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We report an experiment demonstrating the generation of directional thermal radiation with a spectral brightness that is about 9 times greater than that of the ambient pumping reservoir. The experiment is based on the recent proposal for a nontraditional quantum heat engine and uses cold Rb atoms, electromagnetically induced transparency, and photon correlation spectroscopy Phys. Rev. A 94, 053859 (2016)PLRAAN2469-992610.1103/PhysRevA.94.053859.
Zou et al. (Fri,) studied this question.