  The temporal Talbot effect refers to the periodic revivals of a pulse train
propagating in a dispersive medium, and is a temporal analog of the spatial
Talbot effect with group-velocity dispersion in time replacing diffraction in
space. Because of typically large temporal Talbot lengths, this effect has been
observed to date in only single-mode fibers, rather than with freely
propagating fields in bulk dispersive media. Here we demonstrate for the first
time the temporal Talbot effect in free space by employing dispersive
space-time wave packets, whose spatio-temporal structure induces group-velocity
dispersion of controllable magnitude and sign in free space.
