Atom--photon Entanglement with a Single Trapped Cesium Atom

We demonstrate atom–photon entanglement using a single cesium atom trapped in an optical tweezer. Entanglement is generated by resonant excitation and subsequent spontaneous decay, which entangles the atomic Zeeman state with photon polarization. The photon is collected with a high numerical aperture objective (NA = 0.55) and coupled into a single-mode fiber, enabling atom photon measurements and measurement of the Bell-state fidelity. We obtain raw entanglement fidelity of $\mathcal{F}=0.942(16)$ and inferred fidelity of ${\mathcal{F}}_{\mathrm{i}\mathrm{n}\mathrm{f}}=0.962(26)$ after correcting independently characterized atom measurement errors. Compared with related free-space experiments using ${}^{87}$Rb, the multilevel structure of the relevant excited state in ${}^{133}$Cs requires the use of a single short excitation pulse in each entanglement attempt in order to suppress unwanted re-excitation. These results establish a free-space Cs atom–photon interface and provide a step toward dual-species Rb–Cs quantum networking.