  An entangled photon experiment has been performed with a large variation of
the temperature of the non-linear crystal generating the entangled pair by
spontaneous downconversion. The photon pairs are separated by a nonpolarizing
beamsplitter, and the polarization modes are mixed by half wave plates. The
correlation function of the coincidences is studied as a function of the
temperature. In the presence of a narrow interference filter we observe that
the correlation changes between -1 and +1 about seven times within a
temperature interval of about 30 degrees C. We show that the common simplified
single-mode pair representation of entangled photons is insufficient to
describe the results, but that the biphoton description that includes frequency
and phase details gives close to perfect fit with experimental data for two
different choices of interference filters. We explain the main ideas of the
underlying physics, and give an interpretation of the two-photon amplitude
which provides an intuitive understanding of the effect of changing the
temperature and inserting interference filters.
