Markus Pflitsch

Quantum cryptography provides everlasting secure communication - a property unattainable by classical or post-quantum cryptography. Traditionally, quantum cryptography proclaims everlasting security by relying on principles such as the no-cloning theorem, Bell inequalities, or the uncertainty principle, along with the standard assumption that legitimate users have no control over the communication channel. In this study, we demonstrate that other, nontraditional quantum effects and techniques boost quantum cryptography performance while maintaining everlasting security of communication. In particular, legitimate users can exploit the physical properties of optical fiber channels - specifically, that all losses are caused by Rayleigh scattering and are therefore homogeneously distributed. This quantum property can be combined with quantum tomography of the fiber channel, which reveals the spatial loss distribution and potential eavesdropping attempts. Finally, we present a detailed framework for constructing a security statement applicable to arbitrary cryptographic solutions and distinguish it from formal security theorems proven within idealized mathematical models of cryptographic setups.

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.

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.