A unitary operation on a fixed number of qubits used as a component of a quantum circuit. Examples include the Hadamard gate and Controlled-Z gate. Measurements are described separately by quantum measurement; a gate is a reversible operation on its full input state.
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A quantum logic gate is a fundamental building block of quantum computing, analogous to classical logic gates in traditional computing. Quantum gates manipulate individual qubits (quantum bits), which are the basic units of quantum information. Unlike classical bits that can exist in a state of either 0 or 1, qubits can exist in superpositions of these states, allowing for a more complex form of computation.
At Section "Quantum computing is just matrix multiplication" we saw that making a quantum circuit actually comes down to designing one big unitary matrix.
Instead, they use quantum logic gates.