Interaction-free measurement and forward scattering

Interaction-free measurement is discussed on a three-state basis, including the unitary, nondetected phase of absorption. The interaction-free output can be regarded as part of the forward-scattered wave accompanying absorption. For a perfect absorber the forward-scattered wave is locked both in amplitude and in phase. For an imperfect one it has a nontrivial phase of dynamical origin, measurable by interferometry.