Faculty of Physics, University of Warsaw
The quantum back-flow effect states that for a free particle described by a wave function centered in x < 0 consisting entirely of positive momenta, the probability of remaining in x < 0 may nevertheless increase with time. Eliezer et al  reported the observation of an analog effect of quantum back-flow, namely transverse optical back-flow where a beam of light propagating to a specific transverse direction is measured locally to propagate in the opposite direction, by isolating these local regions using a slit. Here, we report the observation of optical back-flow in the sense of Berry , in the superposition of two wide Gaussian beams with unequal intensity. In the plane formed by the transverse direction and the direction of propagation, the x-component of the resultant local wave-vector in the dark fringe, counterintuitively, exceeds the wave-vectors of the two interfering beams. We find this observation simpler to perform and more intuitive for understanding the effect of optical back-flow, than what was previously suggested.
 Y. Eliezer et al, Observation of optical backflow, Optica Vol. 7, No. 1 (2020) (pg 72)
 M.V. Berry, Quantum backflow, negative kinetic energy, and optical retro-propagation, J. Phys. A: Math. Theor. 43 (2010) 415302 (15pp)