Thermodynamics and state preparation in a two-state system of light

The manuscript reports on the observation of a thermalized two-state system of light, with bosonic stimulation here resulting in a ground state preparation efficiency that is distinctly above than what is obtained for the population imbalance in e.g. the field of NMR spin resonance.

Two-state systems in contact to a thermal bath are well investigated for the two-state system being an effective spin-1/2 system, as an electron or a proton in a magnetic field, such that system follows Fermi-Dirac statistics, which prohibits the multiple occupancy of quantum states. A classical example for this are room temperature NMR experiments, where initial state preparation proceeds through contact to the environment, which at feasible magnetic field strengths results in a ~10^-5 state preparation efficiency. Integer spin particles, as photons or many atoms, obey Bose-Einstein statistics, with the possibility for Bose-enhancement, which is responsible for effects as lasing or Bose-Einstein condensation. However, the thermodynamics of a bosonic two-state system has to date not been investigated experimentally, neither with photonic nor with cold atom systems.

Using an optical two-mode microcavity filled with dye solution providing radiatively coupling to the room temperature environment, we experimentally demonstrate a thermalized two-state system of photons. While at low occupation numbers we observe an essentially equal distribution of the populations in the two optical quantum states, as expected from Boltzmann statistics as in the case of room-temperature NMR-experiments, at high occupation numbers, as quantum statistics comes into play despite that the energetic splitting is two orders of magnitude smaller than thermal energy, more than 90% of photons are prepared in the ground state. Our results are in good agreement with theory expectations. Prospects of the observed effects include efficient state preparation in quantum information as well as photonic technology. For example, with the use of birefrigent intracavity materials, efficient thermodynamic polarization control of emitters is expected to become possible when the desired polarization state becomes the energetic ground mode.

Publication: Thermodynamics and state preparation in a two-state system of light. Christian Kurtscheid, Andreas Redmann, Frank Vewinger, Julian Schmitt, and Martin Weitz. Phys. Rev. Lett. – Accepted 30 September, 2025
DOI: https://doi.org/10.1103/kynj-l87s

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