"No, there is NO experimental evidence supporting light travelling as discrete photons"
Actually, there *is*, though certainly not in the experiment we describe in the Sci. American. In fact, it is correct that most of the results of the photoelectric effect can be explained by a semi-classical theory, which combines a classical light field with a quantum mechanical atom (it fails to get energy conservation right though). However, the definitive experiments use true single-photon sources.
Basically, you send a single photon onto a beam splitter, with detectors in both outputs, and you find that at most one of the detectors fire. Where do you get a true single photon? That, it turns out, is not so easy. There are several methods in use today:
1. drive an atomic transition that emits two photons. Detecting the first, you know you have the second.
2. use the process of "spontaneous downconversion", in which a nonlinear optical crystal is used to "split" a high-energy pump photon into exactly two lower-energy daughter photons. Again, detect one and you know you have the other. (By the way, this is the source that is used in all the cool quantum teleportation experiments, and tests of quantum nonlocality.)
3. excite a single atom (or quantum dot) and let it decay. Since there's only one system, it can emit only one photon.
The bottom line -- these experiments have been done now many times, and conclusively demonstrate interference of single photons (and more fundamentally that radiative electromagnetic energy comes in bundles, which we call "photons").
Hopefully this helped to clarify any confusion on this issue.
Best wishes,
Paul Kwiat
PS I'm happy to provide references of published articles for these claims, but I thought it unlikely that many people would actually need/read them.
