Non-Hermitian Optical Four-Wave Mixing in Cold Atoms

We report demonstration of non-Hermitian optical four-wave mixing (FWM) in cold atoms. Particularly we show that anti-Parity-Time (anti-PT) symmetry is inherent in the forward Four-Wave Mixing (FFWM) process without linear gain and loss. Different from the traditional PT/anti-PT experiments that rely on the interplay of gain and loss to exploit non-Hermitian dynamics, in this experiment, we obtain the anti-PT Hamiltonian by making use of phase mismatching and nonlinear photon-atom coupling. By tuning the nonlinear coupling strength, we observe the dynamics of anti-PT phase transition. The work was supported by the Hong Kong Research Grants Council (Project No. 16308118 and No. C6005-17G).