Roman Solař

We demonstrate the feasibility of entanglement-based quantum key distribution (eQKD) in high-security financial infrastructure over a 22 km fiber link with 8 dB loss between two data centers using polarization entanglement. The fully automated system continuously generated secure keys for four months at an average rate of 63.8 kb/s, which were stored into a key management system and consumed to establish a VPN tunnel. The setup achieved 93.7% total up-time, with no downtime caused by the quantum optical components. Active polarization control kept the quantum bit error rate below 2% for 97.4% of the time and timing synchronization based on the entangled photon pairs’ intrinsic temporal correlations achieved sub-300 ps precision. Our standalone system requires neither polarized guide lasers nor external high-precision time references. These results show practical integration of eQKD into operational financial infrastructure.

We investigate the practical requirements for certifying high-dimensional quantum entanglement using existing matrix completion techniques. Focusing on time-bin entangled systems, we simulate the measurement of selected diagonals of the density matrix to identify minimal yet effective configurations. Our results indicate that measuring the main diagonal along with just two off-diagonals is often sufficient to tightly lower-bound the Schmidt number, entanglement of formation, and distillable secret key rate. We further develop a method for selecting an optimal set of diagonals based on the dimension of the system - corresponding to an optimal choice of interferometer delays. This work offers experimental guidance for efficient entanglement certification in high-dimensional quantum systems.

This work introduces a novel approach to certifying high-dimensional quantum entanglement using matrix completion methods. Instead of relying on complete state tomography, our method measures select elements of the density matrix and completes the remaining elements through convex optimization to minimize the entanglement measure. This allows us to compute a lower bound for the Schmidt number and the entanglement of formation, providing a practical alternative to traditional techniques. Our approach is flexible and does not require measurements in specific bases, making it particularly advantageous for time-bin entanglement scenarios.