3 PROPOSED QUANTUM ERASERS

Interaction with a measuring apparatus can cause otherwise interfering paths of a particle to become distinguishable. Under some conditions, however, it is possible to ''erase'' the distinguishability, thereby restoring interference. The concept of;a quantum eraser is useful fog understanding the role of entanglement in interference. Several attemp ts to investigate the phenomenon have been made using the correlated p hotons from spontaneous parametric down-conversion. However, none of t he experiments previously performed in connection with quantum erasure has provided an optimal demonstration. We propose three improved down -conversion schemes, each of which satisfies all the criteria for a tr ue quantum eraser. As the proposed schemes are all modifications or co mbinations of previously completed experiments, they are deemed to be feasible.