Effect of Asymmetry on the Performance of Atomic Ensemble Based Repeater Protocols

Long-distance entanglement distribution is a formidable technological challenge. Quantum repeaters can in theory be used to enable this goal, with much work having been done on the effect of hardware imperfections on their performance. However, these investigations were mostly performed under the assumption of symmetric placement of quantum repeaters and heralding stations, while it is likely that real-world deployment of quantum repeaters will make use of existing fiber networks. This inevitably implies asymmetric node placement. Here, we numerically investigate how asymmetric placement of nodes affects the performance of atomic-ensemble elementary-link and single-repeater setups, building on existing simulations based on the discrete-event quantum-network simulator NetSquid. We find that the secret key rate achievable by an elementary link performing entanglement-based quantum key distribution decreases as the degree of asymmetry increases. Furthermore, we find that this effect can be mitigated by individually optimizing each of the photon sources used for elementary-link generation and we introduce a heuristic that allows us to reduce the search space over which this optimization is performed. We believe that this work constitutes a valuable stepping stone towards realistic investigations on the effect of asymmetry on the feasibility of long-distance entanglement distribution.