  Non-Hermitian optical systems with parity-time (PT) symmetry have recently
revealed many intriguing prospects that outperform conservative structures. The
prevous works are mostly rooted in complex arrangements with controlled
gain-loss interplay. Here, we demonstrate anti-PT symmetry inherent in
nonlinear optical interactions based upon forward optical four-wave mixing in a
laser-cooled atomic ensemble with negligible linear gain and loss. We observe
the pair of frequency modes undergo a nontrivial anti-PT phase transition
between coherent power oscillation and optical parametric amplification in
presence of a large phase mismatch.
