All optical exceptional-line semimetal in thermal atomic vapor.

Dirac and Weyl semimetals exhibit discrete band degeneracies, whereas nodal line semimetals (NDLS) possess a one-dimensional closed loop degeneracy in momentum space. In non-Hermitian systems, the symmetry-protected nodal line is replaced by an exceptional line. In this study, we have conducted experiments that demonstrate the presence of concentric exceptional lines within an atomic vapor system that possesses anti-PT symmetry. Additionally, we have also shown the topological phase transition on these exceptional lines by defining a Z_2 topological index, originating from the spectral topology of the system exhibiting a positive nonlinear refractive index. These findings position our system as a prime candidate for an all-optical exceptional-line semimetal, which has immense applications in quantum gate preparations and sensing.