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Canadian Journal of Physics

Gaussian density operator properties for the harmonic oscillator

Saman Alavi, Robert F. Snider

Published Jun 13, 2025• 0 citations
Publisher

Abstract

An operator whose position representation is a Gaussian function is referred to as a Gaussian operator. In common with the harmonic oscillator Hamiltonian operator, the eigenvectors of the Gaussian operator are a product of Hermite polynomial functions and a Gaussian weighting factor. It is demonstrated that the thermal equilibrium density operator of a one-dimensional harmonic oscillator can be expressed in Gaussian operator form, including its eigenvalues. These can be obtained without first solving for the eigenfunctions of the harmonic oscillator Hamiltonian.

DOI
10.1139/cjp-2025-0055

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An operator whose position representation is a Gaussian function is referred to as a Gaussian operator. In common with the harmonic oscillator Hamiltonian operator, the eigenvectors of the Gaussian operator are a product of Hermite polynomial functions and a Gaussian weighting factor. It is demonstrated that the thermal equilibrium density operator of a one-dimensional harmonic oscillator can be expressed in Gaussian operator form, including its eigenvalues. These can be obtained without first solving for the eigenfunctions of the harmonic oscillator Hamiltonian.
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The eigenvalues and eigenvectors of a normalized gaussian operator do not seem to have been previously considered. I solve this problem for 1-dimensional translational systems. I also address the question as to whether a gaussian operator is a density operator. To answer that question, it is first necessary to be sure what conditions must be satisfied, so a short review of density operators is given. Since position and momentum do not commute in quantum mechanics, it is useful to start with the consequences of the noncommutation, which is generally the Schrödinger-Robertson uncertainty relation, which is also briefly reviewed. It is found that the question of whether a gaussian operator is a density operator is directly tied to this uncertainty relation. Since the Wigner function is the phase space representation of a translational density operator, it is natural to consider the gaussian phase space function associated with a gaussian operator and to compare the phase space and operator properties. Throughout such discussions, the independent parameters in these functions are the first and second moments of position and momentum. The application of this formalism to the free translation and spreading of a gaussian packet is given and shows the formal similarity between classical and quantum behavior, whereas the literature standardly only considers the pure state case (equivalent to a single wavefunction).
The one-dimensional harmonic oscillator is one of the few models in quantum mechanics that can be solved analytically, and it is used to exam the theories and methods of quantum mechanics, possessing rich connotations. Using the Fourier transform and the generating function of Hermite polynomials, the transformation from the eigenfunction in the coordinate representation to that in the momentum representation for a one-dimensional harmonic oscillator has been achieved. The equivalence and connection between the two representations for describing a one-dimensional harmonic oscillator are verified. The eigenenergies and eigenfunctions of the one-dimensional harmonic oscillator in the coordinate representation and momentum representation are also obtained by using the algebraic solution method, respectively. The equivalence of the two representations is further discussed. The idea of psolving physical problems in higher dimensional spaceq is emphasized, which deepens students' understanding of quantum mechanics and stimulates their interest in learning.
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This chapter examines the linear harmonic oscillator in the energy (Fock) representation, emphasizing its algebraic structure and spectral properties. It is shown that the energy spectrum is discrete and bounded from below. The energy raising and lowering (ladder) operators are introduced, and the energy eigenvalues are derived. The eigenstates are constructed algebraically from the ground state by repeated application of the raising operator, forming the Fock basis. These energy eigenstates are then transformed into the coordinate representation, where the corresponding wavefunctions are expressed in terms of Hermite polynomials. This establishes a connection between the algebraic and coordinate formulations of the quantum harmonic oscillator. Metadata/Keywords: linear harmonic oscillator, Fock representation, ladder operators, energy spectrum, energy eigenstates, ground state, Hermite polynomials, raising and lowering operators