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When annealing a 4H silicon carbide (SiC) crystal, a sequence of optically active defect centers occurs among which the TS center is a prominent example. Here, we present low-temperature photoluminescence analyses on the single defect level. They reveal that the three occurring spectral signatures TS1, TS2 and TS3 originate from one single defect. Their polarization dependences expose three different crystallographic orientations in the basal plane, which relate to the projections of the nearest neighbor directions. Accordingly, we find a three-fold level-splitting in ensemble studies, when applying mechanical strain. This dependency is quantitatively calibrated. A complementary electrical measurement, deep level transient spectroscopy, reveals a charge transition level of the TS defect at 0.6 eV above the valence band. For a future identification, this accurate characterization of its optical and electronic properties along with their response to mechanical strain is a milestone., Comment: 8 pages + supporting information

Shallow Nitrogen-Vacancy (NV) centers are promising candidates for high-precision sensing applications; these defects, when positioned a few nanometers below the surface, provide an atomic-scale resolution along with substantial sensitivity. However, the dangling bonds and impurities on the diamond surface result in a complex environment which reduces the sensitivity and is unique to each shallow NV center. To avoid the environment's detrimental effect, we apply feedback-based quantum optimal control. We first show how a direct search can improve the initialization/readout process. In a second step, we optimize microwave pulses for pulsed Optically Detected Magnetic Resonance (ODMR) and Ramsey measurements. Throughout the sensitivity optimizations, we focus on robustness against errors in the control field amplitude. This feature not only protects the protocols' sensitivity from drifts but also enlarges the sensing volume. The resulting ODMR measurements produce sensitivities below 1$\mu$T\,Hz$^{-\frac{1}{2}}$ for an 83\% decrease in control power, increasing the robustness by approximately one third. The optimized Ramsey measurements produce sensitivities below 100\,nT\,Hz$^{-\frac{1}{2}}$ giving a two-fold sensitivity improvement. Being on par with typical sensitivities obtained via single NV magnetometry, the complementing robustness of the presented optimization strategy may provide an advantage for other NV-based applications., Comment: 16 pages, 11 figures

Individual nitrogen vacancy (NV) color centers in diamond are versatile, spin-based quantum sensors. Coherently controlling the spin of NV centers using microwaves in a typical frequency range between 2.5 and 3.5 GHz is necessary for sensing applications. In this work, we present a stripline-based, planar, {\Omega}-shaped microwave antenna that enables to reliably manipulate NV spins. We find an optimal antenna design using finite integral simulations. We fabricate our antennas on low-cost, transparent glass substrate. We demonstrate highly uniform microwave fields in areas of roughly 400 x 400 {\mu}m^2 while realizing high Rabi frequencies of up to 10 MHz in an ensemble of NV centers.

Diamond based quantum technology is a fast emerging field with both scientific and technological importance. With the growing knowledge and experience concerning diamond based quantum systems, comes an increased demand for performance. Quantum optimal control (QOC) provides a direct solution to a number of existing challenges as well as a basis for proposed future applications. Together with a swift review of QOC strategies, quantum sensing and other relevant quantum technology applications of nitrogen-vacancy (NV) centers in diamond, we give the necessary background to summarize recent advancements in the field of QOC assisted quantum applications with NV centers in diamond., Comment: 29 pages, 24 figures, Review article

We present a new approach combining top down fabrication and bottom up overgrowth to create diamond photonic nanostructures in form of single-crystalline diamond nanopyramids. Our approach relies on diamond nanopillars, that are overgrown with single-crystalline diamond to form pyramidal structures oriented along crystal facets. To characterize the photonic properties of the pyramids, color centers are created in a controlled way using ion implantation and annealing. We find very high collection efficiency from color centers close to the pyramid apex. We further show excellent smoothness and sharpness of our diamond pyramids with measured tip radii on the order of 10 nm. Our results offer interesting prospects for nanoscale quantum sensing using diamond color centers, where our diamond pyramids could be used as scanning probes for nanoscale imaging. There, our approach would offer signifikant advantages compared to the cone-shaped scanning probes which define the current state of the art., Comment: 6 pages, 4 figures

