Observation of parity–time symmetry in optics

Abstract

A photonic system that shows behaviour similar to that of a violation of parity–time symmetry provides a convenient test bed to explore this and related phenomena. It could also lead to a new class of optical materials with exotic properties that exploit non-reciprocal light flow. One of the fundamental axioms of quantum mechanics is associated with the Hermiticity of physical observables1. In the case of the Hamiltonian operator, this requirement not only implies real eigenenergies but also guarantees probability conservation. Interestingly, a wide class of non-Hermitian Hamiltonians can still show entirely real spectra. Among these are Hamiltonians respecting parity–time (P T) symmetry2,3,4,5,6,7. Even though the Hermiticity of quantum observables was never in doubt, such concepts have motivated discussions on several fronts in physics, including quantum field theories8, non-Hermitian Anderson models9 and open quantum systems10,11, to mention a few. Although the impact of P T symmetry in these fields is still debated, it has been recently realized that optics can provide a fertile ground where P T-related notions can be implemented and experimentally investigated12,13,14,15. In this letter we report the first observation of the behaviour of a P T optical coupled system that judiciously involves a complex index potential. We observe both spontaneous P T symmetry breaking and power oscillations violating left–right symmetry. Our results may pave the way towards a new class of P T-synthetic materials with intriguing and unexpected properties that rely on non-reciprocal light propagation and tailored transverse energy flow.