= Bell-state measurement
{c}
A <Bell-state measurement> distinguishes the four two-qubit <Bell states>. Ideally its four measurement operators are the rank-one <orthogonal projectors> onto $|\Phi^+\rangle$, $|\Phi^-\rangle$, $|\Psi^+\rangle$ and $|\Psi^-\rangle$. It measures joint parity and relative-phase information, rather than the two qubits separately in one local basis.
A circuit realization applies a <CNOT gate> with the first qubit as control, then a <Hadamard gate> to that qubit, followed by computational-basis measurements of both. These gates map the Bell basis to four distinct computational-basis states. A complete result therefore carries two classical bits and supplies the correction label in ideal <quantum teleportation>. A restricted experimental analyzer may distinguish fewer than all four outcomes.
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