Neutral atom wavelength compatible 780 nm single photons from a trapped ion via quantum frequency conversion

Quantum networks consisting of quantum memories and photonic interconnects can be used for entanglement distribution (L.-M.Duan and H. J. Kimble, PRL 90, 253601 (2003), H. J. Kimble, Nat. 453, 1023 EP (2008)), quantum teleportation (S.Pirandola et.al, Nat. Photon. 9, 641 (2015)), and distributed quantum computing (T.Spiller, et.al., New J. Phys. 8, 30 (2006). Remotely connected two-node networks have been demonstrated using memories of the same type: trapped ion systems (D.Hucul, et.al, Nat. Phys. 11, 37 (2014)), quantum dots (W. B. Gao et. al, Nat. Photon. 9, 363 (2015)), and nitrogen vacancy centers (W. B. Gao et. al, Nat. Photon. 9, 363 (2015), B. Hensen, et. al., Nat. 526, 682 (2015)). Hybrid systems constrained by the need to use photons with the native emission wavelength of the memory, have been demonstrated between a trapped ion and quantum dot (H.Meyer et.al PRL 114, 123001 (2015)) and between a single neutral atom and a Bose-Einstein Condensate (M.Lettner et. al, PRL 106, 210503 (2011)). Most quantum systems operate at disparate and incompatible wavelengths to each other so such two-node systems have never been demonstrated. Here, we use a trapped 138Ba$^{+}$ ion and a periodically poled lithium niobate (PPLN) waveguide, with a fiber coupled output, to demonstrate 19% end-to-end efficient quantum frequency conversion (QFC) of single photons from 493 nm to 780 nm. At the optimal signal-to-noise operational parameter, we use fluorescence of the ion to produce light resonant with the $^{87}$Rb $D_2$ transition. To demonstrate the quantum nature of both the unconverted 493 nm photons and the converted photons near 780 nm, we observe strong quantum statics in their respective second order intensity correlations. This work extends the range of intra-lab networking between ions and networking and communication between disparate quantum memories. A