# Tensors and Groups in Quantum¶

A rank-k tensor $$\hat T_k^q$$ is defined as

$\begin{split}\left[\hat J_z, \hat T_k^1 \right] &= q\hbar \hat T_k^q \\ \left[ \hat J_{\pm}, \hat T_k^q \right] & = \sqrt{(k\mp q)(k\pm q + 1)}\hbar \hat T_{k}^{q\pm 1} .\end{split}$

## Wigner-Eckart Theorem¶

Wigner-Eckart theorem is

$\bra{n'j'm'}\hat T_k^q \ket{njm} = \bra{n'j'}\vert \hat T_k \vert \ket{nj} \braket{j'm';kj}{kq;jm},$

where $$j,j'$$ are the angular momentum quantum numbers and $$n, n'$$ are quantum numbers which are not related to angular momentum.

It seems that tensor $$\hat T_k^q$$ is a source of angular momentum. The maximum angular momentum it can provide is $$k$$.

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