  We propose a polarised intensity interferometry experiment, which measures
the nonlocal Pancharatnam phase acquired by a pair of Hanbury Brown-Twiss
photons. The setup involves two polarised thermal sources illuminating two
polarised detectors. Varying the relative polarisation angle of the detectors
introduces a two photon geometric phase. Local measurements at either detector
do not reveal the effects of the phase, which is an optical analog of the
multiparticle Aharonov-Bohm effect. The geometric phase sheds light on the
three slit experiment and suggests ways of tuning entanglement.