We investigate the influence of plasma treatments, especially a 0V-bias, potentially low damage O$_2$ plasma as well as a biased Ar/SF$_6$/O$_2$ plasma on shallow, negative nitrogen vacancy (NV$^-$) centers. We ignite and sustain using our 0V-bias plasma using purely inductive coupling. To this end, we pre-treat surfaces of high purity chemical vapor deposited single-crystal diamond (SCD). Subsequently, we create $\sim$10 nm deep NV$^-$ centers via implantation and annealing. Onto the annealed SCD surface, we fabricate nanopillar structures that efficiently waveguide the photoluminescence (PL) of shallow NV$^-$. Characterizing single NV$^-$ inside these nanopillars, we find that the Ar/SF$_6$/O$_2$ plasma treatment quenches NV$^-$ PL even considering that the annealing and cleaning steps following ion implantation remove any surface termination. In contrast, for our 0V-bias as well as biased O$_2$ plasma, we observe stable NV$^-$ PL and low background fluorescence from the photonic nanostructures., Comment: accepted for publication in Optical Materials Express

In this manuscript, we outline a reliable procedure to manufacture photonic nanostructures from single-crystal diamond (SCD). Photonic nanostructures, in our case SCD nanopillars on thin (< 1$\mu$m) platforms, are highly relevant for nanoscale sensing. The presented top-down procedure includes electron beam lithography (EBL) as well as reactive ion etching (RIE). Our method introduces a novel type of inter-layer, namely silicon, that significantly enhances the adhesion of hydrogen silsesquioxane (HSQ) electron beam resist to SCD and avoids sample charging during EBL. In contrast to previously used adhesion layers, our silicon layer can be removed using a highly-selective RIE step which is not damaging HSQ mask structures. We thus refine published nanofabrication processes to ease a higher process reliability especially in the light of the advancing commercialization of SCD sensor devices., Comment: v2: accepted for publication in Micromachines 2019, 10(11), 718; https://doi.org/10.3390/mi10110718

Powered by the mutual developments in instrumentation, materials andtheoretical descriptions, sensing and imaging capabilities of quantum emitters insolids have significantly increased in the past two decades. Quantum emitters insolids, whose properties resemble those of atoms and ions, provide alternative waysto probing natural and artificial nanoscopic systems with minimum disturbance andultimate spatial resolution. Among those emerging quantum emitters, the nitrogen-vacancy (NV) color center in diamond is an outstanding example due to its intrinsicproperties at room temperature (highly-luminescent, photo-stable, biocompatible,highly-coherent spin states). This review article summarizes recent advances andachievements in using NV centers within nano- and single crystal diamonds in sensingand imaging. We also highlight prevalent challenges and material aspects for differenttypes of diamond and outline the main parameters to consider when using color centersas sensors. As a novel sensing resource, we highlight the properties of NV centersas light emitting electrical dipoles and their coupling to other nanoscale dipoles e.g.graphene.

Energy transfer between fluorescent probes lies at the heart of many applications ranging from bio-sensing and -imaging to enhanced photo-detection and light harvesting. In this work, we study F\"orster resonance energy transfer (FRET) between shallow defects in diamond --- nitrogen-vacancy (NV) centers --- and atomically-thin, two-dimensional materials --- tungsten diselenide (WSe$_2$). By means of fluorescence lifetime imaging, we demonstrate the occurrence of FRET in the WSe$_2$/NV system. Further, we show that in the coupled system, NV centers provide an additional excitation pathway for WSe$_2$ photoluminescence. Our results constitute the first step towards the realization of hybrid quantum systems involving single-crystal diamond and two-dimensional materials that may lead to new strategies for studying and controlling spin transfer phenomena and spin valley physics., Comment: v2: accepted for publication in Advanced Quantum Technologies

