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Spin qubits in diamond nanocavities: Towards spin-photon interfaces


Diamond based quantum spin systems like the negatively charged nitrogen-vacancy center (NV) represent a promising platform for precision measurements and quantum information applications. Most of these applications rely on the long electron spin coherence time of the NV ground state of up to 600 ms in bulk. Solid-state cavity systems, on the other hand, have attracted much interest for enhancing light-matter interaction on the nano-scale. Cavity-enhanced light matter interaction can enable the speed up of established quantum protocols and the implementation of novel concepts for quantum networks. A first important step towards more complex systems is the Purcell induced spontaneous emission rate enhancement of an NV inside a cavity. First realizations of cavity-coupled NVs have been reported recently. In these realizations, NVs were randomly distributed with respect to the cavity mode thereby prohibiting a high NV – cavity-mode overlap, hence limiting the spontaneous emission rate enhancement, while spin coherence times were below 1 us.

Here, we present the realization of such systems with improved optical and spin properties. We fabricated photonic crystal cavities (PCC) in high-purity single-crystal diamond by oxygen reactive ion etching using silicon (Si) membranes as etch masks. For single fabrication runs, we find high yield (up to 94 %) of cavities with a mean quality (Q) factor of  up to 6,200 and a maximum Q of 9, 900 ± 200. To couple single NVs with high probability to the mode maxima of such cavities, we demonstrate their spatially deterministic creation inside the cavity region. 15 N implantation through circular apertures, and subsequent annealing result on average in about 1.1 (0.2) NVs per cavity. We show strong Purcell enhancement of the NV’s zero phonon line spontaneous emission rat and demonstrate electron spin coherence times of more than 200µs of cavity coupled NVs. Furthermore we demonstrate efficient integration of photonic diamond structures into semiconductor on-chip photonic circuitry with NV coupling rates exceeding free space collection by several manifolds. These NV-cavity systems pave the way to advanced quantum network implementations.