Credit: “Quantum bits” by Argonne National Laboratory (Openverse)

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.

New analytical framework for pointing error

To address this gap, a study published in the IEEE Journal of Quantum Electronics presents 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 the widely used BB84 QKD protocol and modeled 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.

Key findings and implications for QKD

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 system noise, environmental effects, imperfections, or attempted eavesdropping. QBER is therefore a key performance metric.

The researchers further used QBER to calculate the secret key rate (SKR), which measures the rate at which shared, secure keys can be generated. They analyzed the effects of pointing error caused by 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 favorable for improving performance. The researchers also found that, for achieving a 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

Yalçın Ata et al., “Pointing Error Influence on Quantum Key Distribution,” IEEE Journal of Quantum Electronics (2025). DOI: https://dx.doi.org/10.1109/jqe.2025.3627887

Citation

How pointing errors impact quantum key distribution systems (2026, January 19). Retrieved 19 January 2026 from https://phys.org/news/2026-01-errors-impact-quantum-key.html