The nitrogen vacancy (NV) center in diamond exhibits spin-dependent fluorescence and long spin coherence times under ambient conditions, enabling applications in quantum information processing and sensing 1, 2 . NV centers near the surface can have strong interactions with external materials and spins, enabling new forms of nanoscale spectroscopy 3-6 . However, NV spin coherence degrades within 100 nanometers of the surface, suggesting that diamond surfaces are plagued with ubiquitous defects 7-10 . Prior work on characterizing near-surface noise has primarily relied on using NV centers themselves as probes [7][8][9][10][11][12] ; while this has the advantage of exquisite sensitivity, it provides only indirect information about the origin of the noise. Here we demonstrate that surface spectroscopy methods and single spin measurements can be used as complementary diagnostics to understand sources of noise. We find that surface morphology is crucial for realizing reproducible chemical termination, and use these insights to achieve a highly ordered, oxygen-terminated surface with suppressed noise. We observe NV centers within 10 nm of the surface with coherence times extended by an order of magnitude.Although it is easy to place NV centers near the surface by low-energy ion implantation 8, 9 or delta-doping 7, 8 , the surface itself can host defects that lead to noise that obscures the sensing target ( Fig. 1a). We observe that coherence time degrades with proximity to the surface in numerous samples with different surface conditions ( Fig. 1b), consistent with prior studies 7, 10 , pointing to the need for new techniques to understand and control diamond surfaces. Gaining precise control over diamond surface chemistry is challenging because diamond is a chemically inert material, and also because it is hard to prepare uniform, flat diamond surfaces. Surface morphology is difficult 2 to control because diamond's hardness makes etching and polishing non-trivial. State-of-the-art diamond polishing can achieve surface roughness below 1 nm, but the resulting surface is highly strained. Plasma etching can remove this strained layer 13, 14 , but this process is highly anisotropic and therefore small differences in initial conditions can lead to dramatic differences in final morphology and termination 15, 16 (see Supplementary Information). Therefore, direct characterization of the surface is crucial for establishing that particular protocols reproducibly lead to specific, desired surface terminations.In this work, we characterize the diamond surface by correlating photoelectron spectroscopy, X-ray absorption, atomic force microscopy (AFM), and electron diffraction with measurements of NV spin decoherence and relaxation to identify and eliminate sources of noise at the surface. We find that surface roughness leads to poor NV coherence, and we observe that surface morphology changes the density of electronic defects observed with photoelectron spectroscopy, even for the same nominal chemical termination, implying that it ...

We have developed a single-shot terahertz time-domain spectrometer to perform optical-pump/terahertz-probe experiments in pulsed, high magnetic fields up to 30 T. The single-shot detection scheme for measuring a terahertz waveform incorporates a reflective echelon to create time-delayed beamlets across the intensity profile of the optical gate beam before it spatially and temporally overlaps with the terahertz radiation in a ZnTe detection crystal. After imaging the gate beam onto a camera, we can retrieve the terahertz time-domain waveform by analyzing the resulting image. To demonstrate the utility of our technique, we measured cyclotron resonance absorption of optically excited carriers in the terahertz frequency range in intrinsic silicon at high magnetic fields, with results that agree well with published values.

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We have constructed a unique cluster tool for surface preparation and spectroscopy in ultrahigh vacuum combined with cryogenic, confocal microscopy of single nitrogen vacancy centers in diamond. We modify the diamond surface chemistry and investigate shallow nitrogen vacancy center properties in situ.

Color centers in diamond are widely explored for applications in quantum sensing, computing, and networking. Their optical, spin, and charge properties have extensively been studied, while their interactions with itinerant carriers are relatively unexplored. Here, we show that NV centers situated 10 ± 5 nm of the diamond surface can be converted to the neutral charge state via hole capture. By measuring the hole capture rate, we extract the capture cross section, which is suppressed by proximity to the diamond surface. The distance dependence is consistent with a carrier diffusion model, indicating that the itinerant carrier lifetime can be long, even at the diamond surface. Measuring dynamics of near-surface NV centers offers a tool for characterizing the diamond surface and investigating charge transport in diamond devices.