IEEE study investigates the effects of pointing error on quantum key distribution systems

Quantum key distribution (QKD) is an emerging communication technology that utilizes quantum mechanics principles to ensure highly secure communication between two parties. It enables the sender and receiver to generate a shared secret key over a channel that may be monitored by an attacker. Any attempt to eavesdrop introduces detectable errors in the quantum signals, allowing communicating parties to detect if communication is compromised via QKD protocols.

Among the various parameters that influence the performance of QKD systems, pointing error, a misalignment between the transmitter and receiver, is one of the most important. Such misalignment can arise from mechanical vibrations, atmospheric turbulence, and/or inaccuracies in the alignment mechanisms. Despite its importance, very few studies have examined pointing error in a comprehensive manner for QKD optical wireless communication (OWC) systems.

To address this gap, a new study published in Volume 61, Issue 6 of the IEEE Journal of Quantum Electronics on December 1, 2025, presented a comprehensive analytical framework for modeling the effect of pointing error on QKD OWC system performance. “By combining statistical models of beam misalignment with quantum photon detection theory, we derived analytical expressions for key performance indicators of QKD systems, clarifying the exact role of pointing error in degrading secure key generation,” explains Professor Yalçın Ata from OSTIM Technical University, Turkey.

The researchers focused on widely used BB84 QKD protocol and modelled pointing errors using Rayleigh and Hoyt distributions, which model horizontal and vertical beams better than simplified models used in earlier work. This leads to more accurate characterization of random pointing errors.

Using these statistical models, the researchers first derived analytical expressions for error and sift probabilities under pointing error, a first in the field. These were then used to compute the quantum bit error rate (QBER), which indicates the percentage of bits corrupted due to either system noise, environmental effects, and imperfections or attempted eavesdropping. QBER is therefore, a key performance metric. The researchers further used QBER to calculate the secret key rate (SKR) that measures the rate at which shared, secure keys can be generated. They analysed the effects of pointing error caused due to both symmetric and asymmetric beam alignments.

The results showed that an increased beam waist, and hence, increased pointing error, significantly degrades QKD performance, indicated by higher QBER and decreased SKR. Increasing receiver aperture size can improve performance, but only up to a certain level. Interestingly, asymmetric beam misalignment, where horizontal and vertical deviations are different, was found to be favourable for improving performance. The researchers also found that for achieving non-zero SKR, important for secure communication, increasing average photon numbers is required.

“Our findings, based on Rayleigh and Hoyt framework, are consistent with existing generalized models, while offering new analytical clarity on the role of asymmetry in pointing errors,” concludes Prof. Ata.

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Reference
Authors: Yalçın Ata^1,2 and Kamran Kiasaleh²
DOI: 10.1109/JQE.2025.3627887
Affiliations: ¹ OSTIM Technical University, Turkey
²Erik Jonsson School of Engineering and Computer Science, The University of Texas at Dallas, USA