  A nonclassical feature of the fourth-order interference at a beam splitter,
that genuine photon spin singlets are emitted in predetermined directions even
when incident photons are unpolarized, has been used in a proposal for an
experiment that imposes quantum spin correlation on truly independent photons.
In the experiment, two photons from two such singlets interfere at a beam
splitter, and as a result the other two photons - which nowhere interacted and
whose paths nowhere crossed - exhibit a 100% correlation in polarization, even
when no polarization has been measured in the first two photons. The propsed
experiment permits closure of the remaining loopholes in the Bell theorem proof
and reveals the quantum nonlocality as a property of selection, and pioneers an
experimental procedure for exact preparation of unequal superposition.
