We present, characterize, and apply a photonic quantum interface between the near infrared and telecom spectral regions. A singly resonant optical parametric oscillator (OPO) operated below threshold, in combination with external filters, generates high-rate (> 2.5 · 10 6 s −1 ) narrowband photon pairs (∼ 7 MHz bandwidth); the signal photons are tuned to resonance with an atomic transition in Ca + , while the idler photons are at telecom wavelength. Quantum interface operation is demonstrated through high-rate absorption of single photons by a single trapped ion (∼ 670 s −1 ), heralded by coincident telecom photons.

We report on quantum frequency conversion of near-infrared photons from a wavelength of 854 nm to the telecommunication O-band at 1310 nm with 8 % overall conversion efficiency. Entangled photon pairs at 854 nm are generated via type-II spontaneous parametric down conversion. One photon is mixed with a strong pump field in a nonlinear ridge waveguide for its conversion to 1310 nm. We demonstrate preservation of first and second order coherence of the photons in the conversion process. Based on this we infer the coherence function of the two-photon state and compare it with the actual measured one. This measurement demonstrates preservation of time-energy entanglement of the pair. With 88 % visibility we violate a Bell inequality.

We report on a single-photon-to-single-atom interface, where a single photon generated by Spontaneous Parametric Down Conversion (SPDC) is absorbed by a single trapped ion. The photon is heralded by its time-correlated partner generated in the SPDC process, while the absorption event is heralded by a single photon emitted in its course. Coincidence detection marks doubly-heralded absorption, enabling photon-to-atom quantum state transfer [1,2]. Background in the coincidence measurement is strongly suppressed by a new method that discriminates real absorption events from dark count-induced coincidences.A major goal in quantum technologies is to integrate quantum communication and information processing into quantum networks [3]. One promising approach is using single atoms and photons as nodes and channels of the network [4][5][6]. Single photons are able to transmit quantum information between different nodes of the network, while single atoms serve as high-fidelity, long-storagetime memories; single trapped ions, in particular, may additionally be used as quantum processors [7][8][9][10]. In this context, the use of heralding photons for repeaters and memories schemes is very promising [11,12]: successful entanglement transfer or information storage onto a single atom is signaled by the detection of a photonic herald emitted as a consequence of the process. The detection of such heralds permits one to discriminate successful events and thereby enables high-fidelity protocols, e.g. for photon-to-atom quantum state transfer [1,13] or distant ion entanglement [11].

We demonstrate the multiplexing of a classical coherent and a quantum state of light in a single telecommunciation fiber. For this purpose we make use of spontaneous parametric down conversion and quantum frequency conversion to generate photon pairs at 854 nm and the telecom O-band. The herald photon triggers a telecom C-band laser pulse. The telecom single photon and the laser pulse are combined and coupled to a standard telecom fiber. Low background time correlation of the classical and quantum signal behind the fiber shows successful telecommunication channel multiplexing.

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