Quantum coherence and interaction-free measurements - art. no. 060101

We investigate the extent to which "interaction-free" measurements perturb the state of quantum systems. We show that the absence of energy exchange d uring the measurement is not a sufficient criterion to preserve that state, as the quantum system is subject to measurement-dependent decoherence. Whi le it is possible in general to design interaction-free measurement schemes that do preserve that state, the requirement of quantum coherence preserva tion rapidly leads to a very low efficiency. Our results, which have a simp le interpretation in terms of "which-way" arguments, open up the way to nov el quantum nondemolition techniques.