Charged Nitrogen-vacancy Center In Diamond Research Articles

Appearance of spectral dip in the cathodoluminescence spectrum of negatively charged nitrogen-vacancy centers in diamonds

Negatively charged nitrogen-vacancy (NV−) centers in diamond produce a characteristic optical zero-phonon line (ZPL) at 637 nm. This emission line can be observed under optical excitation (i.e. photoluminescence), but is rarely observed under electron excitation (i.e. cathodoluminescence). This study reports that low-temperature (80 K) and low energy (5 keV) cathodoluminescence spectroscopy is able to detect emission peak or spectral dip at the ZPL of NV− centers. The spectral dip was observed in the diamond samples with high concentration of NV− centers produced by high-energy (2 MeV) e-beam (EB) irradiation. The effects of EB irradiation fluence, NV− centers concentration and substitutional nitrogen concentration on the formation of spectral dip were discussed, and a model based on the NV− absorption of NV0 emission sideband is proposed.

Laser-written waveguide-integrated coherent spins in diamond

Quantum emitters, such as the negatively charged nitrogen-vacancy center in diamond, are attractive for quantum technologies, such as nano-sensing, quantum information processing, and as a non-classical light source. However, it is still challenging to position individual emitters in photonic structures while preserving the spin coherence properties of the defect. In this paper, we investigate single and ensemble waveguide-integrated nitrogen-vacancy centers in diamond fabricated by femtosecond laser writing followed by thermal annealing. Their spin coherence properties are systematically investigated and are shown to be comparable to native nitrogen-vacancy centers in diamond. This method paves the way for the fabrication of coherent spins integrated within photonic devices.

Quantum Vibronic Effects on the Excitation Energies of the Nitrogen-Vacancy Center in Diamond.

We investigated the impact of quantum vibronic coupling on the electronic properties of solid-state spin defects using stochastic methods and first-principles molecular dynamics with a quantum thermostat. Focusing on the negatively charged nitrogen-vacancy center in diamond as an exemplary case, we found a significant dynamic Jahn-Teller splitting of the doubly degenerate single-particle levels within the diamond's band gap, even at 0 K, with a magnitude exceeding 180 meV. This pronounced splitting leads to substantial renormalizations of these levels and, subsequently, of the vertical excitation energies of the doubly degenerate singlet and triplet excited states. Our findings underscore the pressing need to incorporate quantum vibronic effects into first-principles calculations, particularly when comparing computed vertical excitation energies with experimental data. Our study also reveals the efficiency of stochastic thermal line sampling for studying phonon renormalizations of solid-state spin defects.

Correction: Electronic excitations of the charged nitrogen-vacancy center in diamond obtained using time-independent variational density functional calculations

Correction: Electronic excitations of the charged nitrogen-vacancy center in diamond obtained using time-independent variational density functional calculations

Electronic excitations of the charged nitrogen-vacancy center in diamond obtained using time-independent variational density functional calculations

Elucidation of the mechanism for optical spin initialization of point defects in solids in the context of quantum applications requires an accurate description of the excited electronic states involved. While variational density functional calculations have been successful in describing the ground state of a great variety of systems, doubts have been expressed in the literature regarding the ability of such calculations to describe electronic excitations of point defects. A direct orbital optimization method is used here to perform time-independent, variational density functional calculations of a prototypical defect, the negatively charged nitrogen-vacancy center in diamond. The calculations include up to 511 atoms subject to periodic boundary conditions and the excited state calculations require similar computational effort as ground state calculations. Contrary to some previous reports, the use of local and semilocal density functionals gives the correct ordering of the low-lying triplet and singlet states, namely {}^{3}A_2 < {}^{1}E < {}^{1}A_1 < {}^{3}E3A2<1E<1A1<3E. Furthermore, the more advanced meta generalized gradient approximation functionals give results that are in remarkably good agreement with high-level, many-body calculations as well as available experimental estimates, even for the excited singlet state which is often referred to as having multireference character. The lowering of the energy in the triplet excited state as the atom coordinates are optimized in accordance with analytical forces is also close to the experimental estimate and the resulting zero-phonon line triplet excitation energy is underestimated by only 0.15 eV. The approach used here is found to be a promising tool for studying electronic excitations of point defects in, for example, systems relevant for quantum technologies.

