  Almost twenty years ago the light was slowed down to less than $10^{-7}$ of
its vacuum speed in a cloud of ultracold atoms of sodium. Upon a sudden
turn-off of the coupling laser, a slow light pulse can be imprinted on cold
atoms such that it can be read out and converted into photon again. In this
process, the light is stopped by absorbing it and storing its shape within the
atomic ensemble. Alternatively, the light can be stopped at the band edge in
photonic-crystal waveguides, where the group speed vanishes. Here we extend the
phenomenon of stopped light to the new field of parity-time (PT) symmetric
systems. We show that zero group speed in PT symmetric optical waveguides can
be achieved if the system is prepared at an exceptional point, where two
optical modes coalesce. This effect can be tuned for optical pulses in a wide
range of frequencies and bandwidths, as we demonstrate in a system of coupled
waveguides with gain and loss.
