  Since the spatially extended periodic parity-time (PT) symmetric potential
can possess certain unique properties compared to a single PT cell (with only a
pair of coupled gain-loss components), various schemes have been proposed to
realize periodic PT-symmetric potentials based on optical lattices. Here, we
experimentally construct a spatially periodic PT-symmetric optical potential
based on gain-loss arrays induced in a coherently-prepared atomic medium. The
gain and loss arrays are generated in alternating four-level N-type and
three-level $\Lambda$-type configurations in the same atomic medium,
respectively, which do not require discrete diffractions as demonstrated in the
previous work [Phys. Rev. Lett. 117, 123601(2016)] and can be easier to realize
with more relaxed operating conditions. The dynamical behaviors of the system
are investigated by measuring the phase difference between two adjacent gain
and loss channels. The demonstrated PT-symmetric optical lattice with easy
accessibility and better tunability sets a new stage for further exploiting the
peculiar physical properties in periodic non-Hermitian systems.

Optics