Interaction-free measurements of quantum objects

We show that it is possible in principle to realize a unit-efficiency initial state preserving interaction-free measurements (IFM) of quantum objects characterized by their superpositions. Some generalized work is also discussed. c 2001 Optical Society of America OCIS codes: (000.1600) Classical and quantum physics; (270.0270) Quantum optics The concept of Interaction free measurement (IFM) was proposed in 1993 to illustrate the peculiar nonlocal feature of quantum mechanics [1]. It was shown that it is possible to infer the existence of an absorbing object without apparent interaction with the probe photon (hence the name “interaction-free”). As shown in Fig. 1, the experimental setup is a Mach-Zehnder interferometer. The amplitude transmission and reflection coefficients of the two beam splitters are chosen such that an photon incident from the left lower port exists from the right upper port with certainty. However, when an absorbing object (in the original proposal an ultra sensitive optical “bomb”) is in one of the two (upper or lower) optical paths, it blocks any photon following that path and changes completely the interference of the photon wave function. In this case, one finds that there is a finite probability that the photon exits from the right lower port, if the photon is not absorbed by the object. This reveals the presence of the object without it absorbing the probe photon.