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Broadband quasi-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi mathvariant="script">PT</mml:mi></mml:math>-symmetry sustained by inhomogeneous broadening of the spectral line
It is shown that inhomogeneous broadening of the spectral line of active impurities may sustain simultaneously parity ($\mathcal{P}$) and time ($\mathcal{T})$ symmetries of a medium, in a finite range of field frequencies, which is forbidden by the causality principle in media without broadening. If a spectral width of a propagating …