Thermal light sources are necessary for many applications, especially in the infrared. Their design in the past has relied mainly on classical electromagnetics. We instead propose using quantum optics to describe and design thermal sources. Quantum description allows designing the thermal source as a many-resonator system, unlike the classical approach. Many-resonator system brings in new design tools such as phase, symmetry, and topology to achieve thermal sources that are simultaneously bright, spectrally narrow and directional. Also, such an unconventional description unlocks novel many-body physical phenomena in nanophotonics. In the first demonstration of such a device, we observe passive Parity-Time Symmetry and optical phase transition at an exceptional point in a plasmonic-dielectric two-resonator system. This demonstration is based on the non-Hermitian physics necessary to describe light sources. This work paves way for not only novel thermal light sources, but also for any general light source.

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