Pointing errors can break quantum encryption, study finds

Researchers model how alignment errors break quantum encryption

Quantum key distribution (QKD) is a highly secure communication method that uses quantum mechanics to generate encryption keys. Its core security promise is that any attempt to eavesdrop on the quantum signals will disturb them, creating detectable errors.

However, a new study highlights a major practical vulnerability: pointing error. This occurs when the transmitter and receiver are not perfectly aligned, which can severely degrade the system's performance.

A new framework for a critical flaw

Pointing error, caused by factors like mechanical vibration or atmospheric turbulence, has not been thoroughly studied in QKD systems. Researchers from OSTIM Technical University in Turkey have now created a detailed analytical model to measure its impact.

"We derived analytical expressions for key performance indicators of QKD systems, clarifying the exact role of pointing error in degrading secure key generation," said Professor Yaln Ata, the study's author. The work was published in the IEEE Journal of Quantum Electronics.

The team focused on the common BB84 QKD protocol. To model beam misalignment realistically, they used Rayleigh and Hoyt statistical distributions, which better represent random horizontal and vertical beam variations than simpler models used in past research.

How misalignment cripples key generation

Using their new models, the researchers calculated two critical metrics for the first time: the quantum bit error rate (QBER) and the secret key rate (SKR). The QBER measures the percentage of corrupted bits, while the SKR measures how fast secure keys can be generated.

Their analysis revealed a direct performance trade-off:

• As beam misalignment increases, the QBER rises and the SKR falls.
• Larger receiver apertures can help, but only to a point.
• Generating any usable secret key requires increasing the average number of photons sent.

Asymmetric error can be surprisingly beneficial

A key finding was that not all misalignment is equally harmful. The study examined both symmetric and asymmetric beam error, where horizontal and vertical deviations differ.

In some cases, asymmetric misalignment actually performed better than perfectly balanced error, offering a higher secret key rate. This provides new insight for engineers designing stabilization systems for quantum communication links.

"Our findings are consistent with existing generalized models, while offering new analytical clarity on the role of asymmetry in pointing errors," Professor Ata concluded. The work provides a crucial framework for building more reliable real-world quantum encryption systems that must account for physical imperfections.