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CRC Press eBooks

Quantum Mechanical Operators

Ram Yatan Prasad, Pranita

Published Jan 15, 2021• 0 citations
Publisher

Abstract

In the present chapter, before discussing the mechanical operators, operator has been lucidly defined and examples are cited for its clear understanding. It includes linear and non-linear operator, commutator and commutation, and Hermitian operator and its properties. It also consists of Schmidt orthogonalisation, del and del 2 operators, linear momentum operator and it applications, angular momentum vector and commutation, and angular momentum in spherical polar co-ordinates. This chapter also provides a good discussion regarding ladder operator, Hamiltonian operator and its commutation, and relation of angular momentum operators with Hamiltonian operators. It also discusses projection operator clearly and solution of problem related to projection operator. The properties of projection operator have also been given. Suitable references have also been provided for the consultation. A large number of problems and their solutions relating to different operators have also been cited. Questions on concepts have also been provided for exercise purposes. The readers are advised to solve these problems for better understanding.

DOI
10.1201/9781003133605-4

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In the present chapter, before discussing the mechanical operators, operator has been lucidly defined and examples are cited for its clear understanding. It includes linear and non-linear operator, commutator and commutation, and Hermitian operator and its properties. It also consists of Schmidt orthogonalisation, del and del 2 operators, linear momentum operator and it applications, angular momentum vector and commutation, and angular momentum in spherical polar co-ordinates. This chapter also provides a good discussion regarding ladder operator, Hamiltonian operator and its commutation, and relation of angular momentum operators with Hamiltonian operators. It also discusses projection operator clearly and solution of problem related to projection operator. The properties of projection operator have also been given. Suitable references have also been provided for the consultation. A large number of problems and their solutions relating to different operators have also been cited. Questions on concepts have also been provided for exercise purposes. The readers are advised to solve these problems for better understanding.
Abstract In this chapter we introduce the concept of orbital angular momentum into the mathematical language of quantum mechanics. We discuss the commutation relations of the corresponding operators and their roles as generators of rotations in the context of the theory of Lie groups and Lie algebras, focusing on the three-dimensional rotation group SO(3). We further examine how scalar and vector-valued operators transform under spatial rotations. We then turn to the derivation of the eigenfunctions and eigenvalues of the angular-momentum operators, for which we follow a dual approach in analogy to that employed for the harmonic oscillator in Chapter 5. Using both the algebraic ladder-operator method and an analytical method using spherical polar coordinates we discover the relevant azimuthal and magnetic quantum numbers and their relation to the spherical harmonics and the Legendre polynomials
The angular momentum plays an important role in quantum mechanics. It is no longer an ordinary vector, instead it is a vector operator, whose three components do not commute with each other. In quantum mechanics, the angular momentum becomes a vector operator. A general angular momentum is defined as a vector operator whose components are Hermitian operators satisfying the commutation relations similar to those for the orbital angular momentum. In order to obtain the eigenvalues and eigenfunctions, it is more convenient to express the orbital angular momentum operators in spherical polar coordinates. Spin angular momentum is an intrinsic property of elementary particles. In quantum computing, a qubit or quantum bit is a quantum version of the classical binary bit physically realized in a two-state quantum-mechanical system.
Operators represent physically observable quantities, as discussed in Section 2.5. The structure and property of operators depend on the nature of the degree of freedom; operators act on the state space and in particular on the state vector of a given degree of freedom. The significance of operators in the interpretation of quantum mechanics has been discussed in Baaquie (2013e). The operators discussed in this chapter are mostly based on the continuous degree of freedom, which is analyzed in Section 3.1. Hermitian operators represent physically observable properties of a degree of freedom and their mathematical properties are defined in Section 3.3. The coordinate and momentum operators are the leading exemplar of a pair of noncommuting Hermitian operators and these are studied in some detail in Section 3.4. The Weyl operators yield, as in Section 3.5, a finite-dimensional example of the shift and scaling operators; Section 3.8 provides a unitary representation of the coordinate and momentum operators. The term self-adjoint operator is used for Hermitian operators when there is a need to emphasize the importance of the domain of the Hilbert space on which the operators act – a topic not usually discussed in most books on quantum mechanics. Sections 3.10 and 3.11 discuss the concept of self-adjoint operators, in particular the crucial role played by the domain for realizing the property of self-adjointness. It is shown in Section 3.12 how the requirement of self-adjointness yields a non-trivial extension of Hamiltonians that include singular interactions.
An overview of main operators in quantum mechanics is presented. Operators for position, linear momentum, kinetic energy, potential energy, total energy, and angular momentum are introduced. Properties of linear operators are described. Commutative property of operators has profound implications because a pair of non-commutative operators cannot be measured accurately at the same time. Eigenfunctions and eigenvalues of an operator are defined, and operator form of Schrodinger’s equation is given. Most operators in quantum mechanics are of the Hermitian category because they always have real eigenvalues. The condition to test Hermitian nature of operators is specified. Postulates of quantum mechanics are stated to serve as a convenient starting point.
The orbital angular momentum of a particle in quantum mechanics is defined exactly as in classical mechanics. Before developing any properties of the angular momentum operators, students must know the commutation relations they satisfy with themselves, with the momentum operator, and with the position operator. The angular momentum operator is very closely related to rotations of the system. Students should compare the ability of the angular momentum operator to generate infinitesimal rotations, with the corresponding property of the linear momentum p. The chapter provides the theory of the orbital angular momentum sufficiently to study the problem of a particle in a central potential.
This chapter considers the idea of representing physical as well as nonphysical quantities by operators and collect together some frequently used operator techniques. It postulates that any quantity in quantum physics is specified by a linear operator. In a manipulation with operators, it is very important to maintain the order of the operators because operator algebra is not in general commutative for multiplication. An important concept in quantum physics is a commutator. The chapter focuses on to define a special class of operators called Hermitian operators. They are distinguished by the relationship of  and † The Hermitian operators are of great importance in quantum physics, as they represent physical (measurable) quantities. The chapter defines a very important property of wave functions: orthogonality. This property, for example, allows to identify whether the given wave functions belong to the same operator or not. The orthogonality condition of two functions is analogous to the orthogonality condition of two vectors.