Defects for quantum information processing

The idea of utilizing quantum systems to perform complicated quantum mechanical simulations and calculations was raised by Richard Feynman in 1982. This basic idea has initiated new directions in many disciplines and influenced different fields of nowadays physics. By now it has been widely accepted that quantum computation and information processing applications may possess extraordinary features and of great importance for future technologies.

Research groups at the very frontiers of physics are actively working on explaining the behavior and creating viable designs of the building blocks, such as quantum bits (qubits), quantum gates, etc. of would-be quantum computers. One of the most promising candidates for the realization of a qubit is the spin of a single point defect in semiconductors and insulators which can be considered as a good compromise compared to the solution of other contenders and has great technical advantages because of the large amount of accumulated experiences with materials processing. The extraordinary properties of defects such as the famous NV center in diamond allow optical control of the electron and nuclear spins. With applying static and radio and microwave electromagnetic fields to them, several quantum operations have been carried out by these systems so far.

Furthermore, thanks to the achieved ultimate control of electron spins, these point defects often can be utilized as nanoscale sensing tools. High sensitivity of their finely-tuned properties to environmental perturbations is the key feature behind this idea. This recent field of research and application is often called as nanometrology.

Our group actively contributes to the development of these new fields by first-principles and analytical characterization of well-established point defects and promising candidates thereof.

Nitrogen-vacancy center in diamond

The nitrogen-vacancy (NV) center in diamond is a leading contender for realizing the solid state spin qubits concept in quantum information. Our group significantly contributed to the understanding of optical and magnetic properties of this defect, and is continuously investigating those as a function of external perturbations, temperature, surface effects or quantum confinement.

We successfully determined the hyperfine tensors of the nuclei with non-zero nuclei spins and identified the qubits realized by proximate ¹³C isotopes <bib id=""/>. We calculated the corresponding hyperfine tensors in the electronic excited states too and analyzed their role in the so-called excited state level anti-crossing mechanism leading to the nuclear spin polarization in NV center <bib id=""/>. Later the nature of the hyperfine interaction of these nuclei was analyzed in detail <bib id="PhysRevB.88.075202"/>.

We analyzed the electronic structure and the hyperfine signals of the neutral NV defect in detail <bib id=""/> and proposed a scheme to utilize it for quantum information processing. Later on we proposed a model on the dynamical charge switching between the neutral NV defect and the negatively charged NV defect upon strong laser illumination based on quantum mechanical calculations. We calculated the radiative lifetimes and non-radiative Auger-rates <bib id=""/>.

Silicon-vacancy center in diamond

Comments.

In a cooperation with partners in Russia and Germany, we have taken part in the work <bib id="Vlasov2014" /> of predicting and validating the properties of silicon-vacancy colour centres in nanosized diamond particles. We have contributed with time-dependent DFT calculations, not carried out on such large systems so far. Nanoparticles for validating experiments have been extracted from a meteorite sample, but production of similar artificial nanoparticles seems to be very prospective in e.g. biosensing. This article has been selected for review<bib id="becher2014fluorescent" /> in Nature News and Views.

Silicon carbide defects

Comments.

The extraordinary properties of NV-center in diamond exhibit great potential for the investigation of point defect based quantum information processing systems. However, from the application point of view the extreme hardness of diamond may cause difficulties in fabrication and can hinder the spreading of possible applications. Among the first, our group proposed the idea of considering silicon carbide (SiC) as a new target of research, which is a device friendly material but may host promising defects for quantum information processing. The idea of using SiC to host solid state quantum bits was first openly suggested by Adam Gali at International Conference of SiC and Related Materials 2009 (Nürnberg, Germany, 2009) reference where divacancy was the primary candidate where later other candidates were also theoretically studied references: Weber, Gali. The first breakthrough has been achieved for the coherent manipulation of ensemble divacancy and related defect spins in 4H polytype of SiC by the Awschalom group reference.

Investigation of new candidates in SiC is a rapidly developing direction of nowadays research. In this respect, the most actively studied defects in SiC are the divacancy references, the silicon vacancy references, and the carbon antisite-vacancy pair references.

In an international cooperation with Japanese and Australian colleagues, we supported the establishment and identification<bib id="castelletto2014silicon" /> of the carbon antisite–vacancy pair as a world-record bright single-photon emitter in silicon carbide with theoretical work and computational simulation. The results have been highlighted in Nature Physics<bib id="aharonovich2014optical" /> and Nature Photonics<bib id="boretti2014optical" />.

Bibliography

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