 Lev Vaidman

With Avshalom Elitzur, Lev Vaidman showed how an obstructed arm changes the possible outputs of a Mach-Zehnder interferometer. A click in an output forbidden by destructive interference when both arms are open can reveal the obstruction even though the detected photon was not absorbed there. The proposal makes the distinction between a successful conditional quantum measurement and an entire procedure particularly clear: other trials can absorb the photon or give an inconclusive result.^[ref]