Quantitative wave-particle duality and nonerasing quantum erasure

The notion of wave-particle duality may be quantified by the inequality $V^2{+K}^2<~1,$ relating interference fringe visibility V, and path knowledge K. With a single-photon interferometer in which polarization is used to label the paths, we have investigated the relation for various 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.