Tomohiro Yamazaki

Linear-optical entanglement swapping works only probabilistically. Quantum repeater protocols based on it, classified in the first generation, inevitably need classical communication between non-adjacent nodes, which makes the protocols very slow and requires quantum memories with long coherence time. One way to realize faster quantum repeater protocols, classified in the second generation, is to use matter qubits, which enables deterministic entanglement swapping. However, such matter qubits are still experimentally challenging. Here we propose a linear-optical circuit that projects two input qudits of dimension d onto a Bell state defined in a two-qubit subspace with the probability of 1 − d−1, which can be used to realize almost deterministic entanglement swapping for large d. Based on it, we propose a quantum repeater protocol consisting of linear optical elements, photon detectors, high-dimensional quantum memories, and photonic three-qudit GHZ states. In the protocol, the classical communication between non-adjacent nodes becomes unnecessary thanks to the high success probability of the entanglement swapping, making the protocol be categorized into the second generation.