Two-Qubit Gate Matrices

Just as the columns of a single-qubit gate matrix correspond to the idealized states, the columns of a matrix representing a two-qubit gate, such as the Controlled NOT (CNOT) Gate, correspond to the four idealized states—images/_pragprog/svg-71.png, images/_pragprog/svg-74.png, images/_pragprog/svg-318.png, and images/_pragprog/svg-319.png— for two qubits:

The letters are complex numbers with the usual conjugates .

Each column ...

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