In most quantum information experiments, that observable is polarization (more precisely, polarization in some particular direction), which is an exact one-bit measurement result.
In radio telescope observations, it's typically amplitude and wavelength that are measured. I don't know that "the information content of a single photon" is really meaningful for such observations since they don't observe single photons, they observe the overall intensity at different wavelengths.
You can't transfer information by any of these means because you can't control the time or the location or the energy of the photon hitting the screen.
The reason polarization is used to carry information in quantum computers is that you can control it, so you can use it to transfer information. But, as I said, a single photon can only be used to read out one bit in this way.
More than a bit. Between its wavelength, its presence (or absence), polarization, and timing, a photon can carry a great deal more than one bit.