Quantum computing with defects

Abstract

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.semiconductor defects | spintronics | first-principles calculations A quantum computer is a device that would exploit the rules of quantum mechanics to solve certain computational problems more efficiently than allowed by Boolean logic (1). Over the past two decades, qubits have been implemented in a wide variety of materials, including atoms (2), liquids (3), and solids such as superconductors (4), semiconductors (5), and ion-doped insulators (6). Recently, the diamond nitrogen-vacancy (NV −1 ) center has emerged as a leading qubit candidate because it is an individually addressable quantum system that may be initialized, manipulated, and measured with high fidelity at room temperature (7). Interestingly, even though these successes stem largely from the defect's nature as a deep center (a point defect with highly localized electronic bound states confined to a region on the scale of a single lattice constant), no systematic effort has been made to identify other deep centers that might behave similarly. We outline the physical features that such deep centers and their hosts should exhibit and show how these criteria can be used to identify potential qubit candidates within a large class of defects structurally analogous to the diamond NV −1 . To aid in the illustration of these points, we compare density functional theory (DFT) calculations of the diamond NV −1 with those of several defects found in 4H-SiC.Searching for deep centers that behave like the diamond NV −1 is worthwhile for several reasons. From an engineering perspective, it is currently quite difficult to grow and fabricate devices from diamond. The discovery of a similar defect in a more technologically mature host material might allow for more sophisticated implementations of single-and multiqubit devices. Additionally, because deep centers and semiconductors as a whole exhibit a diverse set of physical characteristics, innovative areas of device functionality may potentially arise once the quantum properties of these...

Defects as qubits in3C−and4H−SiC

“…3 These favorable properties have stimulated a search for analogous defects in materials other than diamond. 4 One obvious candidate material is silicon carbide, a compound that is closely related to diamond in structure and bonding.…”

“…7 Recent theoretical work has identified the N C -V Si center in 4H-SiC as a promising candidate for qubit applications. 4,5 First-principles calculations and experimental measurements have also indicated that the neutral divacancy in 4H-SiC has desirable properties 8-10 (see Fig. 1): photoluminescence, angleresolved magneto-luminescence, and continuous-wave optically-detected magnetic resonance (ODMR) measurements have demonstrated room-temperature spin control, with spin-coherence properties comparable to those of the NV center in diamond.…”

Frontiers in diffraction unlimited optical methods for spin manipulation, magnetic field sensing and imaging using diamond nitrogen vacancy defects

“…Finally NV centres, even if not discussed here, occupies a special place as possible fl ying qubit for remote entangling of spin qubits in solid state, due to its optical-spin polarization properties and its possible integration in quantumhybrid-spin systems (e.g., NV and magnetic single molecules). Other deep centres as NV centre in diamond and in similar wide-bandgap semiconductors, such as SiC are under investigation [18,66] ; we are witnessing either a new era of research of another NV-like centre in solid state or the establishing of novel advanced technologies in the above-mentioned fi elds.…”

“…Several studies have addressed this issue both experimentally [14,15] and theoretically [16] . Furthermore, other atom-like defects can potentially be engineered in diamond [17] and other materials [18] with similar or perhaps better properties suitable for the desired applications.…”

Near-band-gap photoinduced nuclear spin dynamics in semi-insulating GaAs: Hyperfine- and quadrupolar-driven relaxation

“…1 Examples of nuclear-electron spin complexes include those formed within self-assembled semiconductor quantum dots, 2 in the vicinity of crystal defects such as the nitrogen-vacancy center in diamond, 3 the divacancy and silicon-vacancy centers in SiC, 4,5 substitutional phosphorous 6 and bismuth 7 in silicon, rare-earth ions, 8 and others. 9 Systems formed by trapped photoexcited electrons and neighboring nuclear spins in semiconductors form a subclass of spin complexes with interesting properties. For example, because these defects often change from paramagnetic to magnetically inactive once the trapped photoelectron recombines, nuclear spins in the vicinity are inherently protected against electron-spin-induced decoherence.…”