Master's defense: Camilla Birch Okkels

Theoretical optimizations of quantum repeaters based on atomic ensembles

Quantum repeaters are a solution to the problem of sending quantum information over long distances. They take advantage of entanglement and using so-called swapping processes to distribute entanglement. This thesis explores the effect of imperfections on quantum repeater protocols. A particular imperfection this thesis focuses on exploring is the multi-photon error arising from a photon source having a probability of emitting more than one photon. Imperfections greatly reduce the efficiency and accuracy of repeater protocols. To mitigate the effect of imperfections more advanced repeater protocols can be thought of that are more robust e.g. the two-click swapping schemes compared to the simpler one-click swapping scheme. Two protocols are introduced. The Jiang protocol using the one-click entanglement generation, one type of two-click swap for the first swapping and a second type of two-click swap for all subsequent swaps. The second protocol is a modified version using one-click entanglement generation and the first type of two-click swapping for all swaps. Comparing the two protocols the modified version proved more robust towards imperfections. This was due to a higher fidelity of the final state.