A unitary operation that manipulates qubit states — 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: Hadamard on qubit 0, then CNOT from 0 to 1, then CNOT from 1 to 2 — 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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