Bell state

Etymology
Named after (1928–1990), Irish physicist.

Noun

 * 1)  One of a set of four entangled states, namely $$|\Phi^+\rangle = {1\over \sqrt{2}} ( |00\rangle + |11\rangle ) $$, $$ |\Phi^-\rangle = {1\over \sqrt{2}} (|00\rangle - |11\rangle )$$,    $$|\Psi^+\rangle = {1\over \sqrt{2}} (|01\rangle + |10\rangle )$$, and $$|\Psi^-\rangle = {1 \over \sqrt{2}} (|01\rangle - |10\rangle )$$, where $$ |xy\rangle := |x\rangle \otimes |y\rangle $$; these are the simplest examples of entangled states.