There is no duality, it is all waves

[pgkwiat]

"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.

[timo]

I'm happy to provide references of published articles for these claims [about single-photon experiments], but I thought it unlikely that many people would actually need/read them.

If you have some experimental data (not a sketch of the setup, that seems pretty clear) at hand then I'd be interested in taking a look at it.

[bascule]

I've always liked this idea. From the conclusion of A Brief History of Time:

 

Quantum mechanics deals with [uncertainty] via a class of quantum theories in which particles don't have well - defined positions and velocities , but are represented by a wave. These quantum theories are deterministic in the sense that they give laws for the evolution of the wave with time. Thus if one knows the wave at one time , one can calculate it at any other time. The unpredictable , random elements comes in only when we try to interpret the wave in terms of position and velocity of particles. But maybe that is our mistake : maybe there are no particle positions and velocities , only waves. It is just that we try to fit the waves to our preconceived ideas of position and velocities. The resulting mismatch is the cause of the apparent unpredictability.

[pgkwiat]

Hi,

You can see data (and setup) in at least these articles:

"Observation of a Nonclassical Berry's Phase for the Photon", Phys. Rev. Lett. 66, 588 (1991) (something I did a long time ago)

 

V. Jacques, et al. Science 315, 966 (2007)

(a recent experiment that basically demonstrates all the features of the article, but with a single-photon source).

 

Enjoy,

Paul K.

[bombus]

If you have some experimental data (not a sketch of the setup, that seems pretty clear) at hand then I'd be interested in taking a look at it.

 

Try this, it's great!