Gates & Circuits
CNOT Gate
A two-qubit gate that flips a target qubit only if a control qubit is in state |1⟩, the primary way entanglement is created.
Definition
The CNOT (Controlled-NOT) gate is a fundamental two-qubit gate: it flips the state of a 'target' qubit if and only if a 'control' qubit is in state |1⟩, leaving it unchanged if the control is |0⟩. Applied to qubits in superposition, it's the standard method for generating entanglement.
Technical Definition
Combined with the Hadamard gate, CNOT forms a universal set of operations sufficient (along with arbitrary single-qubit rotations) to construct any quantum circuit — making it one of the most important two-qubit gates in quantum computing theory and practice.
Visual Explanation: An Analogy
Think of the CNOT gate like a light switch (the target) wired to only respond if a separate sensor (the control) detects 'on' — when the control qubit is in superposition, the resulting entanglement links the two qubits' fates together.
Real-World Use Cases
- The standard method for creating Bell states and other entangled states
- Used throughout virtually every quantum algorithm that requires entanglement
- A key benchmark gate for measuring two-qubit gate fidelity across hardware platforms