In this chapter we shall come back to the issue of the undulatory nature of matter. Interferometer experiments with a blocked path show that this character cannot be taken in a classical sense. Indeed, quantum probabilities are deeply affected by the presence of features. The possibility of measurements without interaction shows that such probabilities have in quantum mechanics an ontological import. We then state a complementarity between local events and non-local features. Delayed choice experiments sets these aspects in new and enlightening terms.
In this paper we analyze the entropy and entropy production of a nonisolated quantum system described within the quantum Brownian motion framework. This is a very general and paradigmatic framework for describing nonisolated quantum systems and can be used in any kind of coupling regime. We start by considering the application of von Neumann entropy to an arbitrarily damped quantum system making use of its reduced density operator. We argue that this application is formally valid and develop a path-integral method to evaluate that quantity analytically. We apply this technique to a harmonic oscillator in contact with a heat bath and obtain an exact form for its entropy. Then we study the entropy production of this system and enlighten important characteristics of its thermodynamical behavior on the pure quantum realm and also address their transition to the classical limit.