Nitrogen-vacancy (NV) centers feature outstanding properties like a spin coherence time of up to one second as well as a level structure offering the possibility to initialize, coherently manipulate and optically read-out the spin degree of freedom of the ground state. However, only about three percent of their photon emission are channeled into the zero phonon line (ZPL), limiting both the rate of indistinguishable single photons and the signal-to-noise ratio (SNR) of coherent spin-photon interfaces. We here report on the enhancement of the SNR of the optical spin read-out achieved by tuning the mode of a two-dimensional photonic crystal (PhC)cavity into resonance with the NV-ZPL. PhC cavities are fabricated by focused ion beam (FIB) milling in thin reactive ion (RIE) etched ultrapure single crystal diamond membranes featuring modes with Q-factors of up to 8250 at mode volumes below one cubic wavelength. NV centers are produced in the cavities in a controlled fashion by a high resolution atomic force microscope (AFM) implantation technique. On cavity resonance we observe a lifetime shortening from 9.0ns to 8.0ns as well as an enhancement of the ZPL emission by almost one order of magnitude. Although on resonance the collection efficiency of ZPL photons and the spin-dependent fluorescence contrast are reduced, the SNR of the optical spin read-out is almost tripled for the cavity-coupled NV centers.

We investigate native nitrogen (NV) and silicon vacancy (SiV) color centers in commercially available, heteroepitaxial, wafer-sized, mm thick, single-crystal diamond. We observe single, native NV centers with a density of roughly 1 NV per $\mu m^3$ and moderate coherence time ($T_2 = 5 \mu s$) embedded in an ensemble of SiV centers. Low-temperature spectroscopy of the SiV zero phonon line fine structure witnesses high crystalline quality of the diamond especially close to the growth surface, consistent with a reduced dislocation density. Using ion implantation and plasma etching, we verify the possibility to fabricate nanostructures with shallow color centers rendering our diamond material promising for fabrication of nanoscale sensing devices. As this diamond is available in wafer-sizes up to $100 mm$ it offers the opportunity to up-scale diamond-based device fabrication.

The negatively-charged nitrogen-vacancy center (NV) in diamond forms a versatile system for quantum sensing applications. Combining the advantageous properties of this atomic-sized defect with scanning probe techniques such as atomic force microscopy (AFM) enables nanoscale imaging of e.g. magnetic fields. To form a scanning probe device, we place single NVs shallowly (i.e. < 20 nm) below the top facet of a diamond nanopillar, which is located on a thin diamond platform of typically below 1 \mu m thickness. This device can be attached to an AFM head, forming an excellent scanning probe tip. Furthermore, it simultaneously influences the collectible photoluminescence (PL) rate of the NV located inside. Especially sensing protocols using continuous optically-detected magnetic resonance (ODMR) benefit from an enhanced collectible PL rate, improving the achievable sensitivity. This work presents a comprehensive set of simulations to quantify the influence of the device geometry on the collectible PL rate for individual NVs. Besides geometric parameters (e.g. pillar length, diameter and platform thickness), we also focus on fabrication uncertainties such as the exact position of the NV or the taper geometry of the pillar introduced by imperfect etching. As a last step, we use these individual results to optimize our current device geometry, yielding a realistic gain in collectible PL rate by a factor of 13 compared to bulk diamond and 1.8 compared to our unoptimized devices., Comment: 20 pages, 12 figures

Many promising applications of single crystal diamond and its color centers as sensor platform and in photonics require free-standing membranes with a thickness ranging from several micrometers to the few 100 nm range. In this work, we present an approach to conveniently fabricate such thin membranes with up to about one millimeter in size. We use commercially available diamond plates (thickness 50 $\mu$m) in an inductively coupled reactive ion etching process which is based on argon, oxygen and SF$_6$. We thus avoid using toxic, corrosive feed gases and add an alternative to previously presented recipes involving chlorine-based etching steps. Our membranes are smooth (RMS roughness <1 nm) and show moderate thickness variation (central part: <1 $\mu$m over $\approx \,$200x200 $\mu$m$^2$). Due to an improved etch mask geometry, our membranes stay reliably attached to the diamond plate in our chlorine-based as well as SF$_6$-based processes. Our results thus open the route towards higher reliability in diamond device fabrication and up-scaling., Comment: 9 pages, 4 figures, version 2 accepted for publication in MDPI micromachines

Individual, luminescent point defects in solids so called color centers are atomic-sized quantum systems enabling sensing and imaging with nanoscale spatial resolution. In this overview, we introduce nanoscale sensing based on individual nitrogen vacancy (NV) centers in diamond. We discuss two central challenges of the field: First, the creation of highly-coherent, shallow NV centers less than 10 nm below the surface of single-crystal diamond. Second, the fabrication of tip-like photonic nanostructures that enable efficient fluorescence collection and can be used for scanning probe imaging based on color centers with nanoscale resolution., Comment: Overview paper on sensing with defects in diamond, we focus on creation of shallow NV centers and nanostructures, Final Version published in Crystals

