Color centers in diamond are important quantum emitters for a broad range of applications ranging from quantum sensing to quantum optics. Understanding the internal energy level structure is of fundamental importance for future applications. We experimentally investigate the level structure of an ensemble of few negatively charged silicon-vacancy (SiV − ) and germanium-vacancy (GeV − ) centers in bulk diamond at room temperature by photoluminescence (PL) and excitation (PLE) spectroscopy over a broad wavelength range from 460 nm to 650 nm and perform power-dependent saturation measurements. For SiV − our experimental results confirm the presence of a higher energy transition at ∼ 2.31 eV. By comparison with detailed theoretical simulations of the imaginary dielectric function we interpret the transition as a dipoleallowed transition from 2 E g -state to 2 A 2u -state where the corresponding a 2u -level lies deeply inside the diamond valence band. Therefore, the transition is broadened by the diamond band. At higher excitation power of 10 mW we indicate signs of a parityconserving transition at ∼ 2.03 eV supported by saturation measurements. For GeV − we demonstrate that the PLE spectrum is in good agreement with the mirror image of the PL spectrum of the zero-phonon line (ZPL). Experimentally we do not observe a higher lying energy level up to a transition wavelength of 460 nm. The observed PL spectra are identical, independent of excitation wavelength, suggesting a rapid decay to 2 E u excited state and followed by optical transition to 2 E g ground state. Our investigations convey important insights for future quantum optics and quantum sensing experiments based on SiV − -center and GeV − -center in diamond.

We realize a potential platform for an efficient spin-photon interface, namely negatively-charged silicon-vacancy centers in a diamond membrane coupled to the mode of a fully-tunable, fiber-based, optical resonator. We demonstrate that introducing the thin (∼ 200 nm), single crystal diamond membrane into the mode of the resonator does not change the cavity properties, which is one of the crucial points for an efficient spin-photon interface. In particular, we observe constantly high Finesse values of up to 3000 and a linear dispersion in the presence of the membrane. We observe cavity-coupled fluorescence from an ensemble of SiV − centers with an enhancement factor of ∼ 1.9. Furthermore from our investigations we extract the ensemble absorption and extrapolate an absorption cross section of (2.9 ± 2) · 10 −12 cm 2 for a single SiV − center, much higher than previously reported.

Realization of quantum photonic devices requires coupling single quantum emitters to the mode of optical resonators. In this work, a hybrid system consisting of defect centers in few‐layer hexagonal boron nitride (hBN) grown by chemical vapor deposition and a fiber‐based Fabry–Pérot cavity is presented. The sub 10‐nm thickness of hBN and its smooth surface enable efficient integration into the cavity mode. This hybrid platform is operated over a broad spectral range larger than 30 nm and its tuneability is used to explore different coupling regimes. Consequently, very large cavity‐assisted signal enhancement up to 50‐fold and strongly narrowed linewidths are achieved, which is owing to cavity funneling, a record for hBN‐cavity systems. Additionally, an excitation and readout scheme is implemented for resonant excitation that allows to establish cavity‐assisted photoluminescence excitation (PLE) spectroscopy. This work marks an important milestone for the deployment of 2D materials coupled to fiber‐based cavities in practical quantum technologies.

The creation of single, negatively charged silicon vacancy (SiV−) centers in well-defined diamond layers close to the host surface is a crucial step for the development of diamond-based quantum optic devices with many applications in nanophotonics, quantum sensing, or quantum information science. Here, we report on the creation of shallow (10 nm below the surface), single SiV− centers in diamond using low energy Si+ ion implantation with subsequent high temperature annealing at 1500 °C. We show transition linewidths down to 99 MHz and narrow inhomogeneous distributions. Furthermore, we achieved a reduction of homogeneous linewidths by a factor of 2 after removing subsurface damage using oxygen plasma processing. These results not only give insights into the formation process of SiV− centers but also indicate a favorable processing method to fabricate shallow single quantum emitters in diamond perfectly suited for coupling to nanostructures on the diamond surface.

Optical coupling enables intermediate-and long-range interactions between distant quantum emitters. Such interaction may be the basic element in bottom-up approaches of coupled spin systems or for integrated quantum photonics and quantum plasmonics. Here, we prepare nanodiamonds carrying single, negatively-charged silicon-vacancy centers for evanescent optical coupling with access to all degrees of freedom by means of atomic force nanomanipulation. The color centers feature excellent optical properties, comparable to silicon-vacancy centers in bulk diamond, resulting in a resolvable fine structure splitting, a linewidth close to the Fourier-Transform limit under resonant excitation and a good polarization contrast. We determine the orbital relaxation time T 1 of the orbitally split ground states and show that all optical properties are conserved during translational nanomanipulation. Furthermore, we demonstrate the rotation of the nanodiamonds. In contrast to the translational operation, the rotation leads to a change in polarization contrast. We utilize the change in polarization contrast before and after nanomanipulation to determine the rotation angle. Finally, we evaluate the likelihood for indistinguishable, single photon emission of silicon-vacancy centers located in different nanodiamonds. Our work enables ideal evanescent, optical coupling of distant nanodiamonds containing silicon-vacancy centers with applications in the realization of quantum networks, quantum repeaters or complex quantum systems.

The difference between state building and nation building is not always appreciated. Simon Chesterman uses only the term state-building, because, he claims, nation-building is specific to post-colonial situations. James Dobbins and his co-authors at RAND solve the problem in an equally sweeping, but opposite fashion by declaring all US involvements in post-conflict reconstruction, from Germany to Iraq, to have been exercises in nationbuilding. Francis Fukuyama acknowledges that there is a distinction between state-building and nation-building but his usage of both terms would suggest that he regards nation-building as a term employed in the language of politics describing what he-in academic discourse-calls statebuilding. See J.

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