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Physical Review B

Spin depolarization effect induced by charge state conversion of nitrogen vacancy center in diamond

Xiang-Dong Chen, Lei‐Ming Zhou, Chang‐Ling Zou, Congcong Li, Yang Dong, Fang‐Wen Sun, Guang‐Can Guo

Published Sep 10, 2015• 42 citations
Publisher

Abstract

The electron spin of the negatively charged the nitrogen vacancy center $({\mathrm{NV}}^{\ensuremath{-}}$) in diamond can be optically polarized through intersystem crossing, which enables the defect to be used for quantum computation and metrology. In this work, we studied the electron spin depolarization effect of the NV center induced by charge state conversion, which was proven to be a spin-independent process. The spin-state initialization fidelity was largely affected by the charge state conversion process. As a result, the optical polarization of the electron spin decreased about $14%(31%$) with a high-power continuous-wave (pulsed) green laser. Moreover, the undefined fluorescence anomalous saturation effect of the NV center was analyzed and explained in detail based on the spin depolarization. The results demonstrated that a weak laser should be used for initialization of the NV center. In addition, the power and polarization of a laser for NV spin detection should be carefully adjusted to obtain the highest fluorescence signal. Our work also provided information that can increase the understanding of the charge state conversion and spin polarization processes of the NV center for quantum information and sensing.

DOI
10.1103/physrevb.92.104301

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The electron spin of the negatively charged the nitrogen vacancy center (NV\ensuremath−({\mathrm{NV}}^{\ensuremath{-}}(NV\ensuremath−) in diamond can be optically polarized through intersystem crossing, which enables the defect to be used for quantum computation and metrology. In this work, we studied the electron spin depolarization effect of the NV center induced by charge state conversion, which was proven to be a spin-independent process. The spin-state initialization fidelity was largely affected by the charge state conversion process. As a result, the optical polarization of the electron spin decreased about 1414%(31%14) with a high-power continuous-wave (pulsed) green laser. Moreover, the undefined fluorescence anomalous saturation effect of the NV center was analyzed and explained in detail based on the spin depolarization. The results demonstrated that a weak laser should be used for initialization of the NV center. In addition, the power and polarization of a laser for NV spin detection should be carefully adjusted to obtain the highest fluorescence signal. Our work also provided information that can increase the understanding of the charge state conversion and spin polarization processes of the NV center for quantum information and sensing.
The nitrogen-vacancy (NV) center in diamond is of high importance in quantum information processing applications. The operation of the NV center relies on the efficient optical polarization of its electron spin. However, the full optical spin-polarization process, which involves the intersystem crossing between the shelving singlet state and the ground-state triplet, is not understood. Here we develop a detailed theory of this process which involves 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 a good agreement with the experimental data. Since the 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.
Nitrogen-vacancy (NV) centers in diamond have been exploited as reliable single-photon emitters, with applications in quantum technologies and metrology. Two charge states are known for NV centers: NV0 and NV–, with the latter being mostly studied due to its long electron spin coherence time. Therefore, control over the charge state of the NV centers is essential. However, the understanding of the dynamics between the different states still remains challenging. Here we show that electron excitation induces the conversion from NV– to NV0 through electron-induced carrier generation. We present for the first time ultrafast pump-probe cathodoluminescence spectroscopy, with electron pulses as pump, and laser pulses as probe, to prepare and read out the NV states. The experimental data is explained with a model considering carrier dynamics (0.8 ns), NV0 spontaneous emission (20 ns) and NV0NV– back transfer (500 ms). Our results provide new insights into the NV–NV0 conversion dynamics, and into the use of pump-probe cathodoluminescence as a nanoscale NV characterization tool.
The photoluminescence of the negatively charged nitrogen-vacancy (${\mathrm{NV}}^{\ensuremath{-}}$) center displays anomalous saturation behavior at high excitation pulse energies. Where the luminescence is expected to approach a maximum value asymptotically, we have found that it instead drops by as much as a factor of two. In this report, we present evidence that these effects are caused by optical spin depolarization of the ${\mathrm{NV}}^{\ensuremath{-}}$ center. We show that the presence of an external magnetic field results in a marked decrease in the anomalous character of the ${\mathrm{NV}}^{\ensuremath{-}}$ centers' luminescence and demonstrate that low-energy pulses applied after a strong depolarizing pulse can repolarize the spin of the center. We also offer a model and derive important parameters describing dynamics in the NV center and its interaction with light, which quantitatively explain all the observations.
The nitrogen vacancy (NV) center in diamond is the most widely studied color center in diamond for its wide range of applications in both physics and biology. The negatively charged state ${\mathrm{NV}}^{\ensuremath{-}}$ is often favored for its better spin-optical properties, whereas the presence of the neutral charge state NV0{\mathrm{NV}}^{0}NV0 is often minimized or neglected by using a single optimized excitation wavelength. In many cases, the use of additional laser wavelengths has an important impact on the NV optical properties and can lead to a dramatically quenched fluorescence. Through measurements of the photophysics of the NV center we show in this work that this quenching mechanism is mediated by charge state interconversion providing additional channels for nonradiative decay. Our results show that this mechanism is also sensitive to the spin state of the ${\mathrm{NV}}^{\ensuremath{-}}$. A better understanding of this physical mechanism will help mitigate the spin and charge state depolarization effects that are detrimental for quantum technology applications.
As a potential candidate for quantum computation and metrology, the nitrogen vacancy (NV) center in diamond presents both challenges and opportunities resulting from charge-state conversion. By utilizing different lasers for the photon-induced charge-state conversion, we achieved subdiffraction charge-state manipulation. The charge-state depletion (CSD) microscopy resolution was improved to 4.1 nm by optimizing the laser pulse sequences. Subsequently, the electron spin-state dynamics of adjacent NV centers were selectively detected via the CSD. The experimental results demonstrated that the CSD can improve the spatial resolution of the measurement of NV centers for nanoscale sensing and quantum information. The nitrogen vacancy (NV) in diamond is useful for quantum optics and spintronics due to the long coherence time of its electron spin state. Scientists in China have now used the techniques of super-resolution microscopy to optically control the charge state of an NV defect with a subdiffraction spatial resolution of 4.1 nm. Xiangdong Chen and co-workers from the University of Science and Technology of China in Hefei used a green (532 nm) laser beam to switch the charge state of the defect between NV0 and NV–. They achieved this manipulation on a subdiffraction scale by using specially shaped doughnut laser beams, in common with the principles of super-resolution stimulated emission depletion microscopy. This achievement represents a significant step towards nanoscale sensing and quantum information processing.
The nitrogen-vacancy (NV) center in diamond enables optical initialization and readout of its electronic spin, forming the basis of a wide range of quantum sensing and metrology applications. A central challenge in such measurements is the coexistence of two charge states, NV- and NV0: While detection protocols rely on the spin-dependent properties of NV-, fluorescence from NV0 does not carry useful contrast and is typically removed as background, reducing the available signal. Here, we show that the origin of NV0 emission depends strongly on the excitation wavelength in nitrogen-containing diamond. Using ensembles of NV centers with varying nitrogen concentrations, we compare excitation at the NV0 zero-phonon line (ZPL) at 575 nm with the commonly used 532 nm. We find that excitation at 575 nm generates NV0 predominantly through spin-selective tunneling from the excited state of NV- to nearby nitrogen donors, such that the NV0 emission follows the spin polarization of NV-. As a result, the NV0 fluorescence contributes to the measurable spin contrast, allowing the full fluorescence signal to be used for detection. This result opens opportunities for improved sensitivity in NV-based sensing applications.