PeterDonis said:
1. It's a topic of debate because, as far as I can tell, you are making claims that go beyond what QM actually says. So one of us must be wrong. ...
2. How do we measure that the photon took a path? Please be specific. Note that just saying, well, we know the source emitted a photon and we detect a photon at detector A, and the source and detector both have well-defined positions, is not sufficient. That's not a measurement of a path.
Also note that phenomena like, for example, being able to point a laser pointer at a specific spot on a screen are not relevant, because a laser pointer does not emit single photons, and we are talking here about experiments involving single photons (or pairs of entangled photons). The point in dispute here is not whether classical light rays have paths, but whether single photons do.
1. I will repeat that this discussion has absolutely nothing to do with the OP. It should therefore be split away, preferably to the Interpretations subforum. At best, this is a gentlemanly semantics discussion; and at worst it is distracting contest of wills that can have no winner.
@PeterDonis: if you aren't willing to provide a hard reference contradicting my reference and commentary below, what does that really say about this discussion anyway? Same question to
@vanhees71... you both are acting as if your words alone are authority when challenged.
------------------------------
2. I have already explained how we know light takes a path, and provided referential support for same. And I have repeatedly specified that ENTANGLED PHOTONS (not a laser pointer) take paths that are determined in advance from a source crystal, where they originate, to a destination detector, where they are destroyed as part of the detection process.
On the other hand:
you refuse to provide any reference for the (absurd) idea that entangled photons lack a path. And yet, check out this paper in which the path length difference is controlled to within a half-wavelength of the detected photons after traveling more than 8 kilometers. That's a ratio of 1:12 billion for the path length for individually detected photons. Hmmm, that's a path!
https://arxiv.org/abs/quant-ph/9806043
This is the entangled photon path, which originates as I described above in Geneva, follows a carefully laid out path and ends an entire city away, at the precise location Tittel et al selected:
"For our Franson-type test of Bell inequalities [16], we produce energy-time entangled photons by parametric downconversion (Fig. 1). Light from a semiconductor laser with an external cavity (10 mW at 655 nm, ∆ν <10MHz) passes through a dispersion prism P to separate out the residual infrared fluorescence light and is focused into a KNbO 3 crystal. The crystal is oriented to ensure degenerate collinear type I phasematching for signal and idler photons at 1310 nm [17]. Behind the crystal, the pump light is separated out by a filter F (RG 1000) while the passing down-converted photons are focused (lens L) into one input port of a standard 3-dB fiber coupler. Therefore half of the pairs are split and exit the source by different output fibers. Using a telecommunications fiber network, the photons are then analysed by all-fiber interferometers located 10.9 km apart from one another in the small villages of Bellevue and Bernex, respectively. The source, located in Geneva, was 4.5 km away from the first analyser and 7.3 km from the second, with connecting fibers of 8.1 and 9.3 km length, resp., as indicated in Fig. 1. Our interferometers use both the Michelson configuration and have a long and a short arm. In order to compensate all birefringence effects in the arms (i.e. to stabilize the polarization), we employ so called Faraday mirrors (FM) to reflect the light [18]. At the input ports, we use optical circulators (C). These devices guide the light from the source to the interferometer, but, thanks to the non-reciprocal nature of the Faraday effect, guide the light reflected back from the interferometer to another fiber, serving as second output port. The output ports of each interferometer are connected to photon counters [19]. We label the ”direct” port as ”+”, the one connected to the circulator ”-”. To control and change the phases ( δ 1, δ 2), the temperature of the interferometers can be varied.
Since the arm length difference is five orders of magnitude larger than the single photon coherence length, there is no single photon interference. However, the path difference in both interferometers is precisely the same, with a sub-wavelengths accuracy. Moreover, this imbalance is two orders of magnitude smaller than the coherence length of the pump laser. Hence, an entangled state can be produced where either both photons pass through the short arms or both use the long arms. Noninterfering possibilities (the photons pass through different arms) can be discarded using a high resolution coincidence technique."
And please take specific note of the fiber path, including the coiled portions, as that is determined by the scientists and the photons are not free to go anywhere else. Other than the PATH it actually takes, of course.
@vanhees71 said: "...in the here discussed specifically quantum properties of entanglement, any reference to paths must lead to confusion". Perhaps you should explain to this "confused" top team that it is not useful to think of these entangled particles as having a path. Because they did exactly the opposite of what you say.
And you asked: "How do we measure that the photon took a path? Please be specific."
So there's my verbatim quoted answer, I don't think it gets any more specific than this solid gold reference. So... what you got that says photons don't take paths? A quoted reference would be appropriate here.
---------------------------
Lastly: I will concede the point, if you can explain the following about entangled photons:
a. I say they have a path, which length can be measured within tolerance (8.1 km +/- 655 nm in the reference).
b. I say they have a frequency/wavelength, which can be measured within tolerance (1310 nm +/- in the reference).
c. I say they have a spin, which can be measured within tolerance (per figure 2 in the reference).
Tell me: what is different about a.path as opposed to b.frequency and c.spin? Photons take paths, have a frequency, and have spin. How is saying any of this misleading in any way?-DrC