  Parity-time (PT)-symmetry in the classical regime has been realized in optics
by introducing loss and gain in electromagnetic wave propagation which has
yielded numerous applications like nonreciprocal propagation and finite
threshold single-mode lasers. However, PT-symmetry in the quantum regime so far
remains elusive. Here, we demonstrate a PT-symmetric zero-threshold
polariton-Raman laser by utilizing stimulated resonant Raman scattering of
polarized exciton-polaritons. By pumping resonantly at the exceptional point of
polariton bands with non-Hermitian topology, a quantum PT-symmetric phase is
realized when the Raman-active phonon mode frequencies match with the polariton
mode frequency difference. The PT-symmetric phase corresponding to
zero-threshold lasing can be switched to PT broken phase showing a finite
threshold via cavity detuning by the variation of bath temperature or pump
polarization. Our realization of PT-symmetry in the quantum regime and
consequently the zero-threshold laser can open up applications in quantum
information and stimulate new research activities in cavity quantum
electrodynamics.
