Non-Hermitian topological phase transition in PT-symmetric mode-locked lasers

The delocalization transition in the Anderson model in the presence of an imaginary gauge field is a landmark example of non-Hermitian topological phase transition. Originally proposed by Hatano and Nelson, its topological nature has been disclosed only very recently. Here it shown that a non-Hermitian phase transition can be observed in the frequency domain of an actively mode-locked laser in a parity-time (PT)-symmetric configuration with incommensurate spacing of cavity axial frequencies.