pilot frames and the QBER feedback allows intelligent parameter reconfiguration. The performance of the simulation using various turbulence regimes shows that the proposed adaptive one boosts the average secret key rate by The simulation results show a 25-40% improvement in secret key rate, reduced QBER, and increased secure transmission range as well as the range and the resilience of transmission, which confirm the efficiency and practical importance of the proposed adaptation technique.
In our recent paper, we proposed using the atmospheric turbulent channel as a source of randomness to simultaneously address various challenges in physical-layer security (PLS), quantum key distribution (QKD), and post-quantum cryptography (PQC). However, the long coherence time of atmospheric turbulence channels limits the achievable secret-key rates (SKRs). To address this issue, we initially leveraged the turbulence effects only during the system’s initialization phase. In this paper, we present a source-based secret key generation scheme that overcomes the limitations imposed by the long coherence time of the turbulent channel, without requiring initialization. The scheme utilizes both atmospheric turbulence and background radiation as sources of randomness. It offers security superior to PQC and is comparable to QKD, while enabling significantly higher SKRs, making it fully compatible with state-of-the-art optical communications.
We investigate a multiple spatial modes based quantum key distribution (QKD) scheme that employs multiple independent parallel beams through a marine free-space optical channel over open ocean. This approach provides the potential to increase secret key rate (SKR) linearly with the number of channels. To improve the SKR performance, we describe a back-propagation mode (BPM) method to mitigate the atmospheric turbulence effects. Our simulation results indicate that the secret key rate can be improved significantly by employing the proposed BPM-based multi-channel QKD scheme.
Security issues of terrestrial free-space optical (FSO) communications over strong atmospheric turbulence are investigated. Based on a subcarrier intensity modulated FSO system model, we first study the scintillation reciprocity of the system from a coherence time perspective. A private secret key-based cryptosystem with key management is introduced to enhance FSO security, and a key agreement approach is proposed based on statistics of the random atmospheric turbulence-induced fading channel measurements. Secret key rate of the key agreement scheme is studied for the strong turbulence channel characterized by the K-distribution. Practical key agreement protocol is designed based on channel identification.