Abstract. This paper is concerned with the Wick symbol of time evolving quantum observables. The time dynamics is following either the Heisenberg equation relative to the Schrödinger Hamiltonian, or the time-dependent Hartree-Fock equation. Under very weak assumptions, we prove that the Wick symbol approximatively follows the classical mechanics laws when the semiclassical parameter h tends to zero. For the Heisenberg equation, this is a form of what is commonly called the Ehrenfest theorem. These statements have to be understood in a weaker sense than usual and in return, we do not assume that the Weyl symbol of the initial observable belongs to a class allowing the use of the Egorov theorem.
IntroductionThe aim of this paper is twofold: to prove, under minimal hypotheses, a version of Ehrenfest theorem, and to give a similar result for the time-dependent HartreeFock equation (TDHF). We first consider a quantum observable (bounded operator) A h (t), (t ∈ IR, h > 0), evolving according to the Heisenberg equation associated to the Schrödinger Hamiltonian (see (1.2)). Generally speaking, the Ehrenfest theorem suggests that the average of this observable A h (t), taken on an h-dependent coherent state, approximatively follows, for small h > 0, the classical mechanics laws [7]. The average of an observable on coherent states is related to its Wick symbol. The first goal of this work is to obtain minimal hypotheses on the initial data A h (0), in order that the idea conveyed by Ehrenfest is asymptotically verified when h tends to 0.A precise statement of Ehrenfest theorem, using Egorov theorem for the proof, is given in [4,13] (see also the bibliography therein). It is also noted ([4] and references therein) that the approximation by the classical mechanics laws is not valid uniformly for all time, but up to a maximal time called the Ehrenfest time. These results are obtained assuming that the family of observables (A h (0)) is a semiclassical pseudodifferential (PDO) operator (see [13] or [17]). Our first goal is therefore to give another precise statement of Ehrenfest theorem, weaker than the one in [13], but in assuming weaker hypotheses on A h (0) (Theorem 1.1). In particular, we do not assume that A h (0) is a PDO operator. It is known that an operator in L 2 (IR n ) is a PDO one in the standard class of Calderón and Vaillancourt, when all its iterated commutators with position and momentum operators are bounded in L 2 (IR n ) (Beals characterization theorem [2]). In our work, the main assumption is that the single commutators of A h (0) with position and momentum operators are bounded, without any hypotheses on the iterated commutators. Then we prove that, in some sense, the