# Defects in Silicon Carbide as Quantum Qubits: Recent Advances in Defect Engineering
## 2 Quantum Qubits  
This section provides an overview of the fundamental properties of quantum qubits. A quantum qubit (or simply \"qubit\") serves as the fundamental unit of quantum information, analogous to a classical bit. However, unlike a classical bit, which can only exist in a definite state of 0 or 1, a qubit can exist in a superposition of both states simultaneously (asgraphically presented in Figure 2). These two basis states are typically represented as \(|0\rangle\) and \(|1\rangle\)[1; 29]. The general state of a qubit is given as follows:  
\[|\ \psi\rangle=\alpha\ |\ \ 0\rangle+\beta\ |\ \ 1\rangle\]  
where \(\alpha\) and \(\beta\) are complex probability amplitudes. The squared magnitudes of these amplitudes, \(|\alpha\,|^{2}\) and \(|\beta|^{2}\), represent the probabilities of measuring the qubit in the \(|0\rangle\) or \(|1\rangle\) state, respectively, and must always sum to one:  
\[|\alpha|^{2}+|\beta|^{2}=1\]  
A qubit remains in its superposition state as long as it is not measured or disturbed by its environment. However, upon measurement, the superposition collapses, and the qubit assumes one of its basis states, either \(|0\rangle\) or \(|1\rangle\), with probabilities determined by \(|\alpha|^{2}\) and \(|\beta|^{2}\). This phenomenon, known as wavefunction collapse, is a fundamental aspect of quantum mechanics and plays a crucial role in quantum computation and information processing [30]. Different physical systems are currently used as qubits. The best known are presented in Figure 3 and briefly described as follows:  
* Superconducting Qubits: Superconducting qubits are one of the most developed and widely used platforms for quantum computing. These qubits are based on superconducting circuits that exhibit quantum behavior. Materials like niobium and aluminum are commonly used in these circuits due to their low resistance at cryogenic temperatures. Superconducting qubits offer fast operation and scalability, but they are sensitive to noise and require ultra-low temperatures for operation, typically in the millikelvin range [31; 32];
* Trapped Ions: Trapped ion qubits are individual ions confined by electromagnetic fields and controlled using laser-based techniques. Commonly used materials include calcium ions (Ca\({}^{+}\)), barium ions (Ba\({}^{+}\)), and ytterbium ions (Yb\({}^{+}\)). The internal states of these ions, such as electronic or hyperfine states, serve as qubits. Trapped ion qubits are known for their long coherence times and high-fidelity gate operations. They are ideal for quantum simulations and algorithms, but the main challenge is scalability, due to the need for precise laser control and individual ion manipulation [33; 34];  
Figure 2: Graphical presentation of (classical) bit and (quantum) qubit. * Defect-Based Qubits: Defect-based qubits are based on point defects in solid-state materials, where the electronic spin states of defects are used to encode information. Examples include the NV center in diamond and VSi or V\({}_{\text{C}}\)-V\({}_{\text{Si}}\) centers in SiC. These defects can operate at room temperature, making them attractive for practical quantum applications. However, challenges remain in defect creation and control, as well as in improving coherence times and gate fidelity [9; 12; 21]. For instance, NV centers in diamond are well-established for quantum sensing and communication applications, while SiC is gaining attention for scalable quantum computing [8; 23];
* Spin-Based Qubits: Spin-based qubits utilize the spin states of electrons or nuclei in materials like silicon and gallium arsenide (GaAs). Quantum dots, such as semiconductor quantum dots made from InAs/GaAs or Si/SiGe systems, are also commonly used. The qubit states are represented by spin-up or spin-down orientations. Spin-based qubits offer long coherence times and are compatible with existing semiconductor technology, making them attractive for scalable quantum computing.