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IEEE Wireless Communications Letters

Backflash Mitigation via Scheduling in FSO Systems With Quantum Key Distribution

Maria Dimitropoulou, Constantinos Psomas, Ioannis Krikidis

Published Jan 1, 2026• 0 citations
Publisher

Abstract

Quantum key distribution (QKD) is a potential technology suitable for fulfiling the stringent requirements of upcoming 6G wireless networks in terms of privacy and security. Nevertheless, QKD systems have several practical limitations in terms of hardware design and implementation, making them vulnerable to malicious attacks. Specifically, we adopt a detection scheme using a single-photon avalanche diode and due to the avalanche effect, a secondary photon is emitted, called backflash, which can be captured by an eavesdropper. To mitigate this security loophole and enhance the secrecy performance, we consider a selection mechanism which schedules a single user for secret key exchange purposes. In particular, we propose an intelligent two-stage selection scheme in a multiuser free-space optical QKD deployment. We derive closed-form expressions for the secrecy success probability by considering a generalized pointing error model. Our results confirm that the proposed scheme yields significant secrecy gains over conventional methods.

DOI
10.1109/lwc.2026.3661400

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Quantum key distribution (QKD) is a potential technology suitable for fulfiling the stringent requirements of upcoming 6G wireless networks in terms of privacy and security. Nevertheless, QKD systems have several practical limitations in terms of hardware design and implementation, making them vulnerable to malicious attacks. Specifically, we adopt a detection scheme using a single-photon avalanche diode and due to the avalanche effect, a secondary photon is emitted, called backflash, which can be captured by an eavesdropper. To mitigate this security loophole and enhance the secrecy performance, we consider a selection mechanism which schedules a single user for secret key exchange purposes. In particular, we propose an intelligent two-stage selection scheme in a multiuser free-space optical QKD deployment. We derive closed-form expressions for the secrecy success probability by considering a generalized pointing error model. Our results confirm that the proposed scheme yields significant secrecy gains over conventional methods.
Recently, there has been growing research interest in quantum technology in general and particularly in quantum key distribution (QKD) that enables two legitimate parties to share a secret key without leaking information to third parties. Practical implementation of QKD has shown limited key rates due to the practical limits of quantum sources and detectors. For example, after each detection event, practical single-photon detectors such as single-photon avalanche diodes (SPADs) undergo a period of inactivity called dead time, which limits the highest photon transmission rate achievable for QKD systems as photon transmission at sub-dead-time regime can generate security loopholes. In this paper, we analyze the performance of a sifting scheme that allows for high-speed operation at sub-dead-time regime both theoretically and numerically. We consider an FSO QKD system, where the effect of background noise can be significant and derive the expressions of sifting probability and sifted bit rate considering both dead time and background noise effects. We present a comprehensive performance analysis of the QKD system in terms of quantum bit error rate (QBER) and secrecy key rate (SKR) and show that operation at sub-dead-time regime can significantly improve the achievable SKR for FSO systems with certain path loss and background noise levels.
This paper proposes a novel two-way free-space quantum key distribution (QKD) protocol, which can be implemented using standard free-space optical (FSO) systems with subcarrier intensity-modulation (SIM) binary phase-shift-keying (BPSK) and direct-detection (DD) receiver. Different eavesdropping threats, including unauthorized receiver, beam-splitting, and intercept-resend attacks, are considered in the security analysis of the proposed protocol. Under the constraints on security requirements, we analytically investigate the design criteria for transmitter and receiver, in particular, the intensity modulation depth at the transmitter and the dual-threshold setting of the receiver in two-way free-space QKD systems. Quantum bit error rate (QBER), the ergodic secret-key rate, and the probability of detecting eavesdropper of the proposed system are analytically derived in closed-form expressions, considering the atmospheric loss, turbulence, and receiver noises.
Quantum key distribution (QKD) allows two users to communicate with information theoretic security by encoding information on single photons. This security is based on the laws of physics and as such can never be broken in theory. However, in practice, components do not always behave according to their theoretical models and these deviations can be exploited by an eavesdropper. In recent years, exposing loopholes in QKD systems, known as quantum hacking, has attracted significant attention. The components most susceptible to being hacked are the single-photon detectors, often avalanche photodiodes (APDs), as they are directly exposed to the optical channel. Whilst measurement-device-independent QKD removes detector vulnerability from the system, secure key rates with this technique can be much lower than point-to-point links. As such, mitigating attacks on QKD systems is a pressing challenge in QKD. In this thesis, the focus is on a special class of detectors, self-differencing APDs (SD-APDs), which have facilitated state-of-the art demonstrations of QKD. The susceptibility of SD-APDs to blinding attacks, the most explored and successful attack to date, was investigated and it was shown that by following best practice for their operation, such an attack would be unsuccessful. We have also proposed and developed a countermeasure such that the onus for appropriate operation could be removed from the user. We have also explored an arguably more dangerous attack, in the form of the after-gate attack. We have shown that delayed detection events, ordinarily considered detrimental in QKD, can provide inherent protection against this attack. Finally, backflashes in GHz-gated APDs were investigated for the first time and it was shown that threat they pose to QKD security is negligible. These results highlight the inherent protection to a number of attacks that self-differencing APDs possess. We stress that the findings presented in this thesis are also applicable to other types of fast-gated InGaAs APDs that don't possess self-differencing circuitry.
This paper proposes a novel free-space quantum key distribution (QKD) system using subcarrier intensity-modulation (SIM) binary phase-shift-keying (BPSK) and dual-threshold/direct-detection (DT/DD) receiver with an avalanche photodiode (APD). The proposed system enables the adoption of well-developed analytical models in conventional free-space optical (FSO) communications and achieves the QKD function with a simple configuration. We analytically investigate the design criteria for transmitter and receiver, in particular, the modulation depth and the setting for dual-threshold in the context of security requirement of QKD systems. The quantum bit error rate (QBER) and the ergodic secret-key rate of the proposed system are analytically derived in closed-form expressions, considering the channel loss, atmospheric turbulence modeled by the log-normal distribution, and receiver noises. Monte-Carlo (M-C) simulations are also implemented to validate the analytical results, and numerical results confirm the feasibility of the proposed system.
In this letter, the performance of a quantum key distribution (QKD) free-space optical (FSO) system is analyzed while taking a generalized pointing error model into account. More specifically, closed-form expressions for the average received powers at both the legitimate receiver and eavesdropper are derived. In addition, their corresponding asymptotic results valid in the high telescope gain regime are also presented. To capture the secure performance, we also investigate the ratio of received powers at the eavesdropper and at the legitimate receiver. Further, in some special cases, we find the optimal telescope gains for the received powers at both the legitimate receiver and eavesdropper, as well as the power ratio, which is important and useful for a secure QKD FSO system design. Finally, some selected numerical results are presented to illustrate the mathematical formalism and validate the accuracy of the derived analytical expressions.