The nitrogen-vacancy (NV) center in diamond has an optically addressable, highly coherent spin. However, an NV center even in high quality single-crystalline material is a very poor source of single photons: extraction out of the high-index diamond is inefficient, the emission of coherent photons represents just a few per cent of the total emission, and the decay time is large. In principle, all three problems can be addressed with a resonant microcavity. In practice, it has proved difficult to implement this concept: photonic engineering hinges on nano-fabrication yet it is notoriously difficult to process diamond without degrading the NV centers. We present here a microcavity scheme which uses minimally processed diamond, thereby preserving the high quality of the starting material, and a tunable microcavity platform. We demonstrate a clear change in the lifetime for multiple individual NV centers on tuning both the cavity frequency and anti-node position, a Purcell effect. The overall Purcell factor $F_{\rm P}=2.0$ translates to a Purcell factor for the zero phonon line (ZPL) of $F_{\rm P}^{\rm ZPL}\sim30$ and an increase in the ZPL emission probability from $\sim 3 \%$ to $\sim 46 \%$. By making a step-change in the NV's optical properties in a deterministic way, these results pave the way for much enhanced spin-photon and spin-spin entanglement rates., Comment: 6 pages, 4 figures

We investigate bright fluorescence of nitrogen (NV)- and silicon-vacancy color centers in pyramidal, single crystal diamond tips which are commercially available as atomic force microscope probes. We coherently manipulate NV electronic spin ensembles with $T_2 = 7.7(3)\,\mu$s. Color center lifetimes in different tip heights indicate effective refractive index effects and quenching. Using numerical simulations, we verify enhanced photon rates from emitters close to the pyramid apex; a situation promising for scanning probe sensing., Comment: 4 pages, 3 figures, for additional information, please see supplementary material uploaded as ancillary file Version 2 to appear (open acces) in Applied Physics Letters

The electronic spin of the nitrogen vacancy (NV) center in diamond forms an atomically sized, highly sensitive sensor for magnetic fields. To harness the full potential of individual NV centers for sensing with high sensitivity and nanoscale spatial resolution, NV centers have to be incorporated into scanning probe structures enabling controlled scanning in close proximity to the sample surface. Here, we present an optimized procedure to fabricate single-crystal, all-diamond scanning probes starting from commercially available diamond and show a highly efficient and robust approach for integrating these devices in a generic atomic force microscope. Our scanning probes consisting of a scanning nanopillar (200 nm diameter, $1-2\,\mu$m length) on a thin ($< 1\mu$m) cantilever structure, enable efficient light extraction from diamond in combination with a high magnetic field sensitivity ($\mathrm{\eta_{AC}}\approx50\pm20\,\mathrm{nT}/\sqrt{\mathrm{Hz}}$). As a first application of our scanning probes, we image the magnetic stray field of a single Ni nanorod. We show that this stray field can be approximated by a single dipole and estimate the NV-to-sample distance to a few tens of nanometer, which sets the achievable resolution of our scanning probes.

We demonstrate the controlled preparation of heteroepitaxial diamond nano- and microstructures on silicon wafer based iridium films as hosts for single color centers. Our approach uses electron beam lithography followed by reactive ion etching to pattern the carbon layer formed by bias enhanced nucleation on the iridium surface. In the subsequent chemical vapor deposition process, the patterned areas evolve into regular arrays of (001) oriented diamond nano-islands with diameters of <500nm and a height of approx. 60 nm. In the islands, we identify single SiV color centers with narrow zero phonon lines down to 1 nm at room temperature., Comment: version 2 accepted for publication in APL

