  Backflow is a counter-intuitive phenomenon in which a forward propagating
quantum particle propagates locally backwards. The actual counter-propagation
property associated with this delicate interference phenomenon has not been
observed to date in any field of physics. Here, we report the observation of
transverse optical backflow where a beam of light propagating to a specific
transverse direction is measured locally to propagate in the opposite
direction. This observation is relevant to any physical system supporting
coherent waves and might lead to unique applications.
