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9.1: Spin Operators

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  • Because spin is a type of angular momentum, it is reasonable to suppose that it possesses similar properties to orbital angular momentum. Thus, by analogy with Section [s8.2], we would expect to be able to define three operators—\(S_x\), \(S_y\), and \(S_z\)—that represent the three Cartesian components of spin angular momentum. Moreover, it is plausible that these operators possess analogous commutation relations to the three corresponding orbital angular momentum operators, \(L_x\), \(L_y\), and \(L_z\). [See Equations ([e8.6])–([e8.8]).] In other words,

    \[\begin{aligned} \label{e10.1x} [S_x, S_y]&= {\rm i}\,\hbar\,S_z,\\[0.5ex] [S_y, S_z]&= {\rm i}\,\hbar\,S_x,\\[0.5ex] [S_z,S_x]&= {\rm i}\,\hbar\,S_y.\label{e10.2x}\end{aligned}\] We can represent the magnitude squared of the spin angular momentum vector by the operator \[S^2 = S_x^{\,2} + S_y^{\,2}+ S_z^{\,2}.\] By analogy with the analysis in Section [s8.2], it is easily demonstrated that \[[S^2, S_x] = [S^2, S_y] = [S^2,S_z] = 0.\] We thus conclude (see Section [smeas]) that we can simultaneously measure the magnitude squared of the spin angular momentum vector, together with, at most, one Cartesian component. By convention, we shall always choose to measure the \(z\)-component, \(S_z\).

    By analogy with Equation ([e8.13]), we can define raising and lowering operators for spin angular momentum: \[S_\pm = S_x \pm {\rm i}\,S_y.\] If \(S_x\), \(S_y\), and \(S_z\) are Hermitian operators, as must be the case if they are to represent physical quantities, then \(S_\pm\) are the Hermitian conjugates of one another: that is,

    \[\label{e10.7} (S_\pm)^\dagger = S_\mp.\] Finally, by analogy with Section [s8.2], it is easily demonstrated that \[\begin{aligned} S_+\,S_- &= S^2-S_z^{\,2}+\hbar\,S_z,\label{e10.7a}\\[0.5ex] S_-\,S_+&= S^2-S_z^{\,2}-\hbar\,S_z,\label{e10.8}\\[0.5ex] [S_+,S_z]&= - \hbar\,S_+,\label{e10.9}\\[0.5ex] [S_-,S_z]&= +\hbar\,S_-.\label{e10.10}\end{aligned}\]

    Contributors and Attributions

    • Richard Fitzpatrick (Professor of Physics, The University of Texas at Austin)

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