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Dynamical decoupling multipulse sequences can be applied to solid state spins for sensing weak oscillating fields from nearby single nuclear spins. By periodically reversing the probing system's evolution, other noises are counteracted and filtered out over the total evolution. However, the technique is subject to intricate interactions resulting in additional resonant responses, which can be misinterpreted with the actual signal intended to be measured. We experimentally characterized three of these effects present in single nitrogen vacancy centers in diamond, where we also developed a numerical simulations model without rotating wave approximations, showing robust correlation to the experimental data. Regarding centers with the $^{15}$N nitrogen isotope, we observed that a small misalignment in the bias magnetic field causes the precession of the nitrogen nuclear spin to be sensed by the electronic spin of the center. Another studied case of ambiguous resonances comes from the coupling with lattice $^{13}$C nuclei, where we reconstructed the interaction Hamiltonian based on echo modulation frequencies and used this Hamiltonian to simulate multipulse sequences. Finally, we also measured and simulated the effects from the free evolution of the quantum system during finite pulse durations. Due to the large data volume and the strong dependency of these ambiguous resonances with specific experimental parameters, we provide a simulations dataset with a user-friendly graphical interface, where users can compare simulations with their own experimental data for spectral disambiguation. Although focused with nitrogen vacancy centers and dynamical decoupling sequences, these results and the developed model can potentially be applied to other solid state spins and quantum sensing techniques., Comment: 15 pages, 9 figures, for associated dataset, see https://figshare.com/articles/dataset/Dataset_for_Ambiguous_Resonances_in_Multipulse_Quantum_Sensing_with_NVs/26245895

While the advantages of photonic quantum computing, including direct compatibility with communication, are apparent, several imperfections such as loss and distinguishability presently limit actual implementations. These imperfections are unlikely to be completely eliminated, and it is therefore beneficial to investigate which of these are the most dominant and what is achievable under their presence. In this work, we provide an in-depth investigation of the influence of photon loss, multi-photon terms and photon distinguishability on the generation of photonic 3-partite GHZ states via established fusion protocols. We simulate the generation process for SPDC and solid-state-based single-photon sources using realistic parameters and show that different types of imperfections are dominant with respect to the fidelity and generation success probability. Our results indicate what are the dominant imperfections for the different photon sources and in which parameter regimes we can hope to implement photonic quantum computing in the near future., Comment: 18 pages, 6 figures, 3 tables

The detection of individual charges plays a crucial role in fundamental material science and the advancement of classical and quantum high-performance technologies that operate with low noise. However, resolving charges at the lattice scale in a time-resolved manner has not been achieved so far. Here, we present the development of an electrometer, leveraging on the spectroscopy of an optically-active spin defect embedded in a solid-state material with a non-linear Stark response. By applying our approach to diamond, a widely used platform for quantum technology applications, we successfully localize charge traps, quantify their impact on transport dynamics and noise generation, analyze relevant material properties, and develop strategies for material optimization., Comment: Main: 9 pages, 5 figures; Supplement: 14 pages, 10 figures

Diamond has emerged as a highly promising platform for quantum network applications. Color centers in diamond fulfill the fundamental requirements for quantum nodes: they constitute optically accessible quantum systems with long-lived spin qubits. Furthermore, they provide access to a quantum register of electronic and nuclear spin qubits and they mediate entanglement between spins and photons. All these operations require coherent control of the color center's spin state. This review provides a comprehensive overview of the state-of-the-art, challenges, and prospects of such schemes, including, high fidelity initialization, coherent manipulation, and readout of spin states. Established microwave and optical control techniques are reviewed, and moreover, emerging methods such as cavity-mediated spin-photon interactions and mechanical control based on spin-phonon interactions are summarized. For different types of color centers, namely, nitrogen-vacancy and group-IV color centers, distinct challenges persist that are subject of ongoing research. Beyond fundamental coherent spin qubit control techniques, advanced demonstrations in quantum network applications are outlined, for example, the integration of individual color centers for accessing (nuclear) multi-qubit registers. Finally, we describe the role of diamond spin qubits in the realization of future quantum information applications.

The coherence characteristics of a tin-vacancy color center in diamond are investigated through optical means including coherent population trapping between the ground state orbital levels and linewidth broadening effects. Due to the large spin-orbit splitting of the orbital ground states, thermalization between the ground states occurs at rates that are impractical to measure directly. Here, spectral information is transformed into its conjugate variable time, providing picosecond resolution and revealing an orbital depolarization timescale of ${\sim30{\rm~ps}}$. Consequences of the investigated dynamics are then used to estimate spin dephasing times limited by thermal effects., Comment: 17 pages, 14 figures, 1 table

The coherent excitation of an optically active spin system is one of the key elements in the engineering of a spin-photon interface. In this work, we use the novel SUPER scheme, employing nonresonant ultrashort optical pulses, to coherently control the main optical transition of a tin-vacancy color center in diamond, a promising emitter that can both be utilized as a quantum memory and a single-photon source. Furthermore, we implement a subpicosecond control scheme using resonant pulses for achieving record short quantum gates applied to diamond color centers. The employed ultrafast quantum gates open up a new regime of quantum information processing with solid-state color centers, eventually enabling multi-gate operations with the optical qubit and efficient spectral filtering of the excitation laser from deterministically prepared coherent photons., Comment: 7 pages, 3 figures

Measurement-based quantum computation relies on single qubit measurements of large multipartite entangled states, so-called lattice-graph or cluster states. Graph states are also an important resource for quantum communication, where tree cluster states are a key resource for one-way quantum repeaters. A photonic realization of this kind of state would inherit many of the benefits of photonic platforms, such as very little dephasing due to weak environmental interactions and the well-developed infrastructure to route and measure photonic qubits. In this work, a linear cluster state and GHZ state generation scheme is developed for group-IV color centers. In particular, this article focuses on an in-depth investigation of the required control operations, including the coherent spin and excitation gates. We choose an off-resonant Raman scheme for the spin gates, which can be much faster than microwave control. We do not rely on a reduced level scheme and use efficient approximations to design high-fidelity Raman gates. We benchmark the spin-control and excitation scheme using the tin vacancy color center coupled to a cavity, assuming a realistic experimental setting. Additionally, the article investigates the fidelities of the Raman and excitation gates in the presence of radiative and non-radiative decay mechanisms. Finally, a quality measure is devised, which emphasizes the importance of fast and high-fidelity spin gates in the creation of large entangled photonic states., Comment: 19 pages, 13 figures

In the rapidly evolving area of integrated photonics, there is a growing need for materials that satisfy the particular requirements of increasingly complex and specialized devices and applications. Present photonic material platforms have made significant progress over the past years; however, each platform still faces specific material and performance challenges. We introduce a novel material for integrated photonics: Aluminum Gallium Nitride (AlGaN) on Aluminum Nitride (AlN) as a platform for developing reconfigurable and nonlinear on-chip optical systems. AlGaN combines compatibility with standard semiconductor fabrication technologies, high electro-optic modulation capabilities, and large nonlinear coefficients while providing a broad and low-loss spectral transmission range, making it a viable material for advanced photonic applications. In this work, we design and grow AlGaN/AlN heterostructures and integrate fundamental photonic building blocks into these chips. In particular, we fabricate edge couplers, low-loss waveguides, directional couplers, and tunable high-quality factor ring resonators to enable nonlinear light-matter interaction and quantum functionality. The comprehensive platform we present in this work paves the way for nonlinear photon-pair generation applications, on-chip nonlinear quantum frequency conversion, and fast electro-optic modulation for switching and routing classical and quantum light fields.

In this work, we provide a comprehensive overview of the microwave-induced manipulation of electronic spin states in negatively charged group-IV color centers in diamond with a particular emphasis on the influence of strain. Central to our investigation is the consideration of the full vectorial attributes of the magnetic fields involved, which are a dc field for lifting the degeneracy of the spin levels and an ac field for microwave control between two spin levels. We observe an intricate interdependence between their spatial orientations, the externally applied strain, and the resultant efficacy in spin state control. In most work to date the ac and dc magnetic field orientations have been insufficiently addressed, which has led to the conclusion that strain is indispensable for the effective microwave control of heavier group-IV vacancies, such as tin- and lead-vacancy color centers. In contrast, we find that the alignment of the dc magnetic field orthogonal to the symmetry axis and the ac field parallel to it can make the application of strain obsolete for effective spin manipulation. Furthermore, we explore the implications of this field configuration on the spin's optical initialization, readout, and gate fidelities., Comment: 13 Pages, 7 figures

Integrated and fiber-packaged magnetic field sensors with a sensitivity sufficient to sense electric pulses propagating along nerves in life science applications and with a spatial resolution fine enough to resolve their propagation directions will trigger a tremendous step ahead not only in medical diagnostics, but in understanding neural processes. Nitrogen-vacancy centers in diamond represent the leading platform for such sensing tasks under ambient conditions. Current research on uniting a good sensitivity and a high spatial resolution is facilitated by scanning or imaging techniques. However, these techniques employ moving parts or bulky microscope setups. Despite being far developed, both approaches cannot be integrated and fiber-packaged to build a robust, adjustment-free hand-held device. In this work, we introduce novel concepts for spatially resolved magnetic field sensing and 2-D gradiometry with an integrated magnetic field camera. The camera is based on infrared absorption optically detected magnetic resonance (IRA-ODMR) mediated by perpendicularly intersecting infrared and pump laser beams forming a pixel matrix. We demonstrate our 3-by-3 pixel sensor's capability to reconstruct the position of an electromagnet in space. Furthermore, we identify routes to enhance the magnetic field camera's sensitivity and spatial resolution as required for complex sensing applications., Comment: Main part: 14 pages, 5 figures

Color centers in diamond are quantum systems with optically active spin-states that show long coherence times and are therefore a promising candidate for the development of efficient spin-photon interfaces. However, only a small portion of the emitted photons is generated by the coherent optical transition of the zero-phonon line (ZPL), which limits the overall performance of the system. Embedding these emitters in photonic crystal cavities improves the coupling to the ZPL photons and increases their emission rate. Here, we demonstrate the fabrication process of "Sawfish" cavities, a design recently proposed that has the experimentally-realistic potential to simultaneously enhance the emission rate by a factor of 46 and couple photons into a single-mode fiber with an efficiency of 88%. The presented process allows for the fabrication of fully suspended devices with a total length of 20.5 $\mu$m and features size as small as 40 nm. The optical characterization shows fundamental mode resonances that follow the behavior expected from the corresponding design parameters and quality (Q) factors as high as 3825. Finally, we investigate the effects of nanofabrication on the devices and show that, despite a noticeable erosion of the fine features, the measured cavity resonances deviate by only 0.9 (1.2)% from the corresponding simulated values. This proves that the Sawfish design is robust against fabrication imperfections, which makes it an attractive choice for the development of quantum photonic networks., Comment: 7 pages, 9 figures

One-way quantum repeaters where loss and operational errors are counteracted by quantum error correcting codes can ensure fast and reliable qubit transmission in quantum networks. It is crucial that the resource requirements of such repeaters, for example, the number of qubits per repeater node and the complexity of the quantum error correcting operations are kept to a minimum to allow for near-future implementations. To this end, we propose a one-way quantum repeater that targets both the loss and operational error rates in a communication channel in a resource-efficient manner using code concatenation. Specifically, we consider a tree-cluster code as an inner loss-tolerant code concatenated with an outer 5-qubit code for protection against Pauli errors. Adopting flag-based stabilizer measurements, we show that intercontinental distances of up to 10,000 km can be bridged with a minimal resource overhead by interspersing repeater nodes that each specializes in suppressing either loss or operational errors. Our work demonstrates how tailored error-correcting codes can significantly lower the experimental requirements for long-distance quantum communication., Comment: 25 pages, 16 figures, 4 tables

Photon loss is one of the key challenges to overcome in complex photonic quantum applications. Photon collection efficiencies directly impact the amount of resources required for measurement-based quantum computation and communication networks. Promising resources include solid-state quantum light sources, however, efficiently coupling light from a single quantum emitter to a guided mode remains demanding. In this work, we eliminate photon losses by maximizing coupling efficiencies in an emitter-to-fiber interface. We develop a waveguide-integrated 'Sawfish' photonic crystal cavity and use finite element simulations to demonstrate that our system transfers, with 97.4% efficiency, the zero-phonon line emission of a negatively-charged tin vacancy center in diamond adiabatically to a single-mode fiber. A surrogate model trained by machine learning provides quantitative estimates of sensitivities to fabrication tolerances. Our corrugation-based design proves robust under state-of-the-art nanofabrication parameters, maintaining an emitter-to-fiber coupling efficiency of 88.6%. To demonstrate its potential in reducing resource requirements, we apply the Sawfish cavity to a recent one-way quantum repeater protocol., Comment: Main part: 16 pages, 7 figures

Optically active solid-state spin defects have the potential to become a versatile resource for quantum information processing applications. Nitrogen-vacancy defect centers (NV) in diamond act as quantum memories and can be interfaced by coherent photons as demonstrated in entanglement protocols. However, in particular in diamond nanostructures, the effect of spectral diffusion leads to optical decoherence hindering entanglement generation. In this work, we present strategies to significantly reduce the electric noise in diamond nanostructures. We demonstrate single NVs in nanopillars exhibiting lifetime-limited linewidth on the time scale of one second and long-term spectral stability with inhomogeneous linewidth as low as 150 MHz over three minutes. Excitation power and energy-dependent measurements in combination with nanoscopic Monte Carlo simulations contribute to a better understanding of the impact of bulk and surface defects on the NV's spectral properties. Finally, we propose an entanglement protocol for nanostructure-coupled NVs providing entanglement generation rates up to hundreds of kHz., Comment: Main part: 13 pages, 5 figures

Nanostructures can be used for boosting the light outcoupling of color centers in diamond; however, the fiber coupling performance of these nanostructures is rarely investigated. Here, we use a finite element method for computing the emission from color centers in inverted nanocones and the overlap of this emission with the propagation mode in a single-mode fiber. Using different figures of merit, the inverted nanocone parameters are optimized to obtain maximal fiber coupling efficiency, free-space collection efficiency, or rate enhancement. The optimized inverted nanocone designs show promising results with 66% fiber coupling or 83% free-space coupling efficiency at the tin-vacancy center zero-phonon line wavelength of 619 nm. Moreover, when evaluated for broadband performance, the optimized designs show 55% and 76% for fiber coupling and free-space efficiencies respectively, for collecting the full tin-vacancy emission spectrum at room temperature. An analysis of fabrication insensitivity indicates that these nanostructures are robust against imperfections. For maximum emission rate into a fiber mode, a design with a Purcell factor of 2.34 is identified. Finally, possible improvements offered by a hybrid inverted nanocone, formed by patterning into two different materials, are investigated, and increases the achievable fiber coupling efficiency to 71%., Comment: The following article has been accepted by Applied Physics Letters. After it is published, it will be found at https://doi.org/10.1063/5.0050338

We report coherent interactions within an ensemble of silicon-vacancy color centers in diamond. The interactions are ascribed to resonant dipole-dipole coupling. Further, we demonstrate control over resonant center pairs by using a driving optical pulse to induce collective, interaction-enabled Rabi-oscillations in the ensemble. Non-resonant center pairs do not undergo collective oscillations.

An ensemble of silicon vacancy centers in diamond (\ce{SiV-}) is probed using two coherent spectroscopy techniques. Two main distinct families of \ce{SiV-} centers are identified using multidimensional coherent spectroscopy, and these families are paired with two orientation groups by comparing spectra from different linear polarizations of the incident laser. By tracking the peak centers in the measured spectra, the full diamond strain tensor is calculated local to the laser spot. Such measurements are made at multiple points on the sample surface and variations in the strain tensor are observed.

Quantum phenomena are typically observable at length and time scales smaller than those of our everyday experience, often involving individual particles or excitations. The past few decades have seen a revolution in the ability to structure matter at the nanoscale, and experiments at the single particle level have become commonplace. This has opened wide new avenues for exploring and harnessing quantum mechanical effects in condensed matter. These quantum phenomena, in turn, have the potential to revolutionize the way we communicate, compute and probe the nanoscale world. Here, we review developments in key areas of quantum research in light of the nanotechnologies that enable them, with a view to what the future holds. Materials and devices with nanoscale features are used for quantum metrology and sensing, as building blocks for quantum computing, and as sources and detectors for quantum communication. They enable explorations of quantum behaviour and unconventional states in nano- and opto-mechanical systems, low-dimensional systems, molecular devices, nano-plasmonics, quantum electrodynamics, scanning tunnelling microscopy, and more. This rapidly expanding intersection of nanotechnology and quantum science/technology is mutually beneficial to both fields, laying claim to some of the most exciting scientific leaps of the last decade, with more on the horizon., Comment: Roadmap article with contributed sections and subsections on: 1. Metrology and sensing 2. Quantum light sources, cavities and detectors 3. Quantum computing with spins 4. Nano and opto-mechanics 5. Low-dimensional systems 6. Molecular devices 7. Nanoplasmonics (47 pages, 25 figures). Contains arXiv:1907.02625, arXiv:1907.07087, arXiv:2001.11119, arXiv:2011.13907

The particle-like nature of light becomes evident in the photon statistics of fluorescence of single quantum systems as photon antibunching. In multichromophoric systems, exciton diffusion and subsequent annihilation occurs. These processes also yield photon antibunching but cannot be interpreted reliably. Here, we develop picosecond time-resolved antibunching (psTRAB) to identify and decode such processes. We use psTRAB to measure the true number of chromophores on well-defined multichromophoric DNA-origami structures, and precisely determine the distance-dependent rates of annihilation between excitons. Further, psTRAB allows us to measure exciton diffusion in mesoscopic H- and J-type conjugated-polymer aggregates. We distinguish between one-dimensional intra-chain and three-dimensional inter-chain exciton diffusion at different times after excitation and determine the disorder-dependent diffusion lengths. Our method provides a new lens through which excitons can be studied at the single-particle level, enabling the rational design of improved excitonic probes such as ultra-bright fluorescent nanoparticles, and materials for optoelectronic devices.

We characterize a high-density sample of negatively charged silicon-vacancy (SiV$^-$) centers in diamond using collinear optical multidimensional coherent spectroscopy. By comparing the results of complementary signal detection schemes, we identify a hidden population of \ce{SiV^-} centers that is not typically observed in photoluminescence, and which exhibits significant spectral inhomogeneity and extended electronic $T_2$ times. The phenomenon is likely caused by strain, indicating a potential mechanism for controlling electric coherence in color-center-based quantum devices.

An ideal point light source is as small and as bright as possible. For fluorescent point light sources, homogeneity of the light sources is important as well as that the fluorescent units inside the light source maintain their photophysical properties which is compromised by dye aggregation. Here we propose DNA origami as a rigid scaffold to arrange dye molecules in a dense pixel array with high control of stoichiometry and dye-dye interactions. In order to find the highest labeling density in a DNA origami structure without influencing dye photophysics we alter the distance of two ATTO647N dyes in single base pair steps and probe the dye-dye interactions on the single-molecule level. For small distances strong quenching in terms of intensity and fluorescence lifetime is observed. With increasing distance, we observe reduced quenching and molecular dynamics. However, energy transfer processes in the weak coupling regime still have a significant impact and can lead to quenching by singlet-dark-state-annihilation. Our study fills a gap of studying the interactions of dyes relevant for superresolution microscopy with dense labeling and for single-molecule biophysics. Incorporating these findings in a 3D DNA origami object will pave the way to bright and homogeneous DNA origami nanobeads.

The capability to embed self-assembled quantum dots (QDs) at predefined positions in nanophotonic structures is key to the development of complex quantum photonic architectures. Here, we demonstrate that QDs can be deterministically positioned in nanophotonic waveguides by pre-locating QDs relative to a global reference frame using micro-photoluminescence ($\mu$PL) spectroscopy. After nanofabrication, $\mu$PL images reveal misalignments between the central axis of the waveguide and the embedded QD of only $(9\pm46$) nm and $(1\pm33$) nm, for QDs embedded in undoped and doped membranes, respectively. A priori knowledge of the QD positions allows us to study the spectral changes introduced by nanofabrication. We record average spectral shifts ranging from 0.1 to 1.1 nm, indicating that the fabrication-induced shifts can generally be compensated by electrical or thermal tuning of the QDs. Finally, we quantify the effects of the nanofabrication on the polarizability, the permanent dipole moment and the emission frequency at vanishing electric field of different QD charge states, finding that these changes are constant down to QD-surface separations of only 70 nm. Consequently, our approach deterministically integrates QDs into nanophotonic waveguides whose light-fields contain nanoscale structure and whose group index varies at the nanometer level., Comment: 26 pages, 9 figures. Updated version of the manuscript, with new appendices and new figures

Waveguide-based spin-photon interfaces on the GaAs platform have emerged as a promising system for a variety of quantum information applications directly integrated into planar photonic circuits. The coherent control of spin states in a quantum dot can be achieved by applying circularly polarized laser pulses that may be coupled into the planar waveguide vertically through radiation modes. However, proper control of the laser polarization is challenging since the polarization is modified through the transformation from the far field to the exact position of the quantum dot in the nanostructure. Here we demonstrate polarization-controlled excitation of a quantum-dot electron spin and use that to perform coherent control in a Ramsey interferometry experiment. The Ramsey interference reveals a pure dephasing time of $ 2.2\pm0.1 $ ns, which is comparable to the values so far only obtained in bulk media. We analyze the experimental limitations in spin initialization fidelity and Ramsey contrast and identify the underlying mechanisms.

Medium-scale ensembles of coupled qubits offer a platform for near-term quantum technologies including computing, sensing, and the study of mesoscopic quantum systems. Atom-like emitters in solids have emerged as promising quantum memories, with demonstrations of spin-spin entanglement by optical and magnetic interactions. Magnetic coupling in particular is attractive for efficient and deterministic entanglement gates, but raises the problem of individual spin addressing at the necessary nanometer-scale separation. Current super-resolution techniques can reach this resolution, but are destructive to the states of nearby qubits. Here, we demonstrate the measurement of individual qubit states in a sub-diffraction cluster by selectively exciting spectrally distinguishable nitrogen vacancy (NV) centers. We demonstrate super-resolution localization of single centers with nanometer spatial resolution, as well as individual control and readout of spin populations. These measurements indicate a readout-induced crosstalk on non-addressed qubits below $4\times10^{-2}$. This approach opens the door to high-speed control and measurement of qubit registers in mesoscopic spin clusters, with applications ranging from entanglement-enhanced sensors to error-corrected qubit registers to multiplexed quantum repeater nodes., Comment: 7 pages, 4 figures + supplementary information

Quantum emitters in solids are being developed for a range of quantum technologies, including quantum networks, computing, and sensing. However, a remaining challenge is the poor photon collection due to the high refractive index of most host materials. Here we overcome this limitation by introducing monolithic parabolic reflectors as an efficient geometry for broadband photon extraction from quantum emitter and experimentally demonstrate this device for the nitrogen-vacancy (NV) center in diamond. Simulations indicate a photon collection efficiency exceeding 75% across the visible spectrum and experimental devices, fabricated using a high-throughput gray-scale lithography process, demonstrate a photon extraction efficiency of $(48\pm 5)$%. This device enables a raw experimental efficiency of $(12\pm 2)$\% with fluorescence detection rates as high as $(4.6 - 5.7)\times 10^6$ counts per second from a single NV center.

Access to the electron spin is at the heart of many protocols for integrated and distributed quantum-information processing [1-4]. For instance, interfacing the spin-state of an electron and a photon can be utilized to perform quantum gates between photons [2,5] or to entangle remote spin states [6-9]. Ultimately, a quantum network of entangled spins constitutes a new paradigm in quantum optics [1]. Towards this goal, an integrated spin-photon interface would be a major leap forward. Here we demonstrate an efficient and optically programmable interface between the spin of an electron in a quantum dot and photons in a nanophotonic waveguide. The spin can be deterministically prepared with a fidelity of 96\%. Subsequently the system is used to implement a "single-spin photonic switch", where the spin state of the electron directs the flow of photons through the waveguide. The spin-photon interface may enable on-chip photon-photon gates [2], single-photon transistors [10], and efficient photonic cluster state generation [11].

We demonstrate the fabrication of photonic crystal nanobeam cavities with rectangular cross section into bulk diamond. In simulation, these cavities have an unloaded quality factor (Q) of over 1 million. Measured cavity resonances show fundamental modes with spectrometer-limited quality factors larger than 14,000 within 1nm of the NV center's zero phonon line at 637nm. We find high cavity yield across the full diamond chip with deterministic resonance trends across the fabricated parameter sweeps.

The controlled creation of defect center---nanocavity systems is one of the outstanding challenges for efficiently interfacing spin quantum memories with photons for photon-based entanglement operations in a quantum network. Here, we demonstrate direct, maskless creation of atom-like single silicon-vacancy (SiV) centers in diamond nanostructures via focused ion beam implantation with $\sim 32$ nm lateral precision and $< 50$ nm positioning accuracy relative to a nanocavity. Moreover, we determine the Si+ ion to SiV center conversion yield to $\sim 2.5\%$ and observe a 10-fold conversion yield increase by additional electron irradiation. We extract inhomogeneously broadened ensemble emission linewidths of $\sim 51$ GHz, and close to lifetime-limited single-emitter transition linewidths down to $126 \pm13$ MHz corresponding to $\sim 1.4$-times the natural linewidth. This demonstration of deterministic creation of optically coherent solid-state single quantum systems is an important step towards development of scalable quantum optical devices.

Single photon emitters play a central role in many photonic quantum technologies. A promising class of single photon emitters consists of atomic color centers in wide-bandgap crystals, such as diamond silicon carbide and hexagonal boron nitride. However, it is currently not possible to grow these materials as sub-micron thick films on low-refractive index substrates, which is necessary for mature photonic integrated circuit technologies. Hence, there is great interest in identifying quantum emitters in technologically mature semiconductors that are compatible with suitable heteroepitaxies. Here, we demonstrate robust single photon emitters based on defects in gallium nitride (GaN), the most established and well understood semiconductor that can emit light over the entire visible spectrum. We show that the emitters have excellent photophysical properties including a brightness in excess of 500x10^3 counts/s. We further show that the emitters can be found in a variety of GaN wafers, thus offering reliable and scalable platform for further technological development. We propose a theoretical model to explain the origin of these emitters based on cubic inclusions in hexagonal gallium nitride. Our results constitute a feasible path to scalable, integrated on-chip quantum technologies based on GaN.

Single-photon sources are of paramount importance in quantum communication, quantum computation, and quantum metrology. In particular, there is great interest in realizing scalable solid-state platforms that can emit triggered photons on demand to achieve scalable nanophotonic networks. We report on a visible-spectrum single-photon emitter in 4H silicon carbide (SiC). The emitter is photostable at room and low temperatures, enabling photon counts per second in excess of 2$\times$10$^6$ from unpatterned bulk SiC. It exists in two orthogonally polarized states, which have parallel absorption and emission dipole orientations. Low-temperature measurements reveal a narrow zero phonon line (linewidth $<0.1~$nm) that accounts for $>30$% of the total photoluminescence spectrum.

The past decade has seen great advances in developing color centers in diamond for sensing, quantum information processing, and tests of quantum foundations. Increasingly, the success of these applications as well as fundamental investigations of light-matter interaction depend on improved control of optical interactions with color centers -- from better fluorescence collection to efficient and precise coupling with confined single optical modes. Wide ranging research efforts have been undertaken to address these demands through advanced nanofabrication of diamond. This review will cover recent advances in diamond nano- and microphotonic structures for efficient light collection, color center to nanocavity coupling, hybrid integration of diamond devices with other material systems, and the wide range of fabrication methods that have enabled these complex photonic diamond systems.

A central goal in quantum information science is to efficiently interface photons with single optical modes for quantum networking and distributed quantum computing. Here, we introduce and experimentally demonstrate a compact and efficient method for the low-loss coupling of a solid-state qubit, the nitrogen vacancy (NV) centre in diamond, with a single-mode optical fibre. In this approach, single-mode tapered diamond waveguides containing exactly one high quality NV memory are selected and integrated on tapered silica fibres. Numerical optimization of an adiabatic coupler indicates that near-unity-efficiency photon transfer is possible between the two modes. Experimentally, we find an overall collection efficiency between 18-40 % and observe a raw single photon count rate above 700 kHz. This integrated system enables robust, alignment-free, and efficient interfacing of single-mode optical fibres with single photon emitters and quantum memories in solids.

A central challenge in developing magnetically coupled quantum registers in diamond is the fabrication of nitrogen vacancy (NV) centers with localization below ~20 nm to enable fast dipolar interaction compared to the NV decoherence rate. Here, we demonstrate the targeted, high throughput formation of NV centers using masks with a thickness of 270 nm and feature sizes down to ~1 nm. Super-resolution imaging resolves NVs with a full-width maximum distribution of $26\pm7$ nm and a distribution of NV-NV separations of $16\pm5$ nm.