Anti-PT symmetry with bound states in the continuum.

Non-Hermitian systems satisfying PT- or anti-PT symmetry have produced many intriguing wave phenomena. Maintaining the Hermitian (non-Hermitian) property and increasing the non-Hermitian (Hermitian) property difference can cause a phase transition from a symmetric phase to a broken phase through the exceptional point (EP). However, it is experimentally difficult to construct a system working at the EP to satisfy both requirements simultaneously. Bound states in the continuum (BICs) are a special lossless wave state despite their coexistence with continuous waves. Deviation from a BIC leads to an intrinsically dissipative quasi-BIC. Here, we propose to attain anti-PT symmetry from a binary quasi-BIC system, where an introduced asymmetry between two quasi-BICs induces an anti-PT phase transition through the EP. Experimentally, we used the thermo-optic effect to accurately control the asymmetry and demonstrated an anti-PT phase transition. We further found that the system can operate at the EP despite fabrication imperfections, evidenced by the slow-light effect with a group index exceeding 40. By harnessing BICs for constructing anti-PT symmetry, our approach opens up a new way for fundamental research and practical applications based on non-Hermitian physics.