The nitrogen-vacancy (NV) colour centre in diamond is an important physical system for emergent quantum technologies, including quantum metrology, information processing and communications, as well as for various nanotechnologies, such as biological and sub-diffraction limit imaging, and for tests of entanglement in quantum mechanics. Given this array of existing and potential applications and the almost 50 years of NV research, one would expect that the physics of the centre is well understood, however, the study of the NV centre has proved challenging, with many early assertions now believed false and many remaining issues yet to be resolved. This review represents the first time that the key empirical and ab initio results have been extracted from the extensive NV literature and assembled into one consistent picture of the current understanding of the centre. As a result, the key unresolved issues concerning the NV centre are identified and the possible avenues for their resolution are examined.

The nitrogen-vacancy colour centre in diamond

The nitrogen vacancy and some nickel related defects in diamond can be observed as single quantum systems in diamond by their fluorescence. The fabrication of single colour centres occurs via generation of vacancies or via controlled nitrogen implantation in the case of the nitrogen vacancy (NV) centre. The NV centre shows an electron paramagnetic ground and optically excited state. As a result electron and nuclear magnetic resonance can be carried out on single defects. Due to the localized nature of the electron spin wavefunction hyperfine coupling to nuclei more than one lattice constant away from the defect as dominated by dipolar interaction. As a consequence the coupling to close nuclei leads to a splitting in the spectrum which allows for optically detected electron nuclear double resonance. The contribution discusses the physics of the NV and other defect centre from the perspective of single defect centre spectroscopy.

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Single defect centres in diamond: A review

Coherent coupling between single quantum objects is at the very heart of modern quantum physics. When the coupling is strong enough to prevail over decoherence, it can be used to engineer quantum entangled states. Entangled states have attracted widespread attention because of applications to quantum computing and long-distance quantum communication. For such applications, solid-state hosts are preferred for scalability reasons, and spins are the preferred quantum system in solids because they offer long coherence times. Here we show that a single pair of strongly coupled spins in diamond, associated with a nitrogen-vacancy defect and a nitrogen atom, respectively, can be optically initialized and read out at room temperature. To effect this strong coupling, close proximity of the two spins is required, but large distances from other spins are needed to avoid deleterious decoherence. These requirements were reconciled by implanting molecular nitrogen into high-purity diamond.

Room-temperature coherent coupling of single spins in diamond

We describe a scanning device where a single spin is used as an ultrasensitive, nanoscale magnetic field sensor. As this “probe spin” we consider a single nitrogen-vacancy defect center in a diamond nanocrystal, attached to the tip of the scanning device. Changes in the local field seen by the probe spin are detected by optically monitoring its electron paramagnetic resonance transition. The room-temperature scanning device may be useful for performing nanoscale magnetic resonance imaging and spectroscopy, and for the characterization of magnetic nanostructures down to the single atom level.

Scanning magnetic field microscope with a diamond single-spin sensor

Electron-spin-echo experiments reveal phase-memory times as long as 58 μs at 300 K for nitrogen-vacancy centers in chemical vapor deposition (CVD) single crystals. The spins were optically polarized and optically detected. Two high-quality CVD samples were studied. From the current results, it is not clear whether these phase-memory times represent a fundamental limit or are limited by an external source of decoherence.

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Long coherence times at 300 K for nitrogen-vacancy center spins in diamond grown by chemical vapor deposition

A concept combining optics and microwave pulses with the negative charge-state of the nitrogen-vacancy (NV-) center in diamond is demonstrated through experiments that are equivalent to single-qubit gates, and decoherencefor this qubit is examined The spin levels of the ground state provide the two-level system Optical excitation provides polarization of these states The polarized state is operated coherently by 35 GHz microwave pulses The final state is read out through the photoluminescence intensity Decoherence arises from different sources for different samples For high-pressure, high-temperature synthetic diamonds, the high concentration of substitutional N limits the phase-memory to a few μs In a single-crystal CVD diamond, the phase memory time is at least 32 μs at 100 K 1 4 N is tightly coupled to the electronic spin and produces modulation of the electron-spin echo decay under certain conditions A two-qubit gate is proposed using this nuclear spin This demonstration provides a great deal of insight into quantum devices in the solid state with some possibility for real application

R. C. Linares

Papers

Long coherence times at 300 K for nitrogen-vacancy center spins in diamond grown by chemical vapor deposition

Single-Qubit Operations with the Nitrogen-Vacancy Center in Diamond