The information rates achievable with a photon-gas model of electromagnetic radiation are studied. At any frequency, information rates over the photon-gas model essentially coincide with the Shannon capacity when the signal-to-noise ratio is below a threshold. Only above the threshold does the photon gas incur in a significant loss in information rates; the loss can amount to half of the capacity. The threshold exceeds $40\mathrm{dB}$ for radio frequencies and vanishes at higher frequencies.

• Received 10 August 2007

DOI:https://doi.org/10.1103/PhysRevA.77.032116