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arXiv:1304.0385

Normal and anti-normal ordered expressions for annihilation and creation operators

Vargas-Martínez, J. M., H. M. Moya-Cessa

Published Apr 1, 2013• 1 citation
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Abstract

We give the normal and anti-normal order expressions of the number operator to the power $k$ by using the commutation relation between the annihilation and creation operators. We use those expressions to give general formulae for functions of the number operator in normal and anti-normal order.

DOI
10.48550/arxiv.1304.0385

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We give the normal and anti-normal order expressions of the number operator to the power kkk by using the commutation relation between the annihilation and creation operators. We use those expressions to give general formulae for functions of the number operator in normal and anti-normal order.
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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.
The expansion of operators as ordered power series in the annihilation and creation operators aaa and ${a}^{\ifmmode\dagger\else\textdagger\fi{}}$ is examined. It is found that normally ordered power series exist and converge quite generally, but that for the case of antinormal ordering the required ccc-number coefficients are infinite for important classes of operators. A parametric ordering convention is introduced according to which normal, symmetric, and antinormal ordering correspond to the values $s=+1,0,\ensuremath{-}1$, respectively, of an order parameter sss. In terms of this convention it is shown that for bounded operators the coefficients are finite when s>0s>0s>0, and the series are convergent when s>12s>\frac{1}{2}s>21​. For each value of the order parameter sss, a correspondence between operators and ccc-number functions is defined. Each correspondence is one-to-one and has the property that the function $f(\ensuremath{\alpha})$ associated with a given operator FFF is the one which results when the operators aaa and ${a}^{\ifmmode\dagger\else\textdagger\fi{}}$ occurring in the ordered power series for FFF are replaced by their complex eigenvalues $\ensuremath{\alpha}$ and ${\ensuremath{\alpha}}^{*}$. The correspondence which is realized for symmetric ordering is the Weyl correspondence. The operators associated by each correspondence with the set of $\ensuremath{\delta}$ functions on the complex plane are discussed in detail. They are shown to furnish, for each ordering, an operator basis for an integral representation for arbitrary operators. The weight functions in these representations are simply the functions that correspond to the operators being expanded. The representation distinguished by antinormal ordering expresses operators as integrals of projection operators upon the coherent states, which is the form taken by the PPP representation for the particular case of the density operator. The properties of the full set of representations are discussed and are shown to vary markedly with the order parameter sss.
Normal-ordered equivalent of the Weyl ordering of q^jp^k\hat{q}^j \hat{p}^kq^​jp^​k
The problem of quantizing a bivariate dynamical system can be reduced to evaluating the ordering of q^jp^k\hat{q}^j \hat{p}^kq^​jp^​k. Here, we consider the Weyl ordering of q^jp^k\hat{q}^j \hat{p}^kq^​jp^​k that is then expressed in term of the annihilation a^\hat{a}a^ and creation a^†\hat{a}^\daggera^† operator. The explicit formula for the normal-ordered equivalent (all a^†\hat{a}^\daggera^†'s preceeding all a^\hat{a}a^'s) of the resulting expression is then given, and some relations are discussed.
A new integral representation is obtained for the star product corresponding to the s -ordering of the creation and annihilation operators. This parametric ordering convention introduced by Cahill and Glauber enables one to vary the type of ordering in a continuous way from normal order to antinormal order. Our derivation of the corresponding integral representation is based on using reproducing formulas for analytic and antianalytic functions. We also discuss a different representation whose kernel is a generalized function and compare the properties of this kernel with those of the kernels of another family of star products which are intermediate between the - and pq -quantization.