Scheme 1. Schematic Representation of the Operation of a Universal CNOT Quantum Logic Gate Acting on a Pair of Coupled Qubits.

The CNOT switches a target bit if and only if a control bit is in a particular state (here defined as state |0⟩). Each qubit state is typically represented by a point on the surface of a Bloch sphere, describing a quantum superposition of states. The quantum gate operation is depicted, for simplicity, on a target qubit in state |0⟩.