  It is shown that the free-carrier (Drude) absorption of circularly polarized
radiation in quantum well structures leads to an electric current flow. The
photocurrent reverses its direction upon switching the light helicity. A pure
orbital mechanism of such a circular photogalvanic effect is proposed that is
based on interference of different pathways contributing to the light
absorption. Calculation shows that the magnitude of the helicity dependent
photocurrent in $n$-doped quantum well structures corresponds to recent
experimental observations.
