Avshalom Elitzur

Avshalom Elitzur and Lev Vaidman proposed detecting an absorbing object through a Mach-Zehnder interferometer adjusted so that destructive interference makes one output dark. Blocking one arm permits a photon to reach that previously dark output. In that successful outcome the photon is detected rather than absorbed by the object, yet its detection establishes that the arm was obstructed. For the balanced arrangement, the success probability is $1/4$ per trial; absorption still occurs with probability $1/2$.^[ref]