Efficient Entanglement Swapping in High-Dimensions with only Linear Optics

Abstract

Entanglement swapping is a fundamental building block for realizing first-generation quantum repeaters, which are essential for building global quantum networks. Current quantum repeater systems still struggle to achieve practical communication rates. High-dimensional (HD) encoding can significantly improve repeater efficiency by boosting information capacity and enhancing noise tolerance and security. However, the experimental demonstration of this protocol so far has been limited only to two-dimensional systems due to the requirement of strong nonlinear interactions. Here, we introduce an efficient linear-optics protocol for HD entanglement swapping that uses ancillary photons and is compatible with arbitrary photonic degrees of freedom (DOFs). We further show that photon-number-resolving detectors substantially enhance the performance of the setup and become especially valuable for dimensions beyond six. For a four-dimensional scenario, we present an experimental design using hyper-entanglement in polarization and time-bin DOFs. This setup resolves the most challenging part of the ancillary photons-based approach, namely the necessary preparation of the ancilla state and analysis of the resulting swapped state.