Multipartite Mixed-Species Entanglement over a Quantum Network

Abstract: We generate multipartite entangled states of two, three and four matter qubits, where the entanglement is distributed over macroscopic distances via a photonic network link. Trapped-ion ${}^{88}\text{Sr}^+$ qubits are entangled directly via the optical fibre link, and the entanglement is subsequently extended to ${}^{43}\text{Ca}^+$ memory qubits co-trapped in each network node, using local mixed-species logic gates. We create remotely entangled $\text{Sr}^+$-$\text{Ca}^+$ and $\text{Ca}^+$-$\text{Ca}^+$ states, as well as mixed-species Greenberger-Horne-Zeilinger (GHZ) states of up to four qubits. We demonstrate storage of the remotely-entangled memory qubits for $\sim10~\text{s}$, more than $100\times$ the creation time.