Observation of a Passive PT Phase Transition

In 1998 Bender and Boettcher discovered that the spectrum of a system with PT-symmetric Hamiltonian can still be entirely real. This subject attracts more and more attention during the last few years. One of the intriguing characteristics of PT-symmetric systems is the possibility of a\textit{ phase transition} beyond which the spectrum ceases to be entirely real. This symmetry breaking occurs suddenly once the imaginary component of the potential exceeds a certain critical level. Here we report the observation of a phase transition in a passive PT-symmetric optical structure once the loss exceeds a certain critical value. This counterintuitive loss-enhanced transmission is purely an outcome of a spontaneous PT symmetry breaking.