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In 1909, Cambridge undergraduate G.I. Taylor published a letter describing his observation of diffraction using light of an extremely low intensity. For years, I've been teaching my students the interpretation of this experiment as demonstrating certain core features of the quantum theory of light. The story was that Taylor did this to prove that a single quantum could be diffracted. I recently learned that this was wrong on both historical and physics grounds.

The actual paper never even mentions quanta. It cites no references, but the only theoretical motivation given is from an obscure theoretical suggestion by Thomson.

Also, it turns out that, for reasons that could not possibly have been anticipated in 1909, the experimental conditions did not really test this aspect of quantum mechanics; the first confirmation of such an effect that is now believed to have been valid was by Grangier et al. in 1986. (Grangier, Roger, and Aspect, "Experimental evidence for a photon anticorrelation effect on a beamsplitter," Europhys. Lett. 1 (1986) 173. Paywalled.)

There apparently was some point in history after 1909 and before 1986 when people did interpret Taylor's experiment as a correct test of quantum mechanics, with a positive result. When was this? Did it play any decisive role in the debate, in which Bohr's followers wanted to quantize the atom but keep the EM field classical?

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It looks like the date when people started (incorrectly) interpreting Taylor as single-photon interference is 1927.

The dominant Bohr school said the EM field wasn't quantized, and nobody really took light quanta seriously until the BKS theory was disproved by Bothe and Geiger in 1925-6. As late as 1925, Slater (the "S" in BKS) wrote up a paper, Phys. Rev. 25, 395, which sounds like the canonical reference on BKS, and which has the first reference to Taylor that I was able to find in a citation index. Although the Slater paper is paywalled, I imagine that he manages to dismiss Taylor's result somehow.

By the 5th Solvay conference in 1927, there was a consensus for photons. (The word was coined in 1926.) 1927 was also the year of an experiment by Dempster and Batho, which was essentially a redo of Taylor under better conditions. The paper is paywalled (90 years later!), but the abstract is available, and they explicitly interpret their result as single-photon interference. They cite Taylor, and this is only the second reference to Taylor that I could find in a citation index.

So it seems that 1927 was when people started to notice Taylor as important and want to interpret it as single-photon interference.

Re the related question of when it was realized that Taylor could not correctly be interpreted as supporting single-photon interference, here's what I dug up. It appears that there was some controversy and confusion about the correct quantum interpretation of the Hanbury-Twiss interferometer. There was a paper on this in 1956, E.M. Purcell, Nature 178, 1449 (1956), in which he apparently correctly analyzed the Bose-Einstein correlations that cause bunching. But it doesn't seem to have been widely understood for another couple of decades, even among specialists, that these same correlations also invalidate the interpretation of Taylor as single-photon interference. There is a 1959 PSSC film that reenacts Taylor with a photomultiplier tube and presents it as single-photon interference. As late as a 1978 review article by Pipkin, even some experts don't seem to have correctly applied this to the historical experiments like Taylor. However, Clauser, Phys. Rev. D 9, 853–860 (1974) seems to have pointed out that bunching invalidated the interpretation. (I'm not 100% certain about all of this, because a lot of the papers are paywalled.)

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  • $\begingroup$ The G.I Taylor experiment is described in the Berkeley Physics Course, Volume 4, Quantum Physics by Eyvind H. Wichmann; it is there used as an example of the indivisible quanta. I remember being stunned by this when I first encountered it, in this text, in 1969. It was many years later that I learned, in a quantum optics class, that it had been reinterpreted. $\endgroup$ – Peter Diehr Jan 2 '18 at 1:27
  • $\begingroup$ @PeterDiehr: Yeah, I first came across it in PSSC Physics, which is a high school book from the same era. The connection with the Berkeley physics course is kind of amusing, because Purcell wrote the E&M volume of that series, and Purcell had apparently correctly analyzed the Bose-Einstein correlations a decade before. $\endgroup$ – Ben Crowell Jan 2 '18 at 16:59
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Googling I came up with

Fields and Particles: An Introduction to Electromagnetic Wave Phenomena and Quantum Physics Francis Bitter, Heinrich Adolf Medicus American Elsevier, 1956 - Science - 688 pages

On page 167 there is a discussion of quanta and a mention of Taylor's paper saying that it wasn't appreciated at the time. I can't see the whole page cleanly since I was using snippets from Goggle Books.

Quoting part...

For example, if we use a photographic plate which we must obviously expose for a long time because of the low light intensity, will we observe interference fringes on it? Such experiments have been performed, the first one(42) as early as 1909 which, however, did not make a great impact on the physicists of those days. The first experiment that was carried out under good conditions was published in 1927(43). Monochromatic light of very low intensity from a helium gas discharge passed through a certain type of interferometer and was registered on a film. It clearly produced interference fringes. Later experiments (44) also confirmed that single quanta ended up predominantly in those regions

(42)G. I. Taylor, Proc. Cambridge Phil. Soc. 15, 114 (1909)

(43) A. J. Dempster and H. F. Batho, Phys. Rev. 30, 644 (1927)

enter image description here

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Searching Google books with "Taylor Dempster Batho" from 1901 through 1950 I came up with

Archives de philosophie - Volumes 17-18 - Page 31, Beauchesne, 1947

En 1909 G. I. Taylor, puis en 1937 Dempster et Batho, utilisant la propriété des plaques photographiques d'accumuler indéfiniment l'action de la lumière, réussirent à photographier- des franges d'interférence avec des sources lumineuses si faibles qu'en tenant compte de l'intensité totale et du temps de pose on est obligé d'admettre, dans la théorie corpusculaire de la lumière, que les photons ont passé séparément, l'un après l'autre,, dans l'interféromètre. En pareil cas la production ...

There were a number of such hits in the mid to late 1950s.

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  • $\begingroup$ The point is I'm not sure who was the absolute first to conflate the notions, but between the mid and late 1950s the misconception seems to have become entrenched. $\endgroup$ – MaxW Dec 26 '17 at 17:32
  • $\begingroup$ Usually on SE it's not a good idea to write multiple answers. It would be more normal to edit your previous answer. $\endgroup$ – Ben Crowell Dec 26 '17 at 17:35

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