Microscopic studies of superconductors and their vortices play a pivotal role in our understanding of the mechanisms underlying superconductivity. Local measurements of penetration depths or magnetic stray-fields enable access to fundamental aspects of superconductors such as nanoscale variations of superfluid densities or the symmetry of their order parameter. However, experimental tools, which offer quantitative, nanoscale magnetometry and operate over the large range of temperature and magnetic fields relevant to address many outstanding questions in superconductivity, are still missing. Here, we demonstrate quantitative, nanoscale magnetic imaging of Pearl vortices in the cuprate superconductor YBCO, using a scanning quantum sensor in form of a single Nitrogen-Vacancy (NV) electronic spin in diamond. The sensor-to-sample distance of ~10nm we achieve allows us to observe striking deviations from the prevalent monopole approximation in our vortex stray-field images, while we find excellent quantitative agreement with Pearl's analytic model. Our experiments yield a non-invasive and unambiguous determination of the system's local London penetration depth, and are readily extended to higher temperatures and magnetic fields. These results demonstrate the potential of quantitative quantum sensors in benchmarking microscopic models of complex electronic systems and open the door for further exploration of strongly correlated electron physics using scanning NV magnetometry., Comment: Main text (5 pages, 4 figures) plus supplementary material (5 pages, 6 figures). Comments welcome. Further information under http://www.quantum-sensing.ch

We report on imaging of microwave (MW) magnetic fields using a magnetometer based on the electron spin of a nitrogen vacancy center in diamond. We quantitatively image the magnetic field generated by high frequency (GHz) MW current with nanoscale resolution using a scanning probe technique. We demonstrate a MW magnetic field sensitivity in the range of a few nT/$\sqrt{\text{Hz}}$, polarization selection and broadband capabilities under ambient conditions and thereby establish the nitrogen vacancy center a versatile and high performance tool for the detection of MW fields. As a first application of this scanning MW detector, we determine the MW current density in a stripline and demonstrate a MW current sensitivity of a few nA/$\sqrt{\text{Hz}}$

Quantum devices for sensing and computing applications require coherent quantum systems which can be manipulated in a fast and robust way. Such quantum control is typically achieved using external electric or magnetic fields which drive the system's orbital or spin degrees of freedom. However, most of these approaches require complex and unwieldy antenna or gate structures, and with few exceptions are limited to the regime of weak driving. Here, we present a novel approach to strongly and coherently drive a single electron spin in the solid state using internal strain fields in an integrated quantum device. Specifically, we study individual Nitrogen-Vacancy (NV) spins embedded in diamond mechanical oscillators and exploit the intrinsic strain coupling between spin and oscillator to strongly drive the spins. As hallmarks of the strong driving regime, we directly observe the energy spectrum of the emerging phonon-dressed states and employ our strong, continuous driving for enhancement of the NV spin coherence time. Our results constitute a first step towards strain-driven, integrated quantum devices and open new perspectives to investigate unexplored regimes of strongly driven multi-level systems and to study exotic spin dynamics in hybrid spin-oscillator devices., Comment: 7 pages, 4 figures. Including supplementary material. Comments welcome. Further information under http://quantum-sensing.ch

We investigate the arrival statistics of Stokes (S) and anti-Stokes (aS) Raman photons generated in diamond membranes. Strong quantum correlations between the S and aS signals are observed, which implies that the two processes share the same phonon, that is, the phonon excited by the S process is consumed in the aS process. We show that the intensity cross-correlation $g_{\rm S,aS}^{(2)}(0)$, which describes the simultaneous detection of Stokes and anti-Stokes photons, decreases steadily with laser power as $1/{\rm P_L}$. Contrary to many other material systems, diamond exhibits a maximum $g_{\rm S,aS}^{(2)}(0)$ at very low pump powers, implying that the Stokes-induced aS photons outnumber the thermally generated aS photons. On the other hand, the coincidence rate shows a quadratic plus cubic power dependence, which indicates a departure from the Stokes-induced anti-Stokes process., Comment: 5 pages, 4 figures

The burgeoning field of nanophotonics has grown to be a major research area, primarily because of the ability to control and manipulate single quantum systems (emitters) and single photons on demand. For many years studying nanophotonic phenomena was limited to traditional semiconductors (including silicon and GaAs) and experiments were carried out predominantly at cryogenic temperatures. In the last decade, however, diamond has emerged as a new contender to study photonic phenomena at the nanoscale. Offering plethora of quantum emitters that are optically active at room temperature and ambient conditions, diamond has been exploited to demonstrate super-resolution microscopy and realize entanglement, Purcell enhancement and other quantum and classical nanophotonic effects. Elucidating the importance of diamond as a material, this review will highlight the recent achievements in the field of diamond nanophotonics, and convey a roadmap for future experiments and technological advancements., Comment: Advanced Optical Materials (2014)

