Duda Jarek

Single photon sources are used for decades ( https://en.wikipedia.org/wiki/Single-photon_source ), or just imagine single deexciting atom.

You need e.g. interference in Mach-Zehnder for single photon. Here is the only article with such claim I have seen: https://arxiv.org/pdf/1709.10344 But personally I believe we need also the wave part of wave-particles duality - to get also other QM-like phenomena e.g. orbit quantization, Casimir effect - see their gathered hydrodynamical analogs: https://www.dropbox.com/s/kxvvhj0cnl1iqxr/Couder.pdf

The "observing average trajectory" experiment is only against "photon is huge because of going all trajectories in interference" type of claims - no, only its wave part of duality, "pilot wave" travels all the trajectories. The particle part of duality, the corpuscle, seems to travel single trajectory - especially for charged indivisible electron, but it seems also true for (tough to split) photon - the question is about size of this part. Attosecond chronoscopy seems very useful to test such models of photons as wavepacket also of EM field. But generally I still miss concrete experimental arguments to estimate this size.

Oh, so you refer not to attosecond chronoscopy, but to "Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer" https://science.sciencemag.org/content/332/6034/1170.full - it is for single photons. On the left you have source of single photons from one of two slits. In the center you have calcite (birefringent crystal) which encodes which-way information in photon polarization. On the right you have measurement of both position and polarization. This is weak measurement: weak enough not to destroy interference, but allowing to obtain averaged trajectories from statistics. They write these trajectories agree with dBB:

They mostly use ultrashort pulse laser or electron wavepackets, plus techniques like frequency comb to get these amazing time resolutions. I don't think we can assume that they use single e.g. photon source (?), however, they use single photon atomic transitions - to be able to assume that only one was absorbed.

from https://en.wikipedia.org/wiki/Photon - [62] is Saleh, B.E.A. & Teich, M.C. (2007). Fundamentals of Photonics. Wiley. ISBN 978-0-471-35832-9. Could anybody elaborate on this not being a short pulse of electromagnetic radiation? So what happens during these observed delays e.g. of photoemission? If it is about interference like Mach-Zehnder, there is both EM wave governed by Maxwell equations, and quantum amplitude governed e.g. by Schrodinger - "pilot" wave for psi=sqrt(rho) exp(-iS/hbar) Madelung substitution( https://en.wikipedia.org/wiki/Pilot_wave_theory#Mathematical_formulation_for_a_single_particle ). For let say Mach-Zehnder interference, there is no doubts that "pilot" wave of quantum amplitudes propagates through both trajectories. However, the question is about corpuscular part of wave-particle duality, does it also propagate through both trajectories, or maybe only through one like in this diagram (from http://redshift.vif.com/JournalFiles/V16NO2PDF/V16N2CRO.pdf ) : In case of interference of electron, it is elementary charge - cannot split in two, in experiments it is always nearly point-like charge ... also, scenarios with electron going through lower or upper arm differ by electric field around such setting influencing surrounding atoms - they are very different. For single photons, there are e.g. these experiments measuring averaged trajectories of interfering photons: https://science.sciencemag.org/content/332/6034/1170.full There are also experiments being able to use both wave and particle part of duality simultaneously, e.g. https://en.wikipedia.org/wiki/Afshar_experiment So while quantum amplitude of single photon: the wave part of wave-particle duality seems to form e.g. plane waves, the corpuscular part seems to travel through a concrete trajectory (e.g. from the screen to our eyes).

While there are still needed theoretical models e.g. of single photon, this ~decade old attosecond chronoscopy brings hope to finally verify them experimentally - gathered: https://scholar.google.pl/scholar?cites=15193546925951882986&as_sdt=2005&sciodt=0,5&hl=en E.g. 2020 "Probing molecular environment through photoemission delays" https://www.nature.com/articles/s41567-020-0887-8

So what happens during such 21as of production of EM field of single optical photon? ( https://science.sciencemag.org/content/328/5986/1658 )

Physics is much more than clicks of detectors - we have also lasers e.g. with attosecond-scale pulses, resonators ... ... or delays also in atomic processes, like observed ~21 attosecod delay in photoemission ( https://science.sciencemag.org/content/328/5986/1658 ) - EM radiation, being mainly response to electron dynamics, propagates ~6nm during this time. So what exactly happens during such 21as of production of EM field of single optical photon? This is "what happens in the center of star" type of question - we might be never able to directly measure its details, but should have self-consistent models of what actually happens there - based on general knowledge e.g. of electromagnetism, verified by indirect consequences.

This gif is from https://en.wikipedia.org/wiki/Wave_packet For optical photon it indeed should have some connection with (e.g. expected value of) EM field - what exactly? The big size question is "how many periods are inside" - the mentioned 3 papers suggest it is approximately one period in propagation direction (and some fraction in perpendicular directions). Also, is the envelope really Gaussian as mathematically convenient, or maybe has some different e.g. tail behavior, or even compact support? These are absolutely fundamental questions of physics ... we know nearly nothing concrete about - due to only blindly repeating "it is quantum" as an excuse for searching for answers.

If you prefer, can be "width" in momentum space - as they are related with Fourier transform/Heisenberg uncertainty. Can we tell anything concrete about any of them - e.g. for optical photon produced by deexcitation of single atom? Also, how to relate such probability distribution with EM field, e.g. expected value of its energy density?

You write "Different experiments give different results", so I ask for elaboration, but instead of giving any example you write further "You should be able to evaluate some basic QM experiments." - which ones? Like creationists claiming "many examples" which cannot be explained by evolution, asking for example they give e.g. eye ... https://en.wikipedia.org/wiki/Evolution_of_the_eye Size of photon makes sense also in QM: e.g. as width of wavepacket, as uncertainty of quantum position operator, through adding terms in WKB approximation etc. Please be more specific.

Great, so please elaborate which experiments you are referring to, and let us try to understand/discuss differences between their evaluations.

From one side, can we directly measure processes inside a star? Rather not, but does it prevent us from searching for models of what actually happens there? Also rather not: we can build self-consistent models based on general knowledge, and tune details of what we don't know to get agreement with measurements of indirect consequences. Similarly with microscopic physics - for which human observer is just a system of atoms governed by the same objective physics - on which we should focus on, like some properties of EM wave created by single atom deexication: optical photon. From the other side, while QM might be the final description, it is built on classical mechanics: as Feynman ensemble, through quantization etc. And we directly experience classical mechanics - emerging from the quantum one, also being its approximation - e.g. the initial one in WKB semiclassical approximation ( https://en.wikipedia.org/wiki/WKB_approximation ). If there are technical difficulties with complete quantum description of shