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Materials for Quantum Technology

Nitrogen-vacancy centers in diamond: discovery of additional electronic states

Minh Tuan Luu, Ali Tayefeh Younesi, Ronald Ulbricht

Published Jul 10, 2024• 16 citations
Publisher

Abstract

Abstract Nitrogen-vacancy (NV) defect centers in diamond are key to applications in quantum sensing and quantum computing. They create localized electronic states in the diamond lattice with distinct population relaxation pathways following photoexcitation that ultimately enable its unique properties. The defect is known to exist in two charge states: neutral and negative, with respectively one and two known optically-active electronic transitions. Here, we report on the observation of a large number of hitherto undiscovered excited electronic states in both charge states as evidenced by distinct optical transitions in the infrared to ultraviolet part of the spectrum. These transitions are observed by monitoring the electronic relaxation of NV centers after photoexcitation using transient absorption spectroscopy, directly probing transient phenomena occurring on timescales from femtoseconds to microseconds. We also for the first time probed the electron transfer dynamics from the 3 E state of NV− to nearby single-substitutional nitrogen defects (N s ) that leads to the well-known effect of NV photoluminescence quenching.

DOI
10.1088/2633-4356/ad61b3

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Negatively charged nitrogen-vacancy (NV${}^{\ensuremath{-}}$) color centers in diamond have generated much interest for use in quantum technology. Despite the progress made in developing their applications, many questions about the basic properties of NV${}^{\ensuremath{-}}$ centers remain unresolved. Understanding these properties can validate theoretical models of NV${}^{\ensuremath{-}}$, improve their use in applications, and support their development into competitive quantum devices. In particular, knowledge of the phonon modes of the ${}^{1}{A}_{1}$ electronic state is key for understanding the optical pumping process. Using pump-probe spectroscopy, we measured the phonon sideband of the ${}^{1}E\ensuremath{\rightarrow}{}^{1}{A}_{1}$ electronic transition in the NV${}^{\ensuremath{-}}$ center. From this we calculated the ${}^{1}E\ensuremath{\rightarrow}{}^{1}{A}_{1}$ one-phonon absorption spectrum and found it to differ from that of the ${}^{3}E\ensuremath{\rightarrow}{}^{3}{A}_{2}$ transition, a result which is not anticipated by previous group-theoretical models of the NV${}^{\ensuremath{-}}$ electronic states. We identified a high-energy 169-meV localized phonon mode of the ${}^{1}{A}_{1}$ level.

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Abstract The nitrogen-vacancy (NV) center in diamond is a promising quantum system for magnetometry applications exhibiting optical readout of minute energy shifts in its spin sub-levels. Key material requirements for NV ensembles are a high NV − concentration, a long spin coherence time and a stable charge state. However, these are interdependent and can be difficult to optimize during diamond growth and subsequent NV creation. In this work, we systematically investigate the NV center formation and properties in bulk chemical vapor deposition (CVD) diamond. The nitrogen flow during growth is varied by over four orders of magnitude, resulting in a broad range of single substitutional nitrogen concentrations of 0.2–20 parts per million. For a fixed nitrogen concentration, we optimize electron-irradiation fluences with two different accelerated electron energies, and we study defect formation via optical characterizations. We discuss a general approach to determine the optimal irradiation conditions, for which an enhanced NV concentration and an optimum of NV charge states can both be satisfied. We achieve spin–spin coherence times T 2 ranging from 45.5 to 549 μ s for CVD diamonds containing 168 to 1 parts per billion NV − centers, respectively. This study shows a pathway to engineer properties of NV-doped CVD diamonds for improved sensitivity.

