# Defects in Silicon Carbide as Quantum Qubits: Recent Advances in Defect Engineering
## 1 Introduction  
Quantum technologies are poised to revolutionize computing, communication, and sensing by harnessing the principles of quantum mechanics. At their foundation lie quantum bits, or qubits, which can exist in superposition states, enabling vastly enhanced computational capabilities compared to classical bits [1]. Various physical systems have been explored for qubit realization, including trapped ions, superconducting circuits, and semiconductor-based defect centers [2, 3]. Among these, solid-state defects in wide-bandgap semiconductors have attracted significant attention due to their long spin coherence times, optical addressability, and compatibility with established semiconductor fabrication techniques [4]. Silicon carbide (SiC), a wide bandgap semiconductor, stands out as a promising host material due to the variety of its point defects. These point defects are commonly referred to as optically active defects or color centers, and all those names will be used in this paper. The pioneering work on and understanding of color centers started with nitrogen-vacancy (NV) centers in diamond [5, 6, 7, 8]. However, color centers in SiC present notable advantages, including compatibility with wafer-scale production, precise doping control, integration with mature complementary metal-oxide semiconductor (CMOS) technology, and access to well-established nanofabrication processes. Among various color centers in SiC, the SiC vacancy (V\({}_{\text{Si}}\)) and the divacancy (V\({}_{\text{C}}\)-V\({}_{\text{Si}}\)) are the most studied [9, 10, 11, 12, 13]. A critical requirement for the successful implementation of SiC-based quantum technologies is the controlled introduction and manipulation of point defects. Various techniques have been developed to create and stabilize these defects, including ion implantation, neutron irradiation, and femtosecond laser processing [14, 15, 16, 17, 18]. Radiation-induced defects,in particular, are of great interest due to their potential for generating well-defined qubit properties through precise defect engineering. However, several challenges remain [2,4]:  
* Defect Concentration and Spatial Control: Achieving a high density of optically active defects while maintaining spatial precision is crucial for scaling quantum devices;
* Charge State Stability: The performance of defect-based qubits depends on stable charge states, and only specific charged states of color centers are suitable for quantum applications;
* High-purity material: Impurities and unintentionally introduced defects create local strain and electric field variations, leading to the inhomogeneous broadening of optical and spin transition lines. Defects in SiC have been identified as promising candidates for various quantum applications, including the following [9, 10, 11, 12, 16, 19, 20, 21]:  
* Quantum Computing: Defect-based qubits can serve as scalable quantum information processors, leveraging SiC's mature fabrication ecosystem;
* Quantum Sensing: The spin properties of SiC color centers enable the ultrasensitive detection of magnetic, electric, and thermal fields;
* Quantum Communication: Color centers in SiC, particularly those operating at telecom wavelengths, offer pathways for secure quantum information transfer over fiber-optic networks. As illustrated in Figure 1, the number of research publications on quantum qubits has increased significantly over the past decade, reflecting the growing interest in defect-based quantum technologies. The data were obtained from the Web of Science Core Collection using the keyword search \"quantum qubits.\" This trend underscores the expanding role of SiC as a key material in the development of next-generation quantum devices. With the rapid growth of quantum-related research, the number of review papers in this field has expanded significantly. These reviews play a crucial role in summarizing recent advancements, identifying key challenges, and outlining future research directions.