Experimental demonstration of a coherent perfect absorber with PT phase transition.

We report the realization of a coherent perfect absorber, using a pair of passive resonators coupled to a microwave transmission line in the background, which can completely absorb light in its parity-time (PT-)symmetric phase but not in its broken phase. Instead of balancing material gain and loss, we exploit the incident waves in the open system as an effective gain so that ideal PT symmetry can be established by using only passive materials. Such a route will be effective to construct PT-symmetric metamaterials and also tunable PT-symmetric optical elements in general. It also provides a flexible platform for studying exceptional-point physics with both electric and magnetic responses.