Diamond for quantum networks
Prof. Dr. Christoph Becher (Universität des Saarlandes)

<p> In recent years color centers in diamond have attracted significant interest for applications in quantum information. For many of these applications, e.g. quantum networks, it is essential to couple single color centers to a cavity mode with high quality-factor Q and small modal volume in order to coherently manipulate, readout and transfer the center's quantum state. <p>

<p> We follow two different routes towards realization of such optical interfaces: coupling to microcavities based on fiber mirrors or microcavities defined within diamond-based photonic crystals (PhC). The fiber based Fabry-Perot cavities consist of a flat mirror onto which diamond nanocrystals are deposited by spin coating and a fiber mirror where a concave impression has been produced on the fiber facet by laser machining. We demonstrate cavities with lengths of a few microns and finesse of few hundred, sufficient for observing modified spontaneous emission. In addition, we present optimized designs based on Fourier- and real space analysis of PhC defect cavity structures. By careful variation of the field envelope, vertically radiated power can be suppressed and the Q-factor can be improved significantly up to Q  320000 at small modal volumes. We discuss routes for fabrication of diamond-based PhC cavities. <p>

<p> Another prerequisite for future quantum networks is efficient single photon transmission via optical fibers which requires photon wavelengths in the low loss band around 1550nm. However, the vast majority of single photon sources realized to date have emission wavelengths in the red or near-infrared spectral region. We propose an experimental scheme for nonlinear downconversion of single photons emitted from a SiV center in diamond (738 nm) into the telecom band around 1550 nm. This is accomplished by difference frequency mixing with a strong cw signal wave at 1410 nm in a PPLN waveguide crystal. Theoretical considerations show that this process should allow for efficient conversion between the pump and idler fields. <p>