The past decade has seen remarkable progress in isolating and controlling quantum coherence using charges and spins in semiconductors. Quantum control has been established at room temperature, and electron spin coherence times now exceed several seconds, a nine-order-of-magnitude increase in coherence compared with the first semiconductor qubits. These coherence times rival those traditionally found only in atomic systems, ushering in a new era of ultracoherent spintronics. We review recent advances in quantum measurements, coherent control, and the generation of entangled states and describe some of the challenges that remain for processing quantum information with spins in semiconductors.

We demonstrate nanometer-precision depth control of nitrogen-vacancy (NV) center creation near the surface of synthetic diamond using an in situ nitrogen delta-doping technique during plasma-enhanced chemical vapor deposition. Despite their proximity to the surface, doped NV centers with depths (d) ranging from 5 - 100 nm display long spin coherence times, T2 > 100 \mus at d = 5 nm and T2 > 600 \mus at d \geq 50 nm. The consistently long spin coherence observed in such shallow NV centers enables applications such as atomic-scale external spin sensing and hybrid quantum architectures.Comment: 14 pages, 4 figures, 11 pages of additional supplementary materia

Hexagonal boron nitride (h-BN) is a tantalizing material for solid-state quantum engineering. Analogously to three-dimensional wide-bandgap semiconductors like diamond, h-BN hosts isolated defects exhibiting visible fluorescence, and the ability to position such quantum emitters within a two-dimensional material promises breakthrough advances in quantum sensing, photonics, and other quantum technologies. Critical to such applications, however, is an understanding of the physics underlying h-BN's quantum emission. We report the creation and characterization of visible single-photon sources in suspended, single-crystal, h-BN films. The emitters are bright and stable over timescales of several months in ambient conditions. With substrate interactions eliminated, we study the spectral, temporal, and spatial characteristics of the defects' optical emission, which offer several clues about their electronic and chemical structure. Analysis of the defects' spectra reveals similarities in vibronic coupling despite widely-varying fluorescence wavelengths, and a statistical analysis of their polarized emission patterns indicates a correlation between the optical dipole orientations of some defects and the primitive crystallographic axes of the single-crystal h-BN film. These measurements constrain possible defect models, and, moreover, suggest that several classes of emitters can exist simultaneously in free-standing h-BN, whether they be different defects, different charge states of the same defect, or the result of strong local perturbations.Defect engineering in solid-state materials is a rapidly progressing field with applications in quantum information science [1, 2], nanophotonics [3], and nanoscale sensing in biology and chemistry [4]. Inspired by the success of the archetypal nitrogen-vacancy center in diamond [5], recent efforts have uncovered analogous systems in other wide-bandgap semiconductors such as silicon carbide [6, 7] which offer exciting new opportunities for defect engineering in three-dimensional materials. However, optically active impurities in low-dimensional materials and thin films can provide unique functionalities due to intrinsic spatial confinement and the ability to create multi-functional layered materials [8, 9]. Within the class of van der Waals materials, hexagonal boron nitride (h-BN) is an ideal candidate for exploring new defect physics due to its large (∼6 eV) bandgap [10] and its unique optical [11], electrical [12, 13], and vibronic properties [14] that may influence the underlying physics of its defects. At present, however, progress is impeded by an incomplete understanding of the electronic and chemical structure of defects responsible for h-BN's visible fluorescence.