Why "diagonal" polarization doesn't decohere?

It is often said, that qubit should be maintained under very low temperature and kept highly isolated not to decohere.

But we can represent $|0rangle$ with vertical polarization of light and $|1rangle$ with horizontal polarization of light. The the superposition $frac{1}{sqrt{2}}|0rangle +frac{1}{sqrt{2}}|1rangle$ will be represented with 45 degree polarization.

And diagonal polarized light will calmly propagate over space without decohering, i.e. without collapsing to $|0rangle$ or $|1rangle$.

Why?