Absorption-free discrimination between semitransparent objects - art. no. 032105

Absorption-free (also known as "interaction-free") measurement aims to dete ct the presence of an opaque object using a test particle without that part icle being absorbed by the object. Here we consider semitransparent objects that have an amplitude alpha of transmitting a particle while leaving the state of the object unchanged and an amplitude beta of absorbing the partic le. The task is to devise a protocol that can decide which of two known tra nsmission amplitudes is present while ensuring that no particle is absorbed by the object. We show that the probabilities of being able to achieve thi s are limited by an inequality. This inequality implies that absorption-fre e distinction between complete transparency and any partial transparency is always possible with probabilities approaching 1, but that two partial tra nsparencies can only be distinguished with probabilities less than 1.