Repeated non-demolition measurements and a detour into quantum noises

Denis Bernard, Ecole Normale, Paris

Tremendous progresses have been achieved in the last decade in realising and manipulating stable and controllable quantum systems, and these made possible to experimentally study fundamental questions posed in the early days of quantum mechanics. We shall discussed theoretical analysis of recent cavity QED experiments on non-demolition quantum measurements and progressive wave function collapses. While they nicely illustrate postulates of quantum mechanics and the possibility to implement efficient quantum state manipulations, these experiments pose a few questions such as: what does it mean to observe a progressive wave function collapse in real time? how to describe it? what do we learn from them? Their analysis will allow us one hand to link these experiments to basics notions of classical probability or information theory (for instance, the rate of collapse is related to some relevant relative entropy), and on the other hand to touch upon, and illustrate, notions of quantum noises or quantum stochastic processes.