Fig. 2
From: An elementary review on basic principles and developments of qubits for quantum computing
Three different qubit types. Natural qubits employ naturally emerging physical systems such as electrons or atoms. Some systems inherently have two quantum states to be occupied (a intrinsic two-level system) while others have a larger state space and the qubit space is defined as a subset (b two-level subset system). Synthetic qubits utilize engineered quantum systems and their quantum states can be tuned by system parameters. (c engineered two-level system)