  Optical activity, the power of a medium to rotate the polarization of a light
beam, has contributed significantly to molecular structure assessments in
stereochemistry, biomolecular science and crystallography. Thus far, it is
commonly believed that optical activity is manifested only in the chiral media
which can give rise to circular birefringence of light. Here, we experimentally
demonstrate that free space can also support the implementation of
non-reciprocal optical activity with Bessel beams by spin-orbital interaction.
Specifically, non-diffractive optically active beams are realized, with their
optical rotatory power readily controlled by simple optical elements. We show
that such free-space optical activity can be exploited to form non-reciprocal
optical components such as polarization rotators, isolators, and circulators.
Our results may bring about new possibilities of media-independent optical
activity to other transverse waves ranging from radio to optical frequencies.
