Robust Ion-Photon Entanglement via Polarization-to-Time-Bin Conversion

Time-bin photonic qubits are well-suited for quantum network applications due to their robustness to polarization instability in fiber links and potential for heterogeneous networks. In this work, we implement the first entanglement-preserving polarization-to-time-bin conversion of a photon qubit in an entangled state with a matter qubit. Photons initially generated with polarization encoding are converted to the time-bin basis through a polarization-discriminating asymmetric Mach-Zehnder interferometer. The photonic qubits are generated via the $1092$ nm transition of a $^{88}$Sr$^{+}$ ion. We measure state fidelity bounds of $0.906 \pm 0.011 \le \mathcal{F} \le 0.934\pm 0.011$, with conversion error $< 0.028$, and find this fidelity is unaffected by depolarizing noise even at full depolarization strength.