Twisted-bilayer graphene (tBLG) exhibits van Hove singularities in the density of states that can be tuned by changing the twisting angle $\theta$. A $\theta$-defined tBLG has been produced and characterized with optical reflectivity and resonance Raman scattering. The $\theta$-engineered optical response is shown to be consistent with persistent saddle-point excitons. Separate resonances with Stokes and anti-Stokes Raman scattering components can be achieved due to the sharpness of the two-dimensional saddle-point excitons, similar to what has been previously observed for one-dimensional carbon nanotubes. The excitation power dependence for the Stokes and anti-Stokes emissions indicate that the two processes are correlated and that they share the same phonon., Comment: 5 pages, 6 figures

We demonstrate the fabrication of single-crystalline diamond nanopillars on a (111)-oriented chemical vapor deposited diamond substrate. This crystal orientation offers optimal coupling of nitrogen-vacancy (NV) center emission to the nanopillar mode and is thus advantageous over previous approaches. We characterize single native NV centers in these nanopillars and find one of the highest reported saturated fluorescence count rates in single crystalline diamond in excess of 10${}^6$ counts per second. We show that our nano-fabrication procedure conserves the preferential alignment as well as the spin coherence of the NVs in our structures. Our results will enable a new generation of highly sensitive probes for NV magnetometry and pave the way toward photonic crystals with optimal orientation of the NV center's emission dipole., Comment: 4 pages original manuscript, 3 pages supplementary material

We report on single electronic spins coupled to the motion of mechanical resonators by a novel mechanism based on crystal strain. Our device consists of single-crystalline diamond cantilevers with embedded Nitrogen-Vacancy center spins. Using optically detected electron spin resonance, we determine the unknown spin-strain coupling constants and demonstrate that our system resides well within the resolved sideband regime. We realize coupling strengths exceeding ten MHz under mechanical driving and show that our system has the potential to reach strong coupling. Our novel hybrid system forms a resource for future experiments on spin-based cantilever cooling and coherent spin-oscillator coupling., Comment: 4 pages, 4 figures and supplementary information. Comments welcome. Further information under http://www.quantum-sensing.physik.unibas.ch/

Colour centres in diamond have emerged as versatile tools for solid-state quantum technologies ranging from quantum information to metrology, where the nitrogen-vacancy centre is the most studied to-date. Recently, this toolbox has expanded to include different materials for their nanofabrication opportunities, and novel colour centres to realize more efficient spin-photon quantum interfaces. Of these, the silicon-vacancy centre stands out with ultrabright single photon emission predominantly into the desirable zero-phonon line. The challenge for utilizing this centre is to realise the hitherto elusive optical access to its electronic spin. Here, we report spin-tagged resonance fluorescence from the negatively charged silicon-vacancy centre. In low-strain bulk diamond spin-selective excitation under finite magnetic field reveals a spin-state purity approaching unity in the excited state. We also investigate the effect of strain on the centres in nanodiamonds and discuss how spin selectivity in the excited state remains accessible in this regime., Comment: Additional data and analysis in PDF format is available for download at the publications section of http://www.amop.phy.cam.ac.uk/amop-ma

We demonstrate a quantum key distribution (QKD) testbed for room temperature single photon sources based on defect centres in diamond. A BB84 protocol over a short free-space transmission line is implemented. The performance of nitrogen-vacancy (NV) as well as silicon-vacancy defect (SiV) centres is evaluated and an extrapolation for next-generation sources with enhanced efficiency is discussed., Comment: 14 pages, 5 figures

We report fluorescence investigations and Raman spectroscopy on colloidal nanodiamonds (NDs) obtained via bead assisted sonic disintegration (BASD) of a polycrystalline chemical vapor deposition film. The BASD NDs contain in situ created silicon vacancy (SiV) centers. Whereas many NDs exhibit emission from SiV ensembles, we also identify NDs featuring predominant emission from a single bright SiV center. We demonstrate oxidation of the NDs in air as a tool to optimize the crystalline quality of the NDs via removing damaged regions resulting in a reduced ensemble linewidth as well as single photon emission with increased purity. We furthermore investigate the temperature dependent zero-phonon-line fine-structure of a bright single SiV center as well as the polarization properties of its emission and absorption., Comment: 7 pages, 6 figures, version 2 accepted for publication in Journal of Applied Physics

