Markus Pflitsch

The primary obstacle to expanding the reach of quantum cryptography lies in the exponential losses within quantum communication channels. We address this challenge by experimentally realizing the Quantum Control-based Key Distribution (QCKD) protocol, which utilizes physical control over signal losses and ensures that leaked quantum states remain substantially non-orthogonal. The present talk will detail our experiments with QCKD over a 1,707 km fiber optic line, showcasing its effectiveness and scalability. The scaling and performance of QCKD mark a significant step toward achieving globally secure, quantum-resistant communication.

The conventional approach to QKD implies that a potential eavesdropper can conduct any manipulations with a quantum channel. In the context of optical fiber implementations of QKD, we demonstrate how most of the manipulations can be detected by conducting a line tomography procedure. It allows legitimate users to accurately estimate the fraction of the signal available to the eavesdropper and, thus, adaptively modify the setup and post-processing parameters. Our approach significantly increases the secret key generation rate for existing device-dependent QKD protocols and potentially enables surpassing the fundamental PLOB bound.