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arXiv:1803.02561 · Physical review. B./Physical review. B

Theory of the optical spin-polarization loop of the nitrogen-vacancy center in diamond

Gergő Thiering, Ádám Gali

Published Aug 29, 2018• 107 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevb.98.085207

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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.
We investigate spin-dependent decay and intersystem crossing (ISC) in the optical cycle of single negatively charged nitrogen-vacancy (NV) centres in diamond. We use spin control and pulsed optical excitation to extract both the spin-resolved lifetimes of the excited states and the degree of optically induced spin polarization. By optically exciting the centre with a series of picosecond pulses, we determine the spin-flip probabilities per optical cycle, as well as the spin-dependent probability for ISC. This information, together with the independently measured decay rate of singlet population, provides a full description of spin dynamics in the optical cycle of NV centres. The temperature dependence of the singlet population decay rate provides information about the number of singlet states involved in the optical cycle.
The negative Nitrogen-Vacancy (NV) colour centre in diamond is an atomic-level impurity that is remarkably well suited for quantum information processing and quantum metrology applications. Underlying these properties is the fact that optical illumination causes the NV centre to preferentially populate one of its three spin levels. The mechanism that gives rise to this spin polarisation has not been well understood, despite intense research interest in the NV centre and its applications. This thesis arrives at an account of the spin polarisation mechanism, although a detailed theoretical treatment remains outstanding. The energy level structure of the NV centre is investigated using techniques of solid state spectroscopy. These include absorption and photoluminescence measurements made on diamonds at cryogenic and room temperatures in the presence of uniaxial stress, magnetic fields, and microwaves. Pulsed light experiments allow the dynamic processes within the NV centre to be measured. It is shown that at room temperature the orbital doublet excited state behaves like an orbital singlet, and this description is elegantly confirmed by single-site observations. This picture of the excited state is inconsistent with cryogenic measurements, and its behaviour is examined across a range of temperatures in order to resolve this tension. It is shown that electron-vibration interaction accounts for the quenching of orbital properties in the excited state with temperature. A new optical transition within the NV centre is reported at a wavelength of 1042 nm. It is established that this transition occurs between the intermediate singlet levels which are thought to provide the decay pathway that produces spin polarisation. The results are consistent with this understanding, and the order and properties of these singlet levels are established. Electron-vibration interaction is observed to be important within the singlet system. It is shown that electron-vibration interaction is an important detail in the NV optical pumping cycle. Taking it into consideration leads to the first plausible physical description of the spin polarisation mechanism. -- provided by Candidate.
The nitrogen-vacancy (NV) center in diamond has emerged as a versatile atom-like system, finding diverse applications at both ambient and cryogenic temperatures. At room temperature, the NV center's spin-triplet electronic ground state has a long coherence time and can be read out and initialized using nonresonant optical excitation, which has enabled a wide range of metrology and quantum information applications. At cryogenic temperatures, the NV center exhibits a variety of narrow, atom-like optical transitions, which has made the NV center a promising platform for quantum optics applications. We use coherent optical manipulation of the NV center at cryogenic temperatures to probe its internal dynamics and aspects of its interactions with its environment.\nFirst, we investigate the intersystem crossing (ISC) process, which is a spin-dependent nonradiative decay mechanism that enables the electronic spin state to be optically initialized and read out at room temperature. We measure the ISC rates from different excited states as well as the rate at which interactions with phonons redistribute population between these states. Based on these measurements, we develop a theoretical model that unifies the phonon-induced mixing and ISC mechanisms. We find that our model is in excellent agreement with experiment and that it can be used to predict unknown elements of the NV center's electronic structure.\nSecond, we investigate a novel method of using coherent optical manipulation of the NV center's electronic spin to control a nearby nuclear spin. Coherent interactions between the NV center and nearby nuclear spins have enabled the development of NV center-based, multi-qubit quantum registers, but these techniques have generally required the application of microwave or radio-frequency radiation. We demonstrate control of the nuclear spin using an all-optical Raman technique and theoretically evaluate the extent to which the intrinsic physics of the NV center limits the coherence of this Raman-based manipulation technique.
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 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.
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.
We present a procedure that makes use of group theory to analyze and predict the main properties of the negatively charged nitrogen-vacancy (NV) center in diamond. We focus on the relatively low temperature limit where both the spin–spin and spin–orbit effects are important to consider. We demonstrate that group theory may be used to clarify several aspects of the NV structure, such as ordering of the singlets in the ( e 2 ) electronic configuration and the spin–spin and spin–orbit interactions in the ( ae ) electronic configuration. We also discuss how the optical selection rules and the response of the center to electric field can be used for spin–photon entanglement schemes. Our general formalism is applicable to a broad class of local defects in solids. The present results have important implications for applications in quantum information science and nanomagnetometry.