Matthias Kreis

The quantum repeater cell is a basic building block for a quantum network, as it allows to overcome the distance limitations due to unavoidable fiber loss in direct transmission. We demonstrate the implementation of a quantum repeater cell, based on two free-space coupled $^{40}$Ca$^+$ ions in the same trap that act as quantum memories. We demonstrate the asynchronous generation of atom-photon and photon-photon entanglement by controlled emission of single photons from the individually addressed ions and entanglement swapping. We discuss the fidelity as well as the scaling of the generated rate.

Entanglement between a stationary quantum system and a flying qubit is an essential ingredient of a quantum-repeater network. It has been demonstrated for trapped ions, trapped atoms, color centers in diamond, or quantum dots. These systems have transition wavelengths in the blue, red or near-infrared spectral regions, whereas long-range fiber-communication requires wavelengths in the low-loss, low-dispersion telecom regime. A proven tool to interconnect flying qubits at visible/NIR wavelengths to the telecom bands is quantum frequency conversion. Here we use an efficient polarization-preserving frequency converter connecting 854$\,$nm to the telecom O-band at 1310$\,$nm to demonstrate entanglement between a trapped $^{40}$Ca$^{+}$ ion and the polarization state of a telecom photon with a high fidelity of 98.2 $\pm$ 0.2$\%$. The unique combination of 99.75 $\pm$ 0.18$\%$ process fidelity in the polarization-state conversion, 26.5$\%$ external frequency conversion efficiency and only 11.4 photons/s conversion-induced unconditional background makes the converter a powerful ion-telecom quantum interface.