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Sample Experiments – Particle Nature of Photons

How do we show the particle nature of Photons? The obvious way seems to be, take a single photon and show that it cannot split up:

We use a beam splitter and single photon detectors at each output and check whether we see a detection event at the same time in both outputs.

A spontaneous parametric down conversion (SPDC) source like the quED does not act as a true single photon source but a so-called “heralded” single photon source. It generates pairs of photons. Because of energy and momentum conservation, there are always pairs produced, so when we see one photon in the one arm, we know that there is one photon in the other arm. To show this, we use one arm of the quED as the trigger (heralding the existence of the photon in the other arm) and do single photon experiments with the second arm. Experimentally, we only look at coincidences between these arms:

The setup looks like this:

In this case, we look at two-fold coincidences between the trigger and the detectors after the beam splitter. These are the (heralded) single photon events.

If the photon could split up (or if we did not have heralded single photons), we would see a lot of 3-fold coincidences between the trigger and both detectors after the beam splitter. This is not the case:

We have lots of two-fold coincidences (our heralded single photons) and (in comparison) very little 3-fold coincidences (where they “split up”).

Why are there still some three-fold coincidences? Do you have an explanation? Is our assumption incorrect? Or is it what experimental physicists call “experimental imperfections”?

Do you know how to characterize the “quality” of a heralded single photon source? How many 2- or 3-fold coincidences would we expect from “classical” light? Is there a way to test that experimentally?

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