We analyse interaction-free measurements on classical and quantum objects. We show the transition from a classical interaction free measurement to a quantum non-demolition measurement of atom number, and discuss the mechanism of the enforcement of complementarity in atom interferometric interaction-free measurements.
It is now well established that the presence of an opaque classical object can be unambiguously determined by an "interaction-free" measurement (IFM), where the object and the probe never directly interact. For quantum objects, we examine "interaction-free" measurement (the object wavefunction is unchanged) and "interaction-free" preparation (the object wavefunction is changed without physical interaction) and find that in general, neither is possible. We propose using high efficiency IFM's as a quantum information bus to connect disjoint quantum systems.
A short exposition is given of the foundation of the causal description in classical physics and the failure of the principle of causality in coping with atomic phenomena. It is emphasized that the individuality of the quantum processes excludes a separation between a behaviour of the atomic objects and their interaction with the measuring instruments denning the conditions under which the phenomena appear. This circumstance forces us to recognize a novel relationship, conveniently termed complementarity, between empirical evidence obtained under different experimental conditions. An appropriate tool for a complementary mode of description is provided by the quantum-mechanical formalism which allows us to account for regularities of definite or statistical character beyond the grasp of classical physical explanation. – N. B.