English: Illustration of photon entanglement.

Two photons are emitted by a source S and are propagated in two opposite directions. At a certain instant, one of them can be found in the region A, to the right of the source and the other in the region B symmetric to A with respect to S (figure G). The photon is called at the right 2 and assume that it possesses a vertical polarization. This can be indicated as the vector state Failed to parse (syntax error): {\displaystyle \left{{!}}2,V\right\rang } . Analogously, suppose that the photon on the left, indicated as 1, has a horizontal polarization, so that it is described by the vector state Failed to parse (syntax error): {\displaystyle \left{{!}}1,H\right\rang } . The entire system is described as the state

Failed to parse (syntax error): {\displaystyle \left{{!}}\Psi\right\rang = \left{{!}}2,V\right\rang \left{{!}}1,H\right\rang }

which corresponds to the single quantum state which asserts that "(one photon is in A with vertical polarization) and (one photon is in B with horizontal polarization)".