Originally posted by sonhouse
Which would be a function of multiple photons, not an individual photon I would assume.
I'm not quite sure what you mean by that, so to be sure I'll cover both bases.
According to the article it's a single photon effect. By brightness they mean something like cross-section. They can produce a directional beam with a single photon. At the one particle level the "Schrodinger" equation is the wave equation one gets from Maxwell's equations - all quantum mechanics adds is the relation between energy and frequency. This changes if we allow for interactions or very high energies - the coupling depends on the energy scale which is something you need a particle accelerator to notice and photons with an energy of more than about 1MeV can become an electron-positron pair - but in this very low energy (~1eV) single particle setting we can safely ignore that. So apart from the amplitude having to give a probability of one of the photon existing the classical field and the photon wave function are the same, provided there is only one photon
. This directionality at the individual photon level causes the detector to be more likely to detect the incoming photon.
Their thing emits one photon at a time. I assume they did a series of trials and measured the number of times they detected the photon coming out in the right direction, and used that as a measure of brightness. So if that was what you meant I think you're right.