A unitary operation that manipulates qubit states. It is the quantum analog of classical logic gates (AND, OR, NOT). Combinations of gates implement quantum algorithms.
Quantum Gates transform qubits' quantum states via unitary matrices. Common single-qubit gates: Hadamard (creates superposition from |0⟩), Pauli-X (quantum NOT), Pauli-Z (phase flip), rotations. Common two-qubit gates: CNOT and CZ (create entanglement between qubits). Universal gate sets (e.g. Clifford + T, or Hadamard + Toffoli) can implement any quantum algorithm. Real hardware has limited gate sets and imperfect implementations, so 'gate fidelity' is a critical hardware metric.
A 3-qubit GHZ-state circuit applies a Hadamard on qubit 0, then CNOT from 0 to 1, then CNOT from 1 to 2, which creates maximal three-way entanglement.
Quantum gates are the building blocks of quantum programs. Every quantum algorithm is fundamentally a sequence of gates applied to qubits, followed by measurement.
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