Extracting photons from defect centers is challenging due to the high refractive index of typical substrates. For nitrogen-vacancy centers in diamond, reaching saturation count rates above 2.5 x 105 counts/s so far requires nanofabricated optics, such as diamond waveguides or solid immersion lenses. Here, we present an experimental and theoretical study of defect center emission at unmodified planar dielectric surfaces by quantitative back-focal-plane imaging and analytical modeling. Our results indicate that photon count rates approaching those of nanofabricated optics can also be achieved by oil-immersion optics. This is due to a dielectric antenna effect, which directs the majority of the emission into the substrate within a narrow angle window suitable for back-side collection. By quantification of the collection efficiency of back-side detection, our work also enables a novel measurement method for the quantum efficiency of shallow defect centers. Its result challenges established values. Possible reasons for the discrepancy are discussed.

Extended two-dimensional (2D) crystals of dye molecules adsorbed on 2D material substrates such as boron nitride have recently become a subject of intense study, with potential applications ranging from quantum technology to optoelectronics. The most established technique for the production of these films is physical vapor transport in a vacuum. We demonstrate that few-layer crystalline films of the organic dye molecule perylenetetracarboxylic diimide (PTCDI) on boron nitride can be produced by microspacing in-air sublimation, a radically simplified technique that does not require complicated vacuum systems. The resulting layers display clearly resolved atomic step terraces in atomic force microscopy and a clear polarization anisotropy in their fluorescence, confirming molecular alignment and long-range order. Using density functional theory and classical molecular dynamics simulations, the canted motif is identified as the most likely building block for the morphology of a PTDCI monolayer on the hBN substrate.

Planar scanning probe microscopy is a recently emerging alternative approach to tip-based scanning probe imaging. It can scan an extended planar sensor, such as a polished bulk diamond doped with magnetic-field-sensitive nitrogen-vacancy (NV) centers, in nanometer-scale proximity of a planar sample. So far, this technique has been limited to optical near-field microscopy and has required nanofabrication of the sample of interest. Here we extend this technique to magnetometry using NV centers and present a modification that removes the need for sample-side nanofabrication. We harness this new ability to perform a hitherto infeasible measurement- direct imaging of the three-dimensional vector magnetic field of magnetic vortices in a thin film magnetic heterostructure, based on repeated scanning with NV centers with different orientations within the same scanning probe. Our result opens the door to quantum sensing using multiple qubits within the same scanning probe, a prerequisite for the use of entanglement-enhanced and massively parallel schemes.

We demonstrate three-dimensional magnetic resonance tomography with a resolution down to 5.9 +/- 0.1 nm. Our measurements use lithographically fabricated microwires as a source of three-dimensional magnetic field gradients, which we use to image NV centers in a densely doped diamond by Fourier-accelerated magnetic resonance tomography. We also demonstrate a compressed sensing scheme, which allows for direct visual interpretation without numerical optimization and implements an effective zoom into a spatially localized volume of interest, such as a localized cluster of NV centers. It is based on aliasing induced by equidistant undersampling of k-space. The resolution achieved in our work is comparable to the best existing schemes of super-resolution microscopy and approaches the positioning accuracy of site-directed spin labeling, paving the way to three-dimensional structure analysis by magnetic-gradient based tomography.

We present and analyze a simple scheme to calibrate single-qubit gates. It determines the amplitude and phase difference between a quadrature pair of drives, as well as their common detuning from the qubit resonance. The method is based on a two-dimensional Rabi oscillation, a sequence of two pulses of varying length sourced from the drive pair. We demonstrate error diagnosis using this scheme on an ensemble of nitrogen-vacancy centers in diamond and point out subtle pitfalls in its implementation.

We present a scheme to neutralize the dephasing effect induced by classical noise on a qubit. The scheme builds upon the key idea that this kind of noise can be recorded by a classical device during the qubit evolution, and that its effect can be undone by a suitable control sequence that is conditioned on the measurement result. We specifically demonstrate this scheme on a nitrogen-vacancy center that strongly couples to current noise in a nearby conductor. By conditioning the readout observable on a measurement of the current, we recover the full qubit coherence and the qubit's intrinsic coherence time T-2. We demonstrate that this scheme provides a simple way to implement single-qubit gates with an infidelity of 10(-2) even if they are driven by noisy sources, and we estimate that an infidelity of 10(-5) could be reached with additional improvements. We anticipate this method to find widespread adoption in experiments using fast control pulses driven from strong currents, in particular, in nanoscale magnetic resonance imaging, where control of peak currents of 100 mA with a bandwidth of 100 MHz is required. Published under an exclusive license by AIP Publishing.

We demonstrate dispersive readout of the spin of an ensemble of nitrogen-vacancy centers in a high-quality dielectric microwave resonator at room temperature. The spin state is inferred from the reflection phase of a microwave signal probing the resonator. Time-dependent tracking of the spin state is demonstrated, and is employed to measure the T (1) relaxation time of the spin ensemble. Dispersive readout provides a microwave interface to solid state spins, translating a spin signal into a microwave phase shift. We estimate that its sensitivity can outperform optical readout schemes, owing to the high accuracy achievable in a measurement of phase. The scheme is moreover applicable to optically inactive spin defects and it is non-destructive, which renders it insensitive to several systematic errors of optical readout and enables the use of quantum feedback.

High-fidelity projective readout of a qubit's state in a single experimental repetition is a prerequisite for various quantum protocols of sensing and computing. Achieving single-shot readout is challenging for solid-state qubits. For Nitrogen-Vacancy (NV) centers in diamond, it has been realized using nuclear memories or resonant excitation at cryogenic temperature. All of these existing approaches have stringent experimental demands. In particular, they require a high efficiency of photon collection, such as immersion optics or all-diamond micro-optics. For some of the most relevant applications, such as shallow implanted NV centers in a cryogenic environment, these tools are unavailable. Here we demonstrate an all-optical spin readout scheme that achieves single-shot fidelity even if photon collection is poor (delivering less than 10(3) clicks/second). The scheme is based on spin-dependent resonant excitation at cryogenic temperature combined with spin-to-charge conversion, mapping the fragile electron spin states to the stable charge states. We prove this technique to work on shallow implanted NV centers, as they are required for sensing and scalable NV-based quantum registers. The NV center in diamond has been used extensively in sensing,. however single shot readout of its spin remains challenging, requiring complex optical setups. Here, Irber et al. demonstrate a more robust scheme that achieves single-shot readout even when using inefficient detection optics.

It has been reported that the conversion yield and coherence time of ion-implanted NV centers improve if the Fermi level is raised or lowered during the annealing step following implantation. Here, we investigate whether surface transfer doping and surface charging, by UV light, can be harnessed to induce this effect. We analyze the coherence times and the yield of NV centers created by ion implantation and annealing, applying various conditions during annealing. Specifically, we study coating diamond with nickel, palladium, or aluminum oxide, to induce positive surface transfer doping, as well as annealing under UV illumination to trigger vacancy charging. The metal-coated diamonds display a two times higher formation yield than the other samples. The coherence time T-2 varies by less than a factor of two between the investigated samples. Both effects are weaker than previous reports, suggesting that stronger modifications of the band structure are necessary to find a pronounced effect. UV irradiation has no effect on the yield and T-2 times.