Nonreciprocity of natural rotatory power.

We have observed that a light beam that passed through an optically active crystal of Bi(12)SiO(20) and that was then ref lected exactly back through it did not recover its initial polarization orientation. The nonreciprocal component of the rotation was of the order of 2 x 10(-3) of the reciprocal, single-pass rotation. This nonreciprocity is unambiguous evidence of broken reversality of the light-matter interaction process.