This experiment demonstrates that individual photons interfere with themselves when they traverse an interferometer. We simultaneously measure both the interference and the second-order coherence g(2)(0). Since we find g(2)(0)<1, this simultaneously demonstrates both particle and wavelike behavior of light.
This is a copy of the slides for an invited talk at the January 2004 meeting of the American Association of Physics Teachers.
The top figure (red) shows raw counts coming out of the interferometer--this demonstrates interference of the downconverted beam.
The middle figure (blue) shows coincidence counts between the interferometer beam and a gate beam--this shows true single photon interference. The visibility of this fringe pattern is 89%.
The bottom figure (purple) shows the measured values of g(2)(0). Since they are all less than 1, this shows that single photons are in the interferometer.
Same as above, but longer scans. The raw downconversion has an extremely broad bandwidth (we collect about 60 nm worth), hence the short coherence length in the top figure. The gate beam has a 10nm bandwidth filter in it, and because the frequencies of the two beams are entangled this effectively decreases the bandwidth (increases the coherence length) of the detected single photons (bottom).
Theory says that the frequencies are entangled, and while the experiment is consistent with this, the experiment does not prove that the frequencies are entangled. The experiment proves only that the frequencies are correlated.
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