  We experimentally study the photonic properties of a cold-atom sample trapped
in a one-dimensional optical lattice under the conditions of
electromagnetically induced transparency. We show that such a medium has two
photonic band gaps. One of them is in the transparency window and gives rise to
a Bragg mirror, which is spectrally very narrow and dynamically tunable. We
discuss the advantages and the limitations of this system. As an illustration
of a possible application we demonstrate a two-port all-optical switch.