arXiv:2111.07981•New Journal of Physics•Feb 25, 2022•78 citations
Abstract Nitrogen-vacancy (NV) defect centers in diamond are key to applications in quantum sensing and quantum computing. They create localized electronic states in the diamond lattice with distinct population relaxation pathways following photoexcitation that ultimately enable its unique properties. The defect is known to exist in two charge states: neutral and negative, with respectively one and two known optically-active electronic transitions. Here, we report on the observation of a large number of hitherto undiscovered excited electronic states in both charge states as evidenced by distinct optical transitions in the infrared to ultraviolet part of the spectrum. These transitions are observed by monitoring the electronic relaxation of NV centers after photoexcitation using transient absorption spectroscopy, directly probing transient phenomena occurring on timescales from femtoseconds to microseconds. We also for the first time probed the electron transfer dynamics from the 3 E state of NV− to nearby single-substitutional nitrogen defects (N s ) that leads to the well-known effect of NV photoluminescence quenching.
The negatively charged nitrogen vacancy ($\mathrm{N}{\mathrm{V}}^{\ensuremath{-}}$) defect in diamond serves as a popular platform for manipulating and exploiting long-lived coherent spin dynamics at room temperature combined with optical readout. The required spin polarization of the spin triplet 3A2^{3}A_{2}3A2​ electronic ground state occurs through a cycle of repetitious optical photoexcitation events to the 3E^{3}E3E electronic excited state that is accompanied by a series of electronic transitions to a 1A1^{1}A_{1}1A1​ and a 1E^{1}E1E electronic state, and back to the 3A2^{3}A_{2}3A2​ state. The timescales of these transitions are largely known, yet for the relaxation time of the $^{1}A_{1}\ensuremath{\rightarrow}^{1}E$ infrared transition, which predominantly occurs via nonradiative recombination, only an upper limit of 1 ns could be determined so far. Here, we employ ultrafast transient absorption spectroscopy to probe the dynamics of the nonradiative relaxation from the 1A1^{1}A_{1}1A1​ to the 1E^{1}E1E state after photoexcitation of the 3E^{3}E3E state and find a relaxation time of 100 ps at a temperature of 78 K.
The charge state stability of nitrogen-vacancy (NV) centers critically affects their application as quantum sensors and qubits. Understanding charge state conversion and equilibration is important not only for NV centers in diamond but also for defects and impurities in wide-bandgap materials in general. The mechanisms by which these centers change charge state upon optical or electronic excitation without the presence of mobile carriers remain unclear. Here, we elucidate this issue for the case of photoionization of NV center ensembles. Using pump-probe spectroscopy, we ionize negatively charged NV centers and monitor the recovery of NV^{-} on timescales of up to several seconds. We find that the recovery rate depends strongly on the concentration of surrounding nitrogen donors. Remarkably, the equilibration dynamics exhibit no discernible dependence on temperature, ruling out thermally activated processes. We develop a multiphonon-assisted electron-tunneling model, supported by density-functional calculations, that explains the measurements and identifies tunneling as the equilibration mechanism. Because tunneling rates depend exponentially on donor separation, the random distribution of nitrogen donors spreads recovery times from nanoseconds to hours or longer.
Nitrogen-vacancy (NV) complex in diamond is one of the most prominent solid state defects as the negatively charged NV defect (NV−) is a leading contender for quantum technologies. In quantum information processing applications, NV− is photoexcited that often leads to photoionization to neutral NV defect, NV0, and re-ionization back to NV− should occur to control the S=1 spin of NV−. As a consequence, understanding the photophysics of NV0 is crucial for controlling NV−. Furthermore, recent studies have shown that the S=1/2 electron spin of NV0 can also be initialized and read out at certain conditions that turns single NV0 a potential quantum bit. Quantum optics protocols rest on detailed knowledge on the electronic structure of the given system, which is obviously missing for NV0 in diamond. In this study, we combine the group theory and density functional theory calculations toward exploring the nature of the ground and excited states of NV0. We show that the effective three-electron system of NV0 leads to high correlation effects that make this system very challenging for ab initio simulations.
Negatively charged nitrogen-vacancy (NV${}^{\ensuremath{-}}$) color centers in diamond have generated much interest for use in quantum technology. Despite the progress made in developing their applications, many questions about the basic properties of NV${}^{\ensuremath{-}}$ centers remain unresolved. Understanding these properties can validate theoretical models of NV${}^{\ensuremath{-}}$, improve their use in applications, and support their development into competitive quantum devices. In particular, knowledge of the phonon modes of the 1A1{}^{1}{A}_{1}1A1​ electronic state is key for understanding the optical pumping process. Using pump-probe spectroscopy, we measured the phonon sideband of the ${}^{1}E\ensuremath{\rightarrow}{}^{1}{A}_{1}$ electronic transition in the NV${}^{\ensuremath{-}}$ center. From this we calculated the ${}^{1}E\ensuremath{\rightarrow}{}^{1}{A}_{1}$ one-phonon absorption spectrum and found it to differ from that of the ${}^{3}E\ensuremath{\rightarrow}{}^{3}{A}_{2}$ transition, a result which is not anticipated by previous group-theoretical models of the NV${}^{\ensuremath{-}}$ electronic states. We identified a high-energy 169-meV localized phonon mode of the 1A1{}^{1}{A}_{1}1A1​ level.
The negatively charged nitrogen-vacancy defect $({\text{NV}}^{\ensuremath{-}})$ 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 ${\text{NV}}^{\ensuremath{-}}$. In this study, we identify the neutral nitrogen-vacancy defect (NV0)({\text{NV}}^{0})(NV0) by ab initio supercell calculations through the comparison of the measured and calculated hyperfine tensors of the 4A2{^{4}A}_{2}4A2​ excited state. Our analysis shows that (i) the spin state can be 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 NV0{\text{NV}}^{0}NV0 is a hope for realizing qubit in diamond without the need of nitrogen donors. In addition, we propose that NV0{\text{NV}}^{0}NV0 may be more sensitive magnetometer than the ultrasensitive ${\text{NV}}^{\ensuremath{-}}$.
Abstract The nitrogen-vacancy (NV) center in diamond is a promising quantum system for magnetometry applications exhibiting optical readout of minute energy shifts in its spin sub-levels. Key material requirements for NV ensembles are a high NV − concentration, a long spin coherence time and a stable charge state. However, these are interdependent and can be difficult to optimize during diamond growth and subsequent NV creation. In this work, we systematically investigate the NV center formation and properties in bulk chemical vapor deposition (CVD) diamond. The nitrogen flow during growth is varied by over four orders of magnitude, resulting in a broad range of single substitutional nitrogen concentrations of 0.2–20 parts per million. For a fixed nitrogen concentration, we optimize electron-irradiation fluences with two different accelerated electron energies, and we study defect formation via optical characterizations. We discuss a general approach to determine the optimal irradiation conditions, for which an enhanced NV concentration and an optimum of NV charge states can both be satisfied. We achieve spin–spin coherence times T 2 ranging from 45.5 to 549 μ s for CVD diamonds containing 168 to 1 parts per billion NV − centers, respectively. This study shows a pathway to engineer properties of NV-doped CVD diamonds for improved sensitivity.