We study single silicon vacancy (SiV) centres in chemical vapour deposition (CVD) nanodiamonds on iridium as well as an ensemble of SiV centres in a high quality, low stress CVD diamond film by using temperature dependent luminescence spectroscopy in the temperature range 5-295 K. We investigate in detail the temperature dependent fine structure of the zero-phonon-line (ZPL) of the SiV centres. The ZPL transition is affected by inhomogeneous as well as temperature dependent homogeneous broadening and blue shifts by about 20 cm-1 upon cooling from room temperature to 5 K. We employ excitation power dependent g(2) measurements to explore the temperature dependent internal population dynamics of single SiV centres and infer almost temperature independent dynamics., Comment: 19 pages, 9 figures, latest version accepted for publication in New Journal of Physics

Single silicon vacancy (SiV) color centers in diamond have recently shown the ability for high brightness, narrow bandwidth, room temperature single photon emission. This work develops a model describing the three level population dynamics of single SiV centers in diamond nanocrystals on iridium surfaces including an intensity dependent de-shelving process. Furthermore, we investigate the brightness and photostability of single centers and find maximum single photon rates of 6.2 Mcps under continuous excitation. We investigate the collection efficiency of the fluorescence and estimate quantum efficiencies of the SiV centers., Comment: 15 pages, 7 figures, version 2 accepted for publication in Optics Express

Photoluminescence (PL) spectra of single silicon vacancy (SiV) centers frequently feature very narrow room temperature PL lines in the near-infrared (NIR) spectral region, mostly between 820 nm and 840 nm, in addition to the well known zero-phonon-line (ZPL) at approx. 738 nm [E. Neu et al., Phys. Rev. B 84, 205211 (2011)]. We here exemplarily prove for a single SiV center that this NIR PL is due to an additional purely electronic transition (ZPL). For the NIR line at 822.7 nm, we find a room temperature linewidth of 1.4 nm (2.6 meV). The line saturates at similar excitation power as the ZPL. ZPL and NIR line exhibit identical polarization properties. Cross-correlation measurements between the ZPL and the NIR line reveal anti-correlated emission and prove that the lines originate from a single SiV center, furthermore indicating a fast switching between the transitions (0.7 ns). g(2) auto-correlation measurements exclude that the NIR line is a vibronic sideband or that it arises due to a transition from/to a meta-stable (shelving) state., Comment: 9 pages, 7 figures, v2 accepted for publication in Phys. Rev. B

The development of solid-state photonic quantum technologies is of great interest for fundamental studies of light-matter interactions and quantum information science. Diamond has turned out to be an attractive material for integrated quantum information processing due to the extraordinary properties of its colour centres enabling e.g. bright single photon emission and spin quantum bits. To control emitted photons and to interconnect distant quantum bits, micro-cavities directly fabricated in the diamond material are desired. However, the production of photonic devices in high-quality diamond has been a challenge so far. Here we present a method to fabricate one- and two-dimensional photonic crystal micro-cavities in single-crystal diamond, yielding quality factors up to 700. Using a post-processing etching technique, we tune the cavity modes into resonance with the zero phonon line of an ensemble of silicon-vacancy centres and measure an intensity enhancement by a factor of 2.8. The controlled coupling to small mode volume photonic crystal cavities paves the way to larger scale photonic quantum devices based on single-crystal diamond.

