{c}

A device implementing a <Bell-basis measurement> on two <qubits>. A complete analyzer distinguishes all four <Bell states>. A basic passive <linear optics> analyzer for two polarization <qubits> uses a <balanced beam splitter>, <polarizing beam splitters> and <single-photon detectors>. Its ideal unambiguous success probability is $1/2$: the two opposite-polarization <Bell states> have distinct click patterns, while the two same-polarization <Bell states> remain mutually ambiguous. Additional physical resources can change this restriction. A destructive analyzer absorbs the measured <photons>; it need not preserve the detected <Bell state>.


 Bell state analyzer (source code)