  In an interferometer, path information and interference visibility are
incompatible quantities. Complete determination of the path will exclude any
possibility of interference, rendering the visibility zero. However, if the
composite object and probe state is pure, it is, under certain conditions,
possible to trade the path information for improved (conditioned) visibility.
Such a procedure is called quantum erasure. We have performed such experiments
with polarization entangled photon pairs. Using a partial polarizer we could
vary the degree of entanglement between object and probe. We could also vary
the interferometer splitting ratio and thereby vary the a priori path
predictability. We have tested quantum erasure under a number of different
experimental conditions and found good agreement between experiments and
theory.
