Towards a quantum network test-bed over deployed fiber with trapped ions*

Quantum networking applications rely on the ability to establish entanglement between distant nodes. Recent experiments have succeeded in establishing photon-mediated entanglement between matter qubits over distances ranging from one to hundreds of meters, using solid-state platforms, cold neutral atoms, and trapped-ion systems. Here, we propose to demonstrate high-fidelity and high-rate remote entanglement between two trapped-ion quantum nodes separated by an absolute distance of roughly one kilometer and connected by a five-kilometer telecom band optical fiber. Our node technology will rely on generating ion-photon entangled pairs via a cavity-mediated Raman process in a near concentric cavity regime with singly ionized calcium ions. We will use a novel surface-trap architecture, to be produced in-house, designed for maximal optical access to the ion and shuttling along the trap axis for multiplexing. The emitted photons will be converted to telecom wavelengths (1550 nm) via a highly efficient quantum frequency conversion process, and subsequently will be transmitted over deployed fibers between Lawrence Berkeley National Laboratory and the campus of University of California, Berkeley. This link will serve as a testbed for protocols which require high-fidelity, high-rate remote entanglement of quantum processors such as quantum teleportation and distributed quantum computing over kilometer-scale distances.