The surprisingly simple flaw that can undermine quantum encryption

Because of this built in detection feature, QKD is considered one of the most secure communication technologies under development.

How Pointing Error Affects QKD Performance

One critical factor that influences how well QKD works is pointing error, which occurs when the transmitter and receiver are not perfectly aligned. Even slight misalignment can interfere with the quantum signals being exchanged. This can happen for several reasons, including mechanical vibrations, atmospheric turbulence, and/or imperfections in alignment systems.

Although pointing error plays a major role in system reliability, it has not been thoroughly studied in QKD optical wireless communication (OWC) systems.

A New Analytical Framework for Beam Misalignment

To better understand this issue, researchers published a study in IEEE Journal of Quantum Electronics that introduces a detailed analytical model to measure how pointing error affects 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 team focused on the widely used BB84 QKD protocol. To model beam misalignment more realistically, they applied Rayleigh and Hoyt distributions. These statistical tools more accurately represent horizontal and vertical beam variations than simplified approaches used in earlier studies, leading to a clearer picture of how random pointing errors behave.

Measuring Error Rates and Secure Key Generation

Using these improved statistical models, the researchers derived analytical expressions for error and sift probabilities under pointing error, marking a first for the field. From there, they calculated the quantum bit error rate (QBER), which reflects the percentage of corrupted bits caused by system noise, environmental conditions, hardware imperfections, or attempted eavesdropping. Because it captures overall system reliability, QBER is a key performance indicator.

They then used QBER to determine the secret key rate (SKR), which measures how quickly secure shared keys can be generated. The analysis considered both symmetric beam misalignment and asymmetric conditions, where horizontal and vertical deviations differ.

What the Results Reveal About Quantum Security

The findings show that as beam waist increases, pointing error also grows, leading to higher QBER and lower SKR. In other words, performance declines as misalignment becomes more pronounced. Expanding the receiver aperture can improve results, but only up to a certain limit.

Interestingly, asymmetric beam misalignment proved to be beneficial in some cases, offering better performance than perfectly balanced errors. The researchers also determined that generating a non zero SKR, which is essential for secure communication, requires increasing the average number of photons transmitted.

"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.