Local Excitations of a Charged Nitrogen Vacancy in Diamond with Multireference Density Matrix Embedding Theory

We investigate the negatively charged nitrogen-vacancy center in diamond using periodic density matrix embedding theory (pDMET). To describe the strongly correlated excited states of this system, the complete active space self-consistent field (CASSCF) followed by n-electron valence state second-order perturbation theory (NEVPT2) was used as the impurity solver. Since the NEVPT2-DMET energies show a linear dependence on the inverse of the size of the embedding subspace, we performed an extrapolation of the excitation energies to the nonembedding limit using a linear regression. The extrapolated NEVPT2-DMET first triplet-triplet excitation energy is 2.31 eV and that for the optically inactive singlet-singlet transition is 1.02 eV, both in agreement with the experimentally observed vertical excitation energies of ∼2.18 eV and ∼1.26 eV, respectively. This is the first application of pDMET to a charged periodic system and the first investigation of the NV- defect using NEVPT2 for periodic supercell models.

High-precision measurements and first-principles explanation of the temperature-dependent C13 and N14 hyperfine interactions of single NV− centers in diamond at room temperature

Revealing the properties of single spin defects in solids is essential for quantum applications based on solid-state systems. However, it is intractable to investigate the temperature-dependent properties of single defects, due to the low precision for single-defect measurements in contrast to defect ensembles. Here we report that the temperature dependence of the Hamiltonian parameters for single negatively charged nitrogen-vacancy centers in diamond at room temperature is precisely measured and the results are in reasonable agreement with first-principles calculations. In particular, the hyperfine interactions with randomly distributed $^{13}\mathrm{C}$ nuclear spins are clearly observed to vary with temperature and the relevant coefficients are measured with hertz-level precision. The temperature-dependent behaviors are attributed to both thermal expansion and lattice vibrations by first-principles calculations. Our results pave the way for taking nuclear spins as more stable thermometers at nanoscale. The methods developed here for high-precision measurements and first-principles calculations can be further extended to other solid-state spin defects.

Nitrogen-Vacancy Singlet-Manifold Ionization Energy

The singlet states of the negatively charged nitrogen-vacancy centers in diamond play a key role in its optical spin control and readout. In this work, the hitherto unknown ionization energy of the singlet is measured experimentally and found to be in the range 1.91--2.25 eV. This is obtained by analyzing photoluminescence measurements incorporating spin control and nitrogen-vacancy charge state differentiation, along with simulations based on the nitrogen-vacancy's master equation. This work establishes a protocol for a more accurate estimate of this ionization energy, which can possibly lead to improved readout methods.

High temperature spectroscopy of ensembles of nitrogen-vacancy centers in diamond

We study the spectroscopy of an ensemble of negatively charged nitrogen-vacancy centers in diamond at high temperatures between room temperature and 700K under high vacuum conditions. Spin resonances are studied using optical detection of magnetic resonance (ODMR), and optical spectroscopy is employed to study radiative transitions. Upon increasing the temperature, the intensity of radiative decay in visible and infrared decreased. In addition, the ODMR resonance frequencies were decreased, and the phonon line emission shifted to higher wavelengths. Density functional theory calculation of the zero-field splitting parameter (D) revealed that thermal expansion is not enough to explain the shift in the ODMR frequencies. Fitting the measured intensity of photo-luminescence with the theoretical predictions of the Mott–Seitz model yields the value of 0.22eV for the energy barrier associated with nonradiative decay.

