A high-fidelity free-space trapped-ion photonic interface

Trapped atomic ions are a leading platform for quantum computing; they possess excellent coherence properties and have been used to demonstrate high-fidelity single and two-qubit operations. However, extending the size of trapped ion systems remains an outstanding challenge. Trapped ions are particularly well suited for photonic interfaces, which can be used to distribute entanglement over large distances, encouraging a modular architecture. We present a novel free-space photonic interface to a trapped ⁸⁸Sr⁺ ion, enabling high-efficiency collection of spontaneously emitted photons whose polarisation is maximally entangled with the electronic state of the ion. We perform quantum state tomography to evaluate its performance and implement a remote "steering" protocol using a high-speed polarisation analyser. Finally, we discuss progress towards measurement-based blind quantum computing on this apparatus.