Quantum entanglement experiments are normally carried out in the regime (hf>kT - where T is the temperature of the instrument) to minimise thermal noise, which means operating in the optical band, or in the lower frequency band (
I would say you should move step by step from photons to biological systems. The next step after photons is massive particles, for example electrons. We got the evidence of spin entanglement between two electrons and it was made for a room temperature. You can check this paper Article Spin-entanglement between two freely propagating electrons: ...
many thanks for your comments and the paper. For me i'm really stuck with photons, and at that, photons in the Rayleigh-Jeans regime. I dare say this may occupy me for sometime. However, since there are so many photons in this regime, i'm considering how i might confirm entanglement with some novel technology in this regime. I'm really after clues where i may find photon entanglement (in nature) in this regime, or create it artificially, so that i may investigate its characteristics.