This year's lecture focused more specifically on introducing a general and powerful formalism to describe the evolution of a quantum system coupled to an external medium responsible for its decoherence. Quantum information systems are in fact always open, interacting with an environment that plays an essential role in their manipulation and measurement.
This openness to the outside world is general to quantum physics. All physical systems, with the possible exception of the Universe as a whole, are open, and are only considered as isolated within the framework of more or less realistic approximations. Open systems are described using the density operator formalism, which replaces the state vector (or wave function) formalism of elementary quantum mechanics. We have described this formalism by analyzing the properties of the density operator of an open system, established on the basis of general assumptions. We have deduced universal properties of the transformations of the system's density operator. We have thus been able to analyze the behavior of a statistical ensemble of systems as well as that of a single open system. We described different types of possible measurements (projective and generalized measurements) and different methods of continuous observation of a single quantum system. Some of the results obtained were reminders of standard quantum mechanics, but the point of view chosen, that of quantum information, was original for a large part of the audience. It shed new light on the decoherence of mesoscopic superpositions and provided us with useful theoretical tools for the rest of this lecture series.