Entanglement between a stationary quantum system and a photonic flying qubit is an essential ingredient of a quantum-repeater network. Most stationary quantum bits, however, have transition wavelengths in the blue, red or near-infrared spectral regions, whereas long-range fiber-communication requires wavelengths in the low-loss telecom regime. A proven tool to interconnect flying qubits at visible/NIR wavelengths to the telecom bands is quantum frequency conversion.
In this talk I will show efficient and low-noise polarization-preserving frequency converters connecting 854 nm – a transition wavelength in a single trapped 40Ca+-ion – to the Telecom O- and C- band at 1310 nm and 1550nm. This enables the observation of ion-telecom-photon entanglement as well as an ion-to-telecom-photon state transfer. Moreover I will present a complete QFC system designed as telecom interface for an elementary Rubidium-atom based quantum network link. As a first result, the entanglement between a single Rb-atom and a telecom photon transmitted through an optical fiber of 20 km length is shown.