  Bell state measurements, in which two quantum bits are projected onto a
maximally entangled state, are an essential component of quantum information
science. We propose and experimentally demonstrate the projection of two
quantum systems with three states (qutrits) onto a generalized maximally
entangled state. Each qutrit is represented by the polarization of a pair of
indistinguishable photons - a biphoton. The projection is a joint measurement
on both biphotons using standard linear optics elements. This demonstration
enables the realization of quantum information protocols with qutrits, such as
teleportation and entanglement swapping.
