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What is the CNOT gate?
The CNOT (controlled-NOT) gate is a two-qubit gate that flips the target qubit when the control qubit is 1; it is the basic tool for creating entanglement.
If the control qubit is 0, CNOT does nothing; if it is 1, it applies X (NOT) to the target. If the control is in superposition, both cases are processed together and the result is an entangled state.
A CNOT after an H gate puts two qubits into a Bell state. On real hardware two-qubit gates are noisier than single-qubit gates, so compilers try to reduce the number of CNOTs in a circuit.
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Related concepts
- Quantum entanglement · Entanglement means several qubits share a joint quantum state, so their measurement results are correlated even when the qubits are far apart.
- Bell state · A Bell state is one of four special states in which two qubits are entangled in the simplest and strongest way; the best known gives only 00 or 11 when measured.
- Quantum gate · A quantum gate is a basic operation that reversibly changes the state of one or more qubits; quantum programs are sequences of gates.