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

Abstract

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, together with the independent work of Haen \textit{et al.} \cite{haen2026telecom}. 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 shows no statistically significant reduction under depolarizing noise, even at full depolarization strength.