If time is substituted in the equation of motion in the Heisenberg picture the results obtained may lead to confusions if the equations are not interpreted correctly. Some examples are given.
We clarify certain confusions in the literature of the density operator in quantum mechanics, and demonstrate that the quantum Liouville theorem has the same form in both the Schrodinger and the Heisenberg pictures.Our starting point is to treat the density operator as an observable which has its specific time dependence in each of the two pictures.It is further shown that such a formulation will provide the exact correspondence between classical and quantum statistical mechanics with the Liouville theorem being interpreted as a conservation law, which is derivable from the equation of motion only in the quantum case.
How is orthodoxy possible? How can an interpretation maintain both the eigenstate–eigenvalue link and indeterminism? Given the former, the properties possessed by a system are completely fixed by its quantum-mechanical state, but the quantum-mechanical state evolves deterministically, as I noted at the end of chapter 1. By themselves, then, the eigenstate–eigenvalue link and the quantum-mechanical equation of motion lead to determinism. Orthodoxy must change one of these things if it wants to maintain indeterminism. Of course, it cannot change the eigenstate–eigenvalue link, lest it no longer be orthdoxy. Hence it changes the equation of motion. In this chapter, I will discuss two ways to change the quantum-mechanical equation of motion: by ‘interupting’ it from time to time with some other (indeterminsitic) equation, or by making a wholesale replacement. The first strategy I discuss in the next section, and the second in the subsequent section. The projection postulate Collapse as an analogue of Lüder's rule Thus far, we have been working in the ‘Schrödinger picture’, according to which states evolve in time (according to the Scrödinger equation) and any given observable is at all times represented by the same operator. The Heisenberg picture reverses things: the states are constant in time and the operators representing observables change.
Open quantum system theory is fundamental for a proper description of quantum systems in different contexts such as chemistry, condensed matter physics, bio-physics and opto-mechamics. Quantum brownian motion which is analytically described by the Hu-Paz-Zhang (HPZ) model represents one of the most important examples of an open quantum system. In this paper we propose a novel derivation of its master equation, based on the Heisenberg picture. We provide an exact and analytic equation both for the operators as well as for the states. The result, equivalent to the one derived originally by HPZ, is expressed in terms of the spectral density, regardless the strength of the coupling between the system and the environment and allows to compute the time evolution of physically relevant quantities in a much easier way, since it is directly expressed in terms of the spectral density. An example is explicitly studied.