We introduce an advanced material system for the production and spectroscopy of single silicon vacancy (SiV) color centers in diamond. We use microwave plasma chemical vapor deposition to synthesize heteroepitaxial nanodiamonds of approx. 160 nm in lateral size with a thickness of approx. 75 nm. These oriented 'nanoislands' combine the enhanced fluorescence extraction from subwavelength sized nanodiamonds with defined crystal orientation. The investigated SiV centers display narrow zero-phonon-lines down to 0.7 nm in the wavelength range 730-750 nm. We investigate in detail the phonon-coupling and vibronic sidebands of single SiV centers, revealing significant inhomogeneous effects. Polarization measurements reveal polarized luminescence and preferential absorption of linearly polarized light., Comment: 9 pages, 9 figures, v3 slightly revised, accepted by Phys. Rev. B

We report on the production of nanodiamonds (NDs) with 70-80 nm size via bead assisted sonic disintegration (BASD) of a polycrystalline chemical vapor deposition (CVD) film. The NDs display high crystalline quality as well as intense narrowband (7 nm) room temperature luminescence at 738 nm due to in situ incorporated silicon vacancy (SiV) centers. The fluorescence properties at room and cryogenic temperatures indicate that the NDs are, depending on preparation, applicable as single photon sources or as fluorescence labels., Comment: The following article has been accepted by Applied Physics Letters. After it is published, it will be found at http://apl.aip.org/. 4 pages, 6 figures

We introduce a process for the fabrication of high quality, spatially isolated nano-diamonds on iridium via microwave plasma assisted CVD-growth. We perform spectroscopy of single silicon-vacancy (SiV)-centres produced during the growth of the nano-diamonds. The colour centres exhibit extraordinary narrow zero-phonon-lines down to 0.7 nm at room temperature. Single photon count rates up to 4.8 Mcps at saturation make these SiV-centres the brightest diamond based single photon sources to date. We measure for the first time the fine structure of a single SiV-centre thus confirming the atomic composition of the investigated colour centres., Comment: 20 pages, 13 figures, accepted by New Journal of Physics

We present investigations on single Ni/Si related color centers produced via ion implantation into single crystalline type IIa CVD diamond. Testing different ion dose combinations we show that there is an upper limit for both the Ni and the Si dose 10^12/cm^2 and 10^10/cm^2 resp.) due to creation of excess fluorescent background. We demonstrate creation of Ni/Si related centers showing emission in the spectral range between 767nm and 775nm and narrow line-widths of 2nm FWHM at room temperature. Measurements of the intensity auto-correlation functions prove single-photon emission. The investigated color centers can be coarsely divided into two groups: Drawing from photon statistics and the degree of polarization in excitation and emission we find that some color centers behave as two-level, single-dipole systems whereas other centers exhibit three levels and contributions from two orthogonal dipoles. In addition, some color centers feature stable and bright emission with saturation count rates up to 78kcounts/s whereas others show fluctuating count rates and three-level blinking., Comment: 7 pages, submitted to Applied Physics B, revised version

Color centers in diamond are very promising candidates among the possible realizations for practical single-photon sources because of their long-time stable emission at room temperature. The popular nitrogen-vacancy center shows single-photon emission, but within a large, phonon-broadened spectrum (~100nm), which strongly limits its applicability for quantum communication. By contrast, Ni-related centers exhibit narrow emission lines at room temperature. We present investigations on single color centers consisting of Ni and Si created by ion implantation into single crystalline IIa diamond. We use systematic variations of ion doses between 10^8/cm^2 and 10^14/cm^2 and energies between 30keV and 1.8MeV. The Ni-related centers show emission in the near infrared spectral range (~770nm to 787nm) with a small line-width (~3nm FWHM). A measurement of the intensity correlation function proves single-photon emission. Saturation measurements yield a rather high saturation count rate of 77.9 kcounts/s. Polarization dependent measurements indicate the presence of two orthogonal dipoles., Comment: 8 pages, published in conference proceedings of SPIE Photonics Europe 2010

We present the design of two-dimensional photonic crystal microcavities in thin diamond membranes well suited for coupling of color centers in diamond. By comparing simulated and ideal field distributions in Fourier and real space and by according modification of air hole positions and size, we optimize the cavity structure yielding high quality factors up to Q = 320000 with a modal volume of V = 0.35 (lambda/n)^3. Using the very same approach we also improve previous designs of a small modal volume microcavity in silicon, gaining a factor of 3 in cavity Q. In view of practical realization of photonic crystals in synthetic diamond films, it is necessary to investigate the influence of material absorption on the quality factor. We show that this influence can be predicted by a simple model, replacing time consuming simulations., Comment: 25 pages, 15 figures