Thermally condensing photons into a coherently split state of light

Techniques to control the quantum state of light play a crucial role in a wide range of fields, from quantum information science to precision measurements. While for electrons in solid state materials complex quantum states can be created by mere cooling, in the field of optics manipulation and control currently builds on non-thermodynamic methods. Using an optical dye-filled microcavity, we have demonstrated that photon wavepackets can be split through thermalization within a potential with two minima subject to tunnel coupling. Even at room temperature, photons condense into a quantum-coherent bifurcated ground state. Fringe signals upon recombination show the relative coherence between the two wells, demonstrating a working interferometer with the non-unitary thermodynamic beamsplitter.

In more recent work, we have started to study the effect of phase fluctuations from grand-canonical condensate properties due to coupling of photons to the photo-excitable dye molecules in the double well system. This effect becomes relevant for a large relative size of the dye reservoir. Current progress will be presented.