Light traveling through a moving medium is dragged along with it. This fascinating phenomenon, known as Fresnel drag, has been a subject of study for almost two centuries. The drag effect is very weak and extremely hard to measure, nonetheless it can be greatly enhanced in a highly dispersive medium supporting slow light - i.e., light that propagates with an extremely slow group velocity. For instance, in a transverse drag configuration, a light pulse passing through a medium moving with velocity v
experiences a lateral displacement approximately given by Δx
, where τ is the time spent by the pulse inside the medium, which in turn is inversely proportional to the group velocity. In this article, the authors put this intuitive expression to the test, employing slow light due to electromagnetically induced transparency in a hot atomic vapor. Remarkably, it is reported that the drag effect induced to light beams with small cross-section can significantly deviate from the intuitive predictions as a result of atomic diffusion over the beam's volume. Employing a model for the drag effect accounting for the Maxwell-Boltzmann distribution of atomic velocities, the authors validate their findings, showing excellent quantitative agreement with experiments. This work has both fundamental and practical importance, for understanding the physics of slow light in complex atomic media and for potential applications in the areas of inertial sensing and navigation.
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