The CNOT (Controlled-NOT or CX) gate is the most important two-qubit quantum gate. It has a control qubit and a target qubit. If the control is |0⟩, the target is unchanged. If the control is |1⟩, the target qubit is flipped (X gate applied). CNOT maps |00⟩→|00⟩, |01⟩→|01⟩, |10⟩→|11⟩, |11⟩→|10⟩. The CNOT gate is entangling: applying H to the control then CNOT creates the Bell state (|00⟩ + |11⟩)/√2. Together with single-qubit gates, CNOT forms a universal gate set for quantum computation. In superconducting hardware, the native two-qubit gate is often CX or ECR; in trapped ions it is typically the Mølmer–Sørensen (XX) gate.
Related Terms
Entanglement
FundamentalsA quantum correlation between two or more qubits where their states are linked regardless of distance.
Bell State
FundamentalsOne of four maximally entangled two-qubit states — the simplest example of quantum entanglement.
Quantum Gate
GatesA unitary operation that transforms the state of one or more qubits.
Toffoli Gate
GatesThe three-qubit CCX gate — flips the target qubit only when both control qubits are |1⟩.