Quantitative wave-particle duality and nonerasing quantum erasure

The notion of wave-particle duality may be quantified by the inequality V-2 + K-2 less than or equal to 1, relating interference fringe visibility V, and path knowledge K. With a single-photon interferometer in which-polariza tion is used to label the paths, we have investigated the relation for vari ous situations, including pure, mixed, and partially mixed input states. A quantum-eraser scheme has been realized that recovers interference fringes even when no which way information is available to erase. [S1050-2947(99)02 911-X].