Theory of neutral nitrogen-vacancy center in diamond and its qubit application

The negatively charged nitrogen-vacancy defect (NV)$^-$ in diamond has attracted much attention in recent years in qubit and biological applications. The negative charge is donated from nearby nitrogen donors that could limit or stem the successful application of (NV)$^-$. In this Letter, we unambiguously identify the \emph{neutral} nitrogen-vacancy defect (NV$^0$) by \emph{ab initio} supercell calculations. Our analysis shows that i) the spin state can be \emph{selectively} occupied optically, ii) the electron spin state can be manipulated by time-varying magnetic field, and iii) the spin state may be read out optically. Based on this NV$^0$ is a new hope for realizing qubit in diamond \emph{without} the need of nitrogen donors.Comment: 4 pages, 1 figur

\emph{Ab initio} calculation of spin-orbit coupling for NV center in diamond exhibiting dynamic Jahn-Teller effect

Point defects in solids may realize solid state quantum bits. The spin-orbit coupling in these point defects plays a key role in the magneto-optical properties that determine the conditions of quantum bit operation. However, experimental data and methods do not directly yield this highly important data, particularly, for such complex systems where dynamic Jahn-Teller (DJT) effect damps the spin-orbit interaction. Here, we show for an exemplary quantum bit, nitrogen-vacancy (NV) center in diamond, that \emph{ab initio} supercell density functional theory provide quantitative prediction for the spin-orbit coupling damped by DJT. We show that DJT is responsible for the multiple intersystem crossing rates of NV center at cryogenic temperatures. Our results pave the way toward optimizing solid state quantum bits for quantum information processing and metrology applications.Comment: 5 pages, 3 figures, 1 tabl

Theory of the optical spinpolarization loop of the nitrogen-vacancy center in diamond

The nitrogen-vacancy (NV) center in diamond is of high importance in quantum information processing applications which relies on the efficient optical polarization of its electron spin. However, the full optical spinpolarization process, in particular, the intersystem crossing between the shelving singlet state and the ground state triplet, is not understood. Here we develop a detailed theory on this process which involves strong electron-phonon couplings and correlation of electronic states that can be described as a combination of pseudo and dynamic Jahn-Teller interactions together with spin-orbit interaction. Our theory provides an explanation for the asymmetry between the observed emission and absorption spectra of the singlet states. We apply density functional theory to calculate the intersystem crossing rates and the optical spectra of the singlets and we obtain good agreement with the experimental data. As NV center serves as a template for other solid-state-defect quantum bit systems, our theory provides a toolkit to study them that might help optimize their quantum bit operation.Comment: 13 pages, 6 figures, 1 tabl

Characterization of oxygen defects in diamond by means of density functional theory calculations

Point defects in diamond are of high interest as candidates for realizing solid state quantum bits, bioimaging agents, or ultrasensitive electric or magnetic field sensors. Various artificial diamond synthesis methods should introduce oxygen contamination in diamond, however, the incorporation of oxygen into diamond crystal and the nature of oxygen-related point defects are largely unknown. Oxygen may be potentially interesting as a source of quantum bits or it may interact with other point defects which are well established solid state qubits. Here we employ plane-wave supercell calculations within density functional theory, in order to characterize the electronic and magneto-optical properties of various oxygen-related defects. Beside the trivial single interstitial and substitutional oxygen defects we also consider their complexes with vacancies and hydrogen atoms. We find that oxygen defects are mostly electrically active and introduce highly correlated orbitals that pose a challenge for density functional theory modeling. Nevertheless, we are able to identify the fingerprints of substitutional oxygen defect, the oxygen-vacancy and oxygen-vacancy-hydrogen complexes in the electron paramagnetic resonance spectrum. We demonstrate that first principles calculations can predict the motional averaging of the electron paramagnetic resonance spectrum of defects that are subject to Jahn-Teller distortion. We show that the high-spin neutral oxygen-vacancy defect exhibits very fast non-radiative decay from its optical excited state that might hinder to apply it as a qubit

The (eg⊗eu)⊗Eg(e_g \otimes e_u) \otimes E_g product Jahn-Teller effect in the neutral group-IV--vacancy quantum bits in diamond

The product Jahn-Teller (pJT) effect may occur for such coupled electron-phonon systems in solids where single electrons occupy double degenerate orbitals. We propose that the excited state of the neutral $X$V split-vacancy complex in diamond, where $X$ and V labels a group-IV impurity atom of $X$=Si, Ge, Sn, Pb and the vacancy, respectively, is such a system with $e_g$ and $e_u$ double degenerate orbitals and $E_g$ quasi-localized phonons. We develop and apply \emph{ab initio} theory to quantify the strength of electron-phonon coupling for neutral $X$V complexes in diamond, and find a significant impact on the corresponding optical properties of these centers. Our results show good agreement with recent experimental data on the prospective SiV($0$) quantum bit, and reveals the complex nature of the excited states of neutral $X$V color centers in diamond.Comment: 2 figures, 1 tabl

An \emph{ab initio} study on split silicon-vacancy defect in diamond: electronic structure and related properties

The split silicon-vacancy defect (SiV) in diamond is an electrically and optically active color center. Recently, it has been shown that this color center is bright and can be detected at the single defect level. In addition, the SiV defect shows a non-zero electronic spin ground state that potentially makes this defect an alternative candidate for quantum optics and metrology applications beside the well-known nitrogen-vacancy color center in diamond. However, the electronic structure of the defect, the nature of optical excitations and other related properties are not well-understood. Here we present advanced \emph{ab initio} study on SiV defect in diamond. We determine the formation energies, charge transition levels and the nature of excitations of the defect. Our study unravel the origin of the dark or shelving state for the negatively charged SiV defect associated with the 1.68-eV photoluminescence center.Comment: 8 pages, 5 figures, 1 tabl

Ab initio supercell calculations on nitrogen-vacancy center in diamond: its electronic structure and hyperfine tensors

The nitrogen-vacancy center in diamond is a promising candidate for realizing the spin qubits concept in quantum information. Even though this defect is known for a long time, its electronic structure and other properties have not yet been explored in detail. We study the properties of the nitrogen-vacancy center in diamond through density functional theory within the local spin density approximation, using supercell calculations. While this theory is strictly applicable for ground state properties, we are able to give an estimate for the energy sequence of the excited states of this defect. We also calculate the hyperfine tensors in the ground state. The results clearly show that: (i) the spin density and the appropriate hyperfine constants are spread along a plane and unevenly distributed around the core of the defect; (ii) the measurable hyperfine constants can be found within about 7 A from the vacancy site. These results have important implications on the decoherence of the electron spin which is crucial in realizing the spin qubits in diamond.Comment: 28 pages, 7 figures, 2 table

Hyperfine tensors of nitrogen-vacancy center in diamond from \emph{ab initio} calculations

We determine and analyze the charge and spin density distributions of nitrogen-vacancy (N-V) center in diamond for both the ground and excited states by \emph{ab initio} supercell calculations. We show that the hyperfine tensor of $^{15}$N nuclear spin is negative and strongly anisotropic in the excited state, in contrast to previous models used extensively to explain electron spin resonance measurements. In addition, we detect a significant redistribution of the spin density due to excitation that has serious implications for the quantum register applications of N-V center.Comment: 4 pages, 2 figures, 1 tabl

\emph{Ab initio} theory of nitrogen-vacancy center in diamond

Nitrogen-vacancy center in diamond is a solid state defect qubit with favorable coherence time up to room temperature which could be harnessed in several quantum enhanced sensor and quantum communication applications, and has a potential in quantum simulation and computing. The quantum control largely depends on the intricate details about the electronic structure and states of the nitrogen-vacancy center, radiative and non-radiative rates between these states and the coupling of these states to external spins, electrical, magnetic and strain fields and temperature. In this review paper it is shown how first principles calculations contributed to understanding the properties of nitrogen-vacancy center, and will be briefly discussed the issues to be solved towards full \emph{ab initio} description of solid state defect qubits.Comment: 33 pages, 12 figure

First principles study of charge diffusion between proximate solid state qubits and its implications on sensor applications

Solid state qubits from paramagnetic point defects in solids are promising platforms to realize quantum networks and novel nanoscale sensors. Recent advances in materials engineering make possible to create proximate qubits in solids that might interact with each other, leading to electron spin/charge fluctuation. Here we develop a method to calculate the tunneling-mediated charge diffusion between point defects from first principles, and apply it to nitrogen-vacancy (NV) qubits in diamond. The calculated tunneling rates are in quantitative agreement with previous experimental data. Our results suggest that proximate neutral and negatively charged NV defect pairs can form an NV--NV molecule. A tunneling-mediated model for the source of decoherence of the near-surface NV qubits is developed based on our findings on the interacting qubits in diamond.Comment: 4 figure