
Properties of Qubits 73
some better theory, which had in it variables of the system that were hidden
to our view so far, must be correct.
As for the construction of local, hidden variable theories, the death knell
was sounded when John Bell [5] showed that any such theory must obey certain
inequalities that are NOT obeyed by certain quantum entangled states! We’ll
take a brief look into these in what follows.
4.4.3 Bell’s inequalities and non-locality
Bell’s original work, and many subsequent variants show how quantum
correlations in an entangled state are essentially different from classical ones.
One of the inequalities of Bell applies to a physical ...