Ion-Photon Entanglement in Strontium for City-Scale Quantum Networks

We present a quantum network node based on Strontium ions that emit photons at 1092 nm with a polarization state that is entangled with a meta-stable D-level qubit in the emitting ion. The wavelength transmits through optical fiber with moderate loss allowing for quantum network links of a few kilometers for a city-scale quantum network. We establish a new type of trapped-ion qubit with state-preparation, single-qubit rotations, and readout. We stimulate the emission of infrared photons, capture them with a high-NA objective and analyze their polarization state with a pair of SNSPDs. We use state-tomography to show that we generate the ion-photon entangled state with ~95% fidelity. 

We transmit the photons over commercial optical fiber, 2.8 km in length, deployed in the field, stretching between two buildings in the downtown area of the city of Durham, NC, and show that entanglement persists, meaning the system is capable of establishing a municipal quantum network across multiple locations. We present a detailed analysis of the noise and error mechanisms affecting the measurement and discuss prospects for using this new trapped-ion quantum network node for the testing of quantum applications on a quantum computer network.