Wigner-Eckart theorem

The Wigner-Eckart theorem is a theorem of representation theory and quantum mechanics. It states that matrix elements of spherical tensor operators on the basis of angular momentum eigenstates can be expressed as the product of two factors, one of which is independent of angular momentum orientation, while the other is just a Clebsch-Gordan coefficient.

The Wigner-Eckart Theorem reads

:$langle\; jm|T^k\_q|j\text{'}m\text{'}\; angle\; =langle\; j||T^k||j\text{'}\; angle\; C^\{jm\}\_\{kqj\text{'}m\text{'}\}$

where $T^k\_q$ is a rank $k$ spherical tensor, $|jm\; angle$ and $|j\text{'}m\text{'}\; angle$ are eigenkets of total angular momentum $J^2$ and its z-component $J\_z$, $langle\; j||T^k||j\text{'}\; angle$ has a value which is independent of $m$ and $q$, and $C^\{jm\}\_\{kqj\text{'}m\text{'}\}=langle\; j\text{'}m\text{'};kq|jm\; angle$ is the Clebsch-Gordan coefficient for adding $j\text{'}$ and $k$ to get $j$.

In effect, the Wigner-Eckart theorem says that operating with a spherical tensor operator of rank k on an angular momentum eigenstate is like adding a state with angular momentum k to the state. The matrix element one finds for the spherical tensor operator is proportional to a Clebsch-Gordan coefficient, which arises when considering adding two angular momenta.

Example

Consider the position expectation value $langle\; njm|x|njm\; angle$. This matrix element is the expectation value of a Cartesian operator in a spherically-symmetric hydrogen-atom-eigenstate basis, which is a nontrivial problem. However, using the Wigner-Eckart theorem simplifies the problem. (In fact, we could get the solution right away using parity, but we'll go a slightly longer way.)

We know that $x$ is one component of $vec\; r$, which is a vector. Vectors are rank-1 tensors, so $x$ is some linear combination of $T^1\_q$ for $q=-1,0,1$. In fact, it can be shown that $x=frac\{T\_\{-1\}^\{1\}-T^1\_1\}\{sqrt\{2$. Therefore:$langle\; njm|x|njm\; angle\; =frac\{1\}\{sqrt\{2langle\; nj||T^1||nj\; angle\; (C^\{jm\}\_\{jm11\}-C^\{jm\}\_\{jm1(-1)\})$which is zero since both of the Clebsch-Gordan coefficients are zero.

References

*J. J. Sakurai, (1994). "Modern Quantum Mechanics", Addison Wesley, ISBN 0-201-53929-2.
*mathworld|urlname=Wigner-EckartTheorem|title= Wigner-Eckart theorem
* [http://electron6.phys.utk.edu/qm2/modules/m4/wigner.htm Wigner-Eckart theorem]
* [http://galileo.phys.virginia.edu/classes/752.mf1i.spring03/TensorOperators.htm Tensor Operators]