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Expectation Values of Observables in Time-Dependent Quantum Mechanics
Authors:J M Barbaroux  A Joye
Institution:(1) Centre de Physique Théorique (Unité Propre de Recherche 7061), CNRS Luminy, Case 907, F-13288 Marseille, Cedex 9, France, and;(2) PHYMAT, Université de Toulon et du Var, B.P. 132, F-83957 La Garde Cedex, France;(3) Institut Fourier, Université de Grenoble 1, B.P. 74, 38402 Saint-Martin d'Hères Cedex, France
Abstract:Let U(t) be the evolution operator of the Schrödinger equation generated by a Hamiltonian of the form H 0(t) + W(t), where H 0(t) commutes for all twith a complete set of time-independent projectors 
$$\{ P_j \} _{j = 1}^\infty $$
. Consider the observable A=sumj P jlambdajwhere lambda j sime j mgr, mgr>0, for jlarge. Assuming that the ldquomatrix elementsrdquo of W(t) behave as for p>0 large enough, we prove estimates on the expectation value 
$$\langle U(t)\phi|AU(t)\phi\rangle\equiv\langle A\rangle_\phi(t)$$
for large times of the type where delta>0 depends on pand mgr. Typical applications concern the energy expectation langH0rangphiv(t) in case H 0(t) equiv H 0or the expectation of the position operator langx2rangphiv(t) on the lattice where W(t) is the discrete Laplacian or a variant of it and H 0(t) is a time-dependent multiplicative potential.
Keywords:Time-dependent Hamiltonians  Schrö  dinger operator  quantum stability  quantum dynamics
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