  Non-reciprocal devices, which allow the non-reciprocal signal routing, serve
as the fundamental elements in photonic and microwave circuits and are crucial
in both classical and quantum information processing. The
radiation-pressure-induced coupling between light and mechanical motion in
traveling wave resonators has been exploited to break the Lorentz reciprocity,
realizing non-reciprocal devices without magnetic materials. Here, we
experimentally demonstrate a reconfigurable nonreciprocal device with
alternative functions of either a circulator or a directional amplifier via the
optomechanically induced coherent photon-phonon conversion or gain. The
demonstrated device exhibits considerable flexibility and offers exciting
opportunities for combining reconfigurability, non-reciprocity and active
properties in single photonic structures, which can also be generalized to
microwave as well as acoustic circuits.
