  The recently-developed notion of 'parity-time (PT) symmetry' in optical
systems with a controlled gain-loss interplay has spawned an intriguing way of
achieving optical behaviors that are presently unattainable with standard
arrangements. In most experimental studies so far, however, the implementations
rely highly on the advances of nanotechnologies and sophisticated fabrication
techniques to synthesize solid-state materials. Here, we report the first
experimental demonstration of optical anti-PT symmetry, a counterpart of
conventional PT symmetry, in a warm atomic-vapor cell. By exploiting rapid
coherence transport via flying atoms, our scheme illustrates essential features
of anti-PT symmetry with an unprecedented precision on phase-transition
threshold, and substantially reduces experimental complexity and cost. This
result represents a significant advance in non-Hermitian optics by bridging a
firm connection with the field of atomic, molecular and optical physics, where
novel phenomena and applications in quantum and nonlinear optics aided by
(anti-)PT symmetry could be anticipated.
