  We propose and experimentally demonstrate a method to directly measure the
phase of biphoton states using an entangled mode as a collinear reference. The
technique is demonstrated with entangled photonic spatial modes in the
Laguerre-Gaussian basis, and it is applicable to any pure quantum system
containing an exploitable reference state in its entanglement spectrum. As one
particularly useful application, we use the new methodology to directly measure
the geometric phase accumulation of entangled photons.
