Nitrogen-vacancy center

The Nitrogen-vacancy center (N-V center) is a crystallographic defect in the structure of a diamond that can be exploited to capture the electron spin and create a spintronic device. Utilized with the emerging technology of spintronics, the N-V center may facilitate an atomic-scale system for solid information processing^[1] numerous important applications quantum electronics and quantum cryptography. The existence of N-V centers was discovered in 1997 by a research team led by Jorg Wrachtrup, at the Chemnitz University of Technology in Chemnitz, Germany.^[2] Spintronics describes technology with the ability to change or influence the quantum spin state of electrons. Electrons absorb or emit photons (quanta of electromagnetic energy) to change valence orbits, and they lose spin coherence by interacting with mutually resonant photon frequencies, causing the electrons to spin flip by energy transfer, through mutual spin-orbit coupling, and through photon emission.

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Vacancies

See also: Crystallographic defects in diamond#Vacancies

A vacancy is an empty position in a diamond's lattice. Vacancies may be affected or created by radiation damage—high-energy subatomic particles knock carbon atoms out of the lattice. The vacancies interact with interstitial atoms ("extra" atoms, most commonly nitrogen, which occupy space between carbon atoms rather than substituting for them) and act as color centers by absorbing visible light, thus producing green or blue colors in Type I, and brown colors in Type IIa diamond. Radiation-induced vacancies can be detected by ultraviolet fluorescence, as well as by a characteristic absorption line at 741.2 nm, termed the GR 1 (General Radiation) line.^[3][4][5]

The annealing process (or the heat of the earth over geological timescales) also allows carbon atoms neighboring a vacancy to jump into a vacant place and leave an empty position in the diamond lattice; by this process a vacancy can migrate through the diamond, and can form compound defects with other vacancies, interstitial atoms or nitrogenous defects (NV centers). The newly-formed compound defects are optically active, producing strong yellows, pinks, and reds, the precise color dependent on the annealing time and type of pre-existing defects present. Vacancies can also be created or modified by HTHP treatment.

Structure

The Nitrogen-vacancy center is found as a defect in the tetrahedral lattice structure of a single crystal diamond. The impurity is based in the lattice of four carbon atoms, where two adjacent sites are altered - one carbon atom replaced with a nitrogen atom and the other space left vacant.^[2] The electrons in the N-V center move in orbits that span the vacancy and the three adjacent carbon atoms but do not approach near the nitrogen atom. While the N-V center may exist randomly within a diamond crystal, ion implantation techniques can enable its artificial creation in predetermined positions.^[6]

Properties

N-V centers are visible as red spots when treated by laser and the brightest where spin states are the highest.^[7] The quantum spin states of impurities in diamonds can retain their quantum character for comparatively long time periods. Laser treatment excites electrons near the N-V center, emitting a single photon in quantum state, a property that is utilized in quantum cryptography. The N-V center is relatively insulated from magnetic interference from other spins; thus, the quantum state of the N-V center exists at room temperature, enabling the efficient transmission of information and quantum functions in normal laboratory conditions.

Applications

The emission of photons of the N-V center has several applications in quantum cryptography, which uses the transmission of information in form of a single photon of one quibit size, made considerably easier by the existence of quantum states with N-V centers at room temperatures. Quantum computations are made possible by the creation of a controlled NOT logic gate by the flipping of N-V qubit through the application of light waves (normally through laser), while logic computations are made possible by the addition of a second nitrogen atom near the N-V center to create a common system.^[7]

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