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Science

In search of the right diamond defect

Jelena Stajic

Published Jul 5, 2018• 0 citations
Publisher

Abstract

Quantum Information Certain defects in diamond are among the most promising physical implementations of qubits, the building blocks of quantum computers. However, identifying a defect with balanced properties is tricky: Nitrogen vacancy centers have a long lifetime but comparatively poor optical properties, whereas negatively charged silicon vacancy centers have the opposite characteristics. Rose et al. used careful materials engineering to stabilize the neutral charge state of silicon vacancy centers and found that they combine long coherence times with excellent optical properties. Science , this issue p. [60][1] [1]: /lookup/doi/10.1126/science.aao0290

DOI
10.1126/science.361.6397.38-c

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Quantum Information
Certain defects in diamond are among the most promising physical implementations of qubits, the building blocks of quantum computers. However, identifying a defect with balanced properties is tricky: Nitrogen vacancy centers have a long lifetime but comparatively poor optical properties, whereas negatively charged silicon vacancy centers have the opposite characteristics. Rose et al. used careful materials engineering to stabilize the neutral charge state of silicon vacancy centers and found that they combine long coherence times with excellent optical properties.
Science , this issue p. 60
In search of the right diamond defect Certain defects in diamond are among the most promising physical implementations of qubits, the building blocks of quantum computers. However, identifying a defect with balanced properties is tricky: Nitrogen vacancy centers have a long lifetime but comparatively poor optical properties, whereas negatively charged silicon vacancy centers have the opposite characteristics. Rose et al. used careful materials engineering to stabilize the neutral charge state of silicon vacancy centers and found that they combine long coherence times with excellent optical properties. Science , this issue p. 60
Quantum Information Like magnetic storage in today's classical computers, tiny “magnets” associated with electronic and nuclear states—spins—are promising qubits (quantum bits) for the future. Electronic spins in special defects in diamond called nitrogen-vacancy (NV) centers are one example. Whereas most applications focus on the least energetic (ground) state of an NV center, Bassett et al. explored the properties of the higher-energy (excited) state (see the Perspective by Childress). They used light pulses to bring the system into the excited state and to vary the time it stayed there. In this way, they could both deduce the electronic structure of the excited state and manipulate the ground state spin. Similar methods may be applicable to other quantum information systems. Science , this issue p. [1333][1]; see also p. [1247][2] [1]: /lookup/doi/10.1126/science.1255541 [2]: /lookup/doi/10.1126/science.1257840
Point defects in semiconductors are becoming central to quantum technologies. They can be used as spin qubits interfacing with photons, which are fundamental for building quantum networks. Currently, the most prominent quantum defect in diamond is the nitrogen-vacancy (NV) center. However, it suffers from spectral diffusion that negatively impacts optical coherence and is due to the coupling of the emission energy with uncontrolled electric fields. The group IV vacancy complexes on the other hand have shown to be significantly more spectrally-stable as they are centrosymmetric and thus immune to the linear Stark shift. They however suffer from several issues ranging from low operation temperature to low optical efficiency due to dark states and difficulty in stabilizing the right defect charge state. Here we search for alternative to the group IV vacancy complex in diamond by systematically evaluating all possible vacancy complex using high-throughput first-principles computational screening. We identify the defects that combine centrosymmetry, emission in the visible range, as well as favorable and achievable electronic structure promoting higher operation temperature and defect levels well within the band gap. We find ZnV−2V^{-2}V−2 to be especially appealing.
Color centers in diamond are point defects within the diamond host lattice that absorb and emit light at optical frequencies. Besides contributing to the striking visual characteristics of "fancy colored" diamonds as gemstones, the centers--particularly the negatively charged nitrogen-vacancy (NV) center and silicon-vacancy (SiV-) center--offer a number of possibilities for quantum computation and quantum information processing. In this talk, I will summarize recent progress made in characterizing negatively charged silicon-vacancy centers in diamond using the technique of optical multidimensional coherent spectroscopy (MDCS). By comparing photoluminescence-based and heterodyne-detection based signal collection schemes in a high-density SiV- center sample, we have selectively identified a population of long-lived and nonradiative silicon-vacancy centers in diamond with more than 40 times as much inhomogeneous spectral broadening as the radiative silicon-vacancy center states that are more commonly observed using photoluminescence. Estimates of the degree of inhomogeneity and overall sample characteristics indicate that strain is likely to play a large role in the formation of these nonradiative states. The findings open possibilities for being able to actively tune the degree of radiative coupling in silicon-vacancy center systems, opening possibilities for new types of quantum-optical devices.
The negatively charged nitrogen-vacancy defect (NV)−^-− 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 (NV)−^-−. In this Letter, we unambiguously identify the \emph{neutral} nitrogen-vacancy defect (NV0^00) by \emph{ab initio} supercell calculations. Our analysis shows that i) the spin state can be \emph{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 NV0^00 is a new hope for realizing qubit in diamond \emph{without} the need of nitrogen donors.
Identifying and designing physical systems for use as qubits, the basic units of quantum information, are critical steps in the development of a quantum computer. Among the possibilities in the solid state, a defect in diamond known as the nitrogen-vacancy (NV(-1)) center stands out for its robustness--its quantum state can be initialized, manipulated, and measured with high fidelity at room temperature. Here we describe how to systematically identify other deep center defects with similar quantum-mechanical properties. We present a list of physical criteria that these centers and their hosts should meet and explain how these requirements can be used in conjunction with electronic structure theory to intelligently sort through candidate defect systems. To illustrate these points in detail, we compare electronic structure calculations of the NV(-1) center in diamond with those of several deep centers in 4H silicon carbide (SiC). We then discuss the proposed criteria for similar defects in other tetrahedrally coordinated semiconductors.