# Defects in Silicon Carbide as Quantum Qubits: Recent Advances in Defect Engineering
## 3 Defects in Silicon Carbide  
SiC is a wide-bandgap (\(\sim\)3.26 eV in 4H-SiC) semiconductor with exceptional properties, making it a key material for power electronics and radiation detection applications [41; 42; 43; 44]. It exists in more than 250 polytypes, with 4H-SiC being the most extensively studied. The 4H (hexagonal) polytype is preferred for electronic components due to its high and nearly \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt}} \hline \hline
**Defect Properties** & **Host Material Properties** \\ \hline The defect must have a paramagnetic, long-lived bound state with an energy splitting suitable for manipulation via electron spin resonance. & Wide bandgap: allows for deep defect levels that satisfy the requirement for optical transitions. \\ \hline The defect should have an optical cycle that allows spin initialization via spin-selective decay. & Low spin–orbit coupling: minimizes unwanted spin flips and decoherence. \\ \hline The defect’s fluorescence must change depending on the qubit state, enabling efficient optical readout. & High-quality crystals: availability of bulk or thin-film single crystals to reduce imperfections and impurities. \\ \hline Optical transitions should not be affected by interference from the host material’s electronic states. & Nuclear spin-free isotopes: constituent elements should have isotopes with zero nuclear spin to minimize decoherence effects. \\ \hline Energy separations between defect states must be large enough to prevent thermal excitations that could destroy spin coherence. & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Key properties of defects and host materials for defect-based qubits [4]. Figure 3: A schematic presentation of different physical systems used as quantum qubit platforms. Adapted from Ref. [39]. isotropic carrier mobility, which enhances device performance [41; 44]. A similar trend is observed in quantum applications, where 4H-SiC remains the dominant choice due to its favorable optical and spin properties. However, other polytypes, such as 3C-SiC (cubic) and 6H-SiC (hexagonal), are gaining increasing attention for quantum technologies, as recent studies suggest that they may offer unique advantages for hosting defect-based qubits [45; 46; 47; 48]. For a more detailed discussion of the material properties of SiC and its most commonly used polytypes (3C, 4H, and 6H), we refer readers to Refs. [41; 43; 44; 49], as well as the references within. These handbooks and review papers provide comprehensive insights into the structural, electrical, and optical characteristics of SiC, making them essential reading for those interested in both the fundamental and applied aspects of the material. Among the many remarkable properties of SiC, its crystallographic structure plays a crucial role in defect engineering. Due to the presence of multiple non-equivalent lattice sites in the primitive cell, point defects in SiC can form in different configurations, significantly influencing their electronic and spin properties. Further details on the electronic and optical properties of point defects in SiC, particularly regarding the impact of the Jahn-Teller effect, can be found in Refs. [50; 51]. The 3C polytype, shown in Figure 4a, has cubic symmetry, with a single C and Si atom in the primitive cell. The 4H polytype has hexagonal symmetry with eight atoms in the primitive cell, of which two are non-equivalent for both Si and C (Figure 4b). The 6H polytype is also of hexagonal symmetry, has 12 atoms in the primitive cell, and 3 are non-equivalent for both Si and C (Figure 4c). Therefore, a single-site defect in 4H has two configurations, and three in 6H-SiC. A pair defect then has four and six configurations in 4H and 6H-SiC, respectively. The non-equivalent sites are denoted as h (hexagonal) and k (quasi-cubic) [52; 53]. Figure 4: The primitive cells of (**a**) 3C, (**b**) 4H, and (**c**) 6H-SiC.