I'm reading some quantum optics and I stumbled on the following completeness relation for the polarization vectors of the electromagnetic field.(adsbygoogle = window.adsbygoogle || []).push({});

[tex]\sum_{a} (\epsilon_{\vec{k} a } )_i ( \epsilon^*_{ \vec{k} a} )_j = \delta_{ij} -\frac{k_i k_j}{\vec{k}^2}[/tex]

Does anyone know how to derive this relation?

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# Completeness relation for polarization vectors

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