Abstract. In the paper we have built and examined the properties of quantum systems with nonequidistant energy levels from a point of view of a new introduced approach -the diagonal operator ordering technique (DOOT). In this frame, we examine also the properties of mixed states described by a canonical density operator. We particularize the obtained results for some particular cases (the system with Hamiltonian whose eigenfunctions are the generalized Laguerre functions, as well as the Pöschl-Teller like potentials, and the infinite quantum well).Keywords: Coherent states, operator ordering, density operator, energy spectra.
IntroductionIt is well-known that a most popular and also most applicable quantum model is the one-dimensional harmonic oscillator (HO-1D). An important feature of HO-1D is its equidistant energy levels, which ease the various mathematical characterizations of their different physical properties, especially in the case of mixed (thermal) states. On the other hand, among the quantum systems allowing an exact solution of the nonrelativistic stationary Schrödinger equation, a special place is occupied by the systems with nonequidistant energy levels. Generally, the appearance of non-equidistant energy levels is determined by the anharmonic character of the potential. Such systems are, e.g. the infinitely deep square-well potential, the potential whose eigenfunctions are the generalized Laguerre functions, the Pöschl-Teller like potential, the Morse potential and so on (see, [1], and references therein). In this book were examined the coherent states (CSs) of these potentials in the frame of factorization method. Also, in [2] were built the CSs for systems related to the generalized Laguerre functions. These CSs are known in the quantum optical literature as belonging to the nonlinear coherent states (NCSs) which generally is an overcomplete set of vectors in Hilbert space. On the other hand, the NCSs can be regarded as particular cases of more general CSs, namely the so called generalized hypergeometric coherent states (GH-CSs) whose appellation becomes from their normalization function which is given by a generalized hypergeometric function [3], [4]. Let us denote by, and φ π ∈ 0, 2 , the continuous parameter which labels the CSs and run over a complex domain. Moreover, any set of CSs must fulfill some conditions summarized by Klauder (called "the Klauder's prescriptions"): continuity in the complex label z , non-orthogonality, but normalization, unity operator resolution with positive defined integration measure, temporal stability and action identity [5].In different calculations involving the CSs it is necessary to use some rules for the operator ordering. A useful and practical technique applicable to the canonical CSs related with the HO-1D, namely, the integration within an ordered product (IWOP) technique, was elaborated by H. -Y. Fan (see, e.g. [6] and references therein). Generally, for any pair of lowering − L and raising + L operators, which generate the GH-CSs, previously ...