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 ...

Graphene: Driven to emission Studying the electronic properties of graphene under extreme nonequilibrium conditions has provided a productive testbed to probe and monitor exotic transport phenomena. Andersen et al. report measurements of electron transport in ultraclean graphene devices where the electron drift velocity is extremely high. They found that direct current at high drift velocities generates a large increase in the noise at gigahertz frequencies and that the noise grows exponentially in the direction of the current. The authors attribute the emission mechanism to amplification of acoustic phonons through the Cerenkov effect. Science , this issue p. 154

No abstract

Techniques to mold the flow of light on subwavelength scales enable fundamentally new optical systems and device applications. The realization of programmable, active optical systems with fast, tunable components is among the outstanding challenges in the field. Here, we experimentally demonstrate a few-pixel beam steering device based on electrostatic gate control of excitons in an atomically thin semiconductor with strong light-matter interactions. By combining the high reflectivity of a MoSe2 monolayer with a graphene split-gate geometry, we shape the wavefront phase profile to achieve continuously tunable beam deflection with a range of 10°, two-dimensional beam steering, and switching times down to 1.6 nanoseconds. Our approach opens the door for a new class of atomically thin optical systems, such as rapidly switchable beam arrays and quantum metasurfaces operating at their fundamental thickness limit.

No abstract