= Mirror return through an ordered polarizer-retarder pair
Take a linear <polarizer> $P=\operatorname{diag}(1,0)$ and a reciprocal <quarter-wave plate> $Q=2^{-1/2}\begin{pmatrix}1&i\\i&1\end{pmatrix}$ in fixed physical transverse axes. A near-normal isotropic mirror has $M=-I_2$ up to global <phase>. If the source/detector side encounters P before Q, the round-trip matrix is $PQM QP=-PQ^2P=0$. Reversing the filter gives $QPM P Q=-QPQ$, which transmits half an initially unpolarized <intensity>. The result is double-pass rejection, not optical nonreciprocity; changing direction without changing the input polarization does not preserve the same physical input state.
Back to article page
