= Two-qubit gate
{title2=$U\in U(4)$}
A two-qubit gate is a <unitary operator> on two <qubit> lines, extended by the identity on the others. It may be entangling, as a <CNOT gate> is, or a product of local operations. In a <Hamiltonian simulation> model allowing arbitrary two-qubit gates, the exponential of a Hermitian term supported on two lines counts as one such gate. Compiling it into a fixed discrete <universal quantum gate set> introduces a separate approximation cost.
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