Fiber Cavity coupled Silicon-Vacancy Center for a Quantum Repeater Node
A collaboration of more than 25 institutes and companies in Germany, Q.Link.X is founded by the German Federal Ministry of Education and Research (BMBF) and aims at long-distance distribution of entanglement:
Quantum communication in a future quantum network  offers data encryption and transmission with physically verifiable security. The famous BB84  protocol for sharing secret keys is limited to fiber lengths of 200-300 km . Our goal is to distribute entanglement over a fiber based quantum network between different QR-nodes. With a chain of QR-nodes and using an entanglement swapping protocol  we aim to overcome the length limitation. The quantum channel is subdivided into several segments, each of them connected by a QR-node, as elementary, scalable and fiber based units.
In the subproject in Mainz, we collaborate within Q.Link.X on the platform of color centers in diamond for a QR. A fiber based optical cavity (Fig. 1) serving as an interface between single photons and a single color center in diamond, which serves as local quantum memory. Specifically, we employ a negatively charged Silicon-Vacancy Center (SiV-) in diamond, providing a narrow, stable and efficient emission in the zero-phonon line (ZPL) at 737 nm and long-lived electronic spin states [5,6]. A spin coherence time of >10 ms  is sufficiently long for QR applications, but is obtained only for temperatures well below one Kelvin. Thus, we will cool the diamond sample with a He3/He4 dilution refrigerator to temperatures below 100 mK.
Efficient coupling of the SiV- to the quantum channel is achieved with a fiber based microcavity, produced in collaboration with David Hunger's group from KIT. The cavity consists of a concave fiber facet , coated with a DBR stack and a plane mirror. The microcavity puts tight spatial boundaries to the vacuum light field, causing the SiV- center to emit a photon into the cavity mode (Purcell enhancement). SiV- embedded in thin diamond membranes, provided by Steven Prawer's group in Melbourne [9,10] are bound to the planar mirror. The position of the plane sample mirror can be controlled by nanopositioners, which allow for positioning of SiV- into the cavity mode (Fig. 2).
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