The entropy of single photons is some factor of order unity times the Boltzmann constant. The quantity has been discussed in many theoretical papers. Some are mentioned below.

The photon is assumed to have a known (not fully specified) energy, and no known polarization.

Have measurements of the single-photon entropy ever been performed or published? Google Scholar yields nothing on the issue. The question is thus: has the entropy of a single photon taken out from a photon field ever been measured?

Or is there some other experimental confirmation of single-photon entropy?

EDIT 1 and 2 combined: Here are some theoretical references.

Li, Li and Yang, 2022 [https://www.mdpi.com/1099-4300/24/11/1609#FD2-entropy-24-01609][1] [1]: https://www.mdpi.com/1099-4300/24/11/1609#FD2-entropy-24-01609

van Enk, S.; Nienhuis, G. Entropy production and kinetic effects of light. Phys. Rev. A 1992, 46, 1438–1448.

Kirwan, A.D. Intrinsic photon entropy? The darkside of light. Int. J. Eng. Sci. 2004, 42, 725–734.

Zimmermann, H. Particle Entropies and Entropy Quanta II. The Photon Gas. Zeitschrift für Physikalische Chemie 2000, 214, 347.