Single-excitation entanglement between a time-bin photon qubit and a metastable Sr⁸⁸⁺ ion qubit

Time-bin photonic qubits are well-suited for remote quantum network applications due to their robustness to polarization fluctuations in fiber links. All previous ion-photon entanglement time-bin schemes require double excitation of the ion, leading to an unavoidable rate penalty compared to the single-excitation polarization encoding. In this work, we implement the first single-excitation ion-photon time-bin scheme. Photons are initially generated with polarization encoding and then converted to the time-bin basis in a polarization-discriminating delay-line setup. We achieve the required optical path stabilization on the conversion stage using a nested piezo-mirror lock comprising a fast and a slow actuator. The photonic qubits are generated via the 1092 nm transition on Sr⁸⁸⁺ ions. Their low attenuation in optical fibers results in negligible loss on the delay line and makes metropolitan-scale quantum network applications viable without quantum frequency conversion. We present ion-photon state characterization, indicating that the approach is suitable for practical long-distance quantum networks.