Diamond NV centers for quantum computing and quantum networks
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Introduction The counterintuitive features of quantum mechanics such as superposition (“being at two places at the same time”) and entanglement (leading to “spooky action at a distance”) have been a subject of debate for much of the 20th century. After pioneering experiments with photons1 showed that entanglement is indeed a part of nature, scientists began to wonder how these quantum resources could be put to use. A number of exciting opportunities have emerged: quantum computers, secure quantum communications, and quantum metrology. Building a scalable quantum technology requires meeting two conflicting demands. On the one hand, a well-isolated system is desired that can be controlled with high precision. Arguably the best example is a single trapped atom. Indeed, many pioneering experiments on individual quantum systems were performed with single atoms;2 the 2012 Nobel Prize in Physics recognized the remarkable level of control achieved over neutral atoms and ions. On the other hand, the technology needs to be scalable. Solid-state systems have a clear advantage in this regard, offering the opportunity to adapt nanofabrication techniques developed in the semiconductor industry to build integrated quantum devices.3 Nitrogen-vacancy (NV) centers in diamond have emerged as one of the most promising candidates for implementing quantum technologies because they exhibit atom-like properties—long-lived spin quantum states and well-defined optical transitions—in a robust solid-state device (see the
Introduction article in this issue). The NV center has spin degrees-of-freedom associated with both its bound electrons and nearby nuclear spins (see Figure 1a–b), and, much like atomic states, these spins can be addressed using optical transitions (see Figure 1c). At the same time, the solid-state host allows for fast electrical and magnetic control using on-chip wiring and waveguides; similarly, photonic structures can be fabricated from the diamond crystal (see Figure 1d) to create an efficient optical interface (see the Loncar and Faraon, and Toyli et al. articles in this issue). In the past few years, a number of labs around the world have exploited this combination of atom-like properties and solid-state control to demonstrate a number of key functionalities required for quantum technologies. In this article, we describe a few of these experiments and discuss prospects and challenges for future applications.
Controlling and reading single electron and nuclear spins Much of the excitement about the NV center stems from its robust spin quantum states. In its ground state, the electrons bound to the NV center have a net spin S = 1, and superpositions of the spin sublevels (ms = –1, 0, 1) can exhibit coherent quantum evolution over long time scales. The typical sources of disturbance for spins in semiconductors and insulators—spin-orbit coupling and magnetic nuclei in the crystalline structure—are relatively weak for NV centers in diamond. For most diamond devices, spin coherence is limited by magnetic interactions
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