Nitrogen-vacancy centers: Physics and applications
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Introduction The interaction of light and matter has underpinned some of the most successful experimental techniques and technological advances in the modern era. These discoveries have been acknowledged by several Nobel Prizes, including the 2012 Physics Award to Wineland and Haroche for optical measurement and manipulation of individual quantum systems. The great progress in the study of quantum mechanics and engineering quantum systems has relied heavily on the experimental accessibility of studying the interaction between optical fields and atoms or atom-like systems. Atoms are a natural system for this research due to the restricted degrees of freedom afforded by such a simple system, and optical photons are a convenient probe as they match the typical energy scale of atomic transitions. However, in order for applications based on atom-light interactions to become widespread, they need to be compatible with today’s highly developed large-scale fabrication processes used in, for example, microelectronics. Working with atoms in the gas phase has its limitations. In many cases, vacuum systems must be employed, and scaling these systems down to the nanoscale remains an open challenge. This has prompted a new line of research into what are coined “artificial atoms” in the solid state. These systems, including nonlinear superconducting circuits1 and localized electrons in semiconductors,2 exhibit the quantum mechanical properties of single atoms but are embedded in solids that can be processed using modern-day fabrication techniques.
In this issue of MRS Bulletin, we highlight progress toward applications based on one such solid-state “artificial atom”—the nitrogen-vacancy (NV) center in diamond. The NV center consists of a vacancy, or missing carbon atom, in the diamond lattice lying next to a nitrogen atom, which has substituted for one of the carbon atoms. They can form naturally during diamond growth or artificially using a variety of implantation and annealing techniques. Many of the properties of the NV center can be described by treating it as a system of two unpaired electrons exhibiting trigonal C3v symmetry.3 Figure 1 shows a visualization of the NV electronic wave function. The center’s atom-like properties include a paramagnetic triplet ground state, which interacts strongly with both microwave and optical fields. Further, the diamond lattice is an ideal host for such an artificial atom, as its unique thermal and mechanical properties, biocompatibility, and nearly nuclear spin-free environment are highly favorable for applications in materials and biological engineering. Later we describe how the quantum mechanical properties of NV centers are being exploited for applications in quantum information science and electromagnetic sensing.
Quantum mechanical coherence and its applications One of the key concepts in quantum mechanics is that a particle can simultaneously exist in an arbitrary superposition of discrete quantum states. For example, suppose the atom
Victor Acosta, HP Labs, Palo Alto, CA; Victor.acos
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