UV-induced charge-state conversion from the negatively to neutrally charged nitrogen-vacancy centers in diamond

Under ultraviolet (UV) excitation with photon energy larger than 4.5 eV, a charge-state conversion from negatively to neutrally charged nitrogen-vacancy (NV− to NV0) centers in diamond samples is realized. The UV-induced charge-state conversion is found to strongly depend on the N concentration in the sample and the irradiation fluence of the electron beam. For the samples with high N concentrations, low-fluence (2.5 × 1017 cm−2) 10-MeV electron beam irradiation usually leads to UV-induced charge-state conversion efficiency higher than that of the samples irradiated with high fluences (≥2.5 × 1018 cm−2). For the samples with a few ppm N, however, the charge-state conversion efficiency is relatively low in the cases of irradiation fluences in this work. Meanwhile, UV-induced NV0 luminescence exhibits temperature dependence different from that of visible-light excited NV0 or NV− centers; that is, the photoluminescence intensity does not reach saturation at temperatures lower than 135 K but decreases with the decrease in temperature. Based on the photoluminescence excitation spectra of NV0 centers in the UV region, the UV-induced charge-state conversion is suggested to involve free-hole generation, diffusion, and the capture by ground-state NV− centers.

Sunlight-Driven Quantum Magnetometry

Emerging quantum technologies, such as quantum computations and precise sensing, provide new opportunities in the fields of science and engineering. While energy consumption is a major concern for modern industry and society, it is rarely taken into consideration for quantum technologies. Especially, the potential of quantum technologies directly powered by renewable energy has long been neglected. The initialization, manipulation, and readout of quantum systems generally require high-power-consuming equipment, such as a dilution refrigerator, a microwave-power amplifier, and a high-power laser. Here, we discover a direct utilization path for solar energy to steer the quantum states of negatively charged nitrogen-vacancy centers in diamond, which is one of the most promising solid-state quantum systems in the past decades. Following this method, we demonstrate sunlight-driven quantum magnetometry. The initialization and readout of this quantum magnetometry are achieved directly by sunlight, and conventional microwave manipulation can be removed by using a microwave-free scheme. By utilizing ambient energy directly, our method brings potential solutions to the energy-consumption issue of quantum technologies. This technique can be further extended to multiple quantum systems, and thus opens the door to environmentally sustainable quantum technologies and self-powered quantum sensing in the future.Received 21 March 2022Revised 24 July 2022Accepted 11 August 2022DOI:https://doi.org/10.1103/PRXEnergy.1.033002Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum sensingSolar energyEnergy Science & TechnologyInterdisciplinary PhysicsQuantum Information

Simulating the Electronic Structure of Spin Defects on Quantum Computers

We present calculations of both the ground- and excited-state energies of spin defects in solids carried out on a quantum computer, using a hybrid classical-quantum protocol. We focus on the negatively charged nitrogen-vacancy center in diamond and on the double vacancy in 4H SiC, which are of interest for the realization of quantum technologies. We employ a recently developed first-principles quantum embedding theory to describe point defects embedded in a periodic crystal and to derive an effective Hamiltonian, which is then transformed to a qubit Hamiltonian by means of a parity transformation. We use the variational quantum eigensolver (VQE) and quantum subspace expansion methods to obtain the ground and excited states of spin qubits, respectively, and we propose a promising strategy for noise mitigation. We show that by combining zero-noise extrapolation techniques and constraints on electron occupation to overcome the unphysical-state problem of the VQE algorithm, one can obtain reasonably accurate results on near-term-noisy architectures for ground- and excited-state properties of spin defects.1 MoreReceived 7 December 2021Revised 27 January 2022Accepted 8 February 2022DOI:https://doi.org/10.1103/PRXQuantum.3.010339Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum algorithmsQuantum computationQuantum simulationPhysical SystemsNitrogen vacancy centers in diamondQuantum InformationCondensed Matter, Materials & Applied Physics

Photoionization of negatively charged NV centers in diamond: Theory and ab initio calculations

We present ab-initio calculations of photoionization thresholds and cross sections of the negatively charged nitrogen-vacancy (NV) center in diamond from the ground $^{3}\!A_2$ and the excited $^{3}\!E$ states. We show that after the ionization from the $^{3}\!E$ level the NV center transitions into the metastable $^{4}\!A_2$ electronic state of the neutral defect. We reveal how spin polarization of $\mathrm{NV}^{-}$ gives rise to spin polarization of the $^{4}\!A_2$ state, providing an explanation of electron spin resonance experiments. We obtain smooth photoionization cross sections by employing dense $k$-point meshes for the Brillouin zone integration together with the band unfolding technique to rectify the distortions of the band structure induced by artificial periodicity of the supercell approach. Our calculations provide a comprehensive picture of photoionization mechanisms of $\mathrm{NV}^{-}$. They will be useful in interpreting and designing experiments on charge-state dynamics at NV centers. In particular, we offer a consistent explanation of recent results of spin-to-charge conversion of NV centers.

Battery Characterization via Eddy-Current Imaging with Nitrogen-Vacancy Centers in Diamond

Sensitive and accurate diagnostic technologies with magnetic sensors are of great importance for identifying and localizing defects of rechargeable solid batteries using noninvasive detection. We demonstrate a microwave-free alternating current (AC) magnetometry method with negatively charged NV centers in diamond based on a cross-relaxation feature between nitrogen-vacancy (NV) centers and individual substitutional nitrogen (P1) centers occurring at 51.2 mT. We apply the technique to non-destructively image solid-state batteries. By detecting the eddy-current-induced magnetic field of the battery, we distinguish a defect on the external electrode and identify structural anomalies within the battery body. The achieved spatial resolution is μμμ360μm. The maximum magnetic field and phase shift generated by the battery at the modulation frequency of 5 kHz are estimated as 0.04 mT and 0.03 rad respectively.

Multiconfigurational study of the negatively charged nitrogen-vacancy center in diamond

Deep defects in wide band gap semiconductors have emerged as leading qubit candidates for realizing quantum sensing and information applications. Due to the spatial localization of the defect states, these deep defects can be considered as artificial atoms/molecules in a solid state matrix. Here we show that unlike single-particle treatments, the multiconfigurational quantum chemistry methods, traditionally reserved for atoms/molecules, accurately describe the many-body characteristics of the electronic states of these defect centers and correctly predict properties that single-particle treatments fail to obtain. We choose the negatively charged nitrogen-vacancy (NV$^-$) center in diamond as the prototype defect to study with these techniques due to its importance for quantum information applications and because its properties are well-known, which makes it an ideal benchmark system. By properly accounting for electron correlations and including spin-orbit coupling and dipolar spin-spin coupling in the quantum chemistry calculations, for the NV$^-$ center in diamond clusters, we are able to: (i) show the correct splitting of the ground (first-excited) triplet state into two levels (four levels), (ii) calculate zero-field splitting values of the ground and excited triplet states, in good agreement with experiment, and (iii) calculate the energy differences between ground and exited spin-triplet and spin-singlet states, as well as their ordering, which are also found to be in good agreement with recent experimental data. The numerical procedure we have developed is general and it can screen other color centers whose properties are not well known but promising for applications.

Novel Ultra Localized and Dense Nitrogen Delta-Doping in Diamond for Advanced Quantum Sensing.

We introduce and demonstrate a new approach for nitrogen-vacancy (NV) patterning in diamond, achieving a deterministic, nanometer-thin, and dense delta-doped layer of negatively charged NV centers in diamond. We employed a pure nitridation stage using microwave plasma and a subsequent in situ diamond overgrowth. We present the highest reported nitrogen concentration in a delta-doped layer (1.8 × 1020 cm-3) while maintaining the pristine diamond crystal quality. This result combined with the large optically detected magnetic resonance contrast can pave the way toward highly sensitive NV-based magnetometers. We further employed this delta-doping technique on high-quality fabricated diamond nanostructures for realizing a topographic NV patterning in order to enhance the sensing and hyperpolarization capabilities of NV-based devices.

Density functional theory study on magnetically detecting positively charged nitrogen-vacancy center in diamond

A quantum-chemical study of the positive charge-state of the nitrogen-vacancy center in diamond is presented. Charge control of this promising qubit candidate is a focus of diamond quantum technology research, as currently charge stability relating to surfaces and nearby defects causes some difficulties for quantum applications. To demonstrate full charge control over the nitrogen vacancy, all three charge states should be identified and the processes that lead to charge state changes understood. However, experimental markers for the positive state remain elusive compared to the readily detectable zero-phonon lines of the neutral and negative. In this work we present predicted hyperfine and zero-field splitting tensors as clear signatures of the normally spinless ${\mathrm{NV}}^{+}\phantom{\rule{4pt}{0ex}}(^{1}\mathrm{A}_{1}$ ground state) by probing a long-lived spin-triplet excited-state $\ensuremath{\sim}0.7$ eV above the ${\mathrm{NV}}^{+}$ ground state. We find a relatively narrow excitation energy range of approximately 0.7--1.1 eV between excitation into an $^{3}\mathrm{E}$ state and conversion into the neutral charge state. To provide insight into the thermal stability of the positive charge state, we have calculated binding and diffusion energies for both charged and uncharged systems. We predict, given that the activation energies are only weakly charge state dependent, all three charge states would diffuse in the 1600--$1900{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\text{C}$ range, but the positive state has a significantly lower binding energy, suggesting that it will dissociate at temperatures around $1000{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ rather than migrate.

Critical phenomena and nonlinear dynamics in a spin ensemble strongly coupled to a cavity. I. Semiclassical approach

We present a theoretical study on the nonlinear dynamics and stationary states of an inhomogeneously broadened spin ensemble coupled to a single-mode cavity driven by an external drive with constant amplitude. Assuming a sizeable number of constituents within the ensemble allows us to use a semiclassical approach and to formally reduce the theoretical description to the Maxwell-Bloch equations for the cavity and spin amplitudes. We explore the critical slowing-down effect, quench dynamics, and asymptotic behavior of the system near a steady-state dissipative phase transition accompanied by a bistability effect. Some of our theoretical findings have recently been successfully verified in a specific experimental realization based on a spin ensemble of negatively charged nitrogen-vacancy centers in diamond strongly coupled to a single-mode microwave cavity (see Science Adv. 3, e1701626 (2017)).

Temperature dependence of the C13 hyperfine structure of the negatively charged nitrogen-vacancy center in diamond

The nitrogen-vacancy (NV) center is a well utilized system for quantum technology, in particular quantum sensing and microscopy. Fully employing the NV center's capabilities for metrology requires a strong understanding of the behavior of the NV center with respect to changing temperature. Here, we probe the NV electronic spin density as the surrounding crystal temperature changes from 10 K to 700 K by examining its $^{13}$C hyperfine interactions. These results are corroborated with \textit{ab initio} calculations and demonstrate that the change in hyperfine interaction is small and dominated by a change in the hybridization of the orbitals constituting the spin density. Thus indicating that the defect and local crystal geometry is returning towards an undistorted structure at higher temperature.

Universal holonomic single quantum gates over a geometric spin with phase-modulated polarized light.

We demonstrate universal non-adiabatic non-abelian holonomic single quantum gates over a geometric electron spin with phase-modulated polarized light and 93% average fidelity. This allows purely geometric rotation around an arbitrary axis by any angle defined by light polarization and phase using a degenerate three-level Λ-type system in a negatively charged nitrogen-vacancy center in diamond. Since the control light is completely resonant to the ancillary excited state, the demonstrated holonomic gate not only is fast with low power, but also is precise without the dynamical phase being subject to control error and environmental noise. It thus allows pulse shaping for further fidelity.