  We demonstrate the existence of a non-local geometric phase in the
intensity-intensity correlations of classical incoherent light, that is not
seen in the lower order correlations. This two-photon Pancharatnam phase was
observed and modulated in a Mach-Zehnder interferometer. Using acousto-optic
interaction, independent phase noise was introduced to light in the two arms of
the interferometer to create two independent incoherent classical sources from
laser light. The experiment is the classical optical analogue of the
multi-particle Aharonov-Bohm effect. As the trajectory of light over the
Poincare sphere introduces a phase shift observable only in the
intensity-intensity correlation, it provides a means of deflecting the
two-photon wavefront, while having no effect on single photons.
