Eavesdropper detection in Six-State protocol against partial intercept-and-resend attack

In recent years, significant advancements have been made in the field of quantum key distribution. QKD offers a theoretically unbreakable level of security, making it a promising technology for protecting sensitive communications in an era of increa- sing cyber threats. The purpose of this work is to propose two threshold-based detection methods for the six-state protocol in a regime of a partial intercept-and-resend attack with a constant interception density. In doing so, "system noise" is defined and its effect on the transmission is taken into account In the first part of this work, after a brief introduction to the aforementioned proto- col, the statistical properties of the shared quantum bit error rate are evaluated and used for determining a sample estimation of the interception density of the attacker. Then, in the second part, the statistical properties previously obtained are used to determine an optimal threshold that minimizes the number of false positives and false negatives and can be used for the detection of a potential eavesdropper. In the third part, a simulator developed in Python with the use of the Qiskit libra- ry is used to analyze a system implementing the six-state protocol with backends inspired by real-world IBM quantum machines. The goal of these simulations is to validate both the statistical analysis made for the quantum bit error rate and the applicability of the proposed detection methods. Lastly, the results of these simulations are evaluated and the accuracy of the de- tection methods is obtained. Through this analysis, it is possible to confirm the applicability of these methods.