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Journal of Mathematical Physics

Expansion Theorem for Functions of Operators

Leon W. Cohen

Published Feb 1, 1966• 13 citations
Publisher

Abstract

A method is given for expanding operator functions of q and p, where p = ℏ/i(∂/∂q), such that all q factors are to the left of the p factors. The method is applicable to the rearrangement of creation and annihilation operators.
DOI
10.1063/1.1704925

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Chapter 3

Perturbation Theory

In this chapter we derive the perturbation series for the partition function. Since it is given by a trace of an exponential of an operator H0 +λV , we first expand the exponential with respect to λ. The result is a power series with operator valued coefficients. Then we compute the trace of the n’th order coefficient. This can be done with Wick’s theorem which lies at the bottom of any diagrammatic expansion. It states that the expectation value, with respect to exp[−βH0], of a product of 2n annihilation and creation operators is given by a sum of terms where each term is a product of expectation values of only two annihilation and creation operators. Depending on whether the model is fermionic or bosonic or complex or scalar, this sum is given by a determinant, a pfaffian, a permanent or simply the sum over all pairings which is the bosonic analog of a pfaffian. For the many-electron system, one obtains an n × n determinant. The final result is summarized in Theorem 3.2.4 below.

Chapman & Hall/CRC eBooks•Aug 30, 2004•0 citations
In quantum mechanics, bosonic operators are mathematical objects that are used to represent the creation (a†a^\daggera†) and annihilation (aaa) of bosonic particles. The natural power of a linear combination of bosonic operators represents an operator (a†x+ay)n(a^\dagger x+ay)^n(a†x+ay)n with nnn as the exponent and x, yx,\,yx,y are the variables free from bosonic operators. The normal ordering of these operators is a mathematical technique that arranges the operators so that all the creation operators are to the left of the annihilation operators, reducing the number of terms in the expression. In this paper, we present a general expansion of the natural power of a linear combination of bosonic operators in normal order. We show that the expansion can be expressed in terms of binomial coefficients and the product of the normal-ordered operators using the direct method and than prove it using the fundamental principle of mathematical induction. We also derive a formula for the coefficients of the expansion in terms of the number of bosons and the commutation relation between the creation and annihilation operators. Our results have important applications in the study of many-body systems in quantum mechanics, such as in the calculation of correlation functions and the evaluation of the partition function. The general expansion presented in this paper provides a powerful tool for analyzing and understanding the behavior of bosonic systems, and can be applied to a wide range of physical problems.
Integrable quantum models (IQMs) are a class of interacting quantum field theories in 1+1 dimensions in which the Hilbert space can be described as a Fock space with generalized statistics. Because of this Fock space description, any observables can be expanded in the creation- and annihilation operators with the coefficients of the expansions being the connected matrix elements. One example of such an observable in IQMs without bound states is the so-called wedge-local field (a free field analog with the interacting creation- and annihilation operators) which is wedge-local in the sense that it strongly commutes with its reflection when smeared with a function localized in the left wedge. In this thesis, we study wedge-local observables in integrable quantum models and the effect of bound states on the properties of the connected matrix elements of these observables. This allows us to shine light on the connection between the localization of an observable and the functional analytic properties of its connected matrix elements in models with critical phenomena, in this case models which exhibit bound states. In scalar integrable quantum models with bound states (e.g. the Bullough-Dodd model), we proof that the connected matrix elements of quadratic forms which satisfy a wedge-locality condition are boundary values of meromorphic functions, so-called form factors, which satisfy a number of properties. Conversely, we show that any expansion in such form factors defines a quadratic form which satisfies the wedge-locality condition. In contrast to models without bound states, such an expansion does not terminate due to a recursion relation between the form factors and the wedge-local field, a wedge-local operator which creates one-particle states from the vacuum, does not fall into the class of the characterization theorem. Further, we conjecture a candidate for the wedge-local field in the Z(4)-Ising model. This model is interesting because it has a ”higher-order” bound state structure and no established wedge-local field is known in such models. However, the conjectured candidate comes with some difficulties. Most notably, it is ill-defined as an unbounded operator.
In this chapter we derive the perturbation series for the partition function. Since it is given by a trace of an exponential of an operator H0 +λV , we first expand the exponential with respect to λ. The result is a power series with operator valued coefficients. Then we compute the trace of the n’th order coefficient. This can be done with Wick’s theorem which lies at the bottom of any diagrammatic expansion. It states that the expectation value, with respect to exp[−βH0], of a product of 2n annihilation and creation operators is given by a sum of terms where each term is a product of expectation values of only two annihilation and creation operators. Depending on whether the model is fermionic or bosonic or complex or scalar, this sum is given by a determinant, a pfaffian, a permanent or simply the sum over all pairings which is the bosonic analog of a pfaffian. For the many-electron system, one obtains an n × n determinant. The final result is summarized in Theorem 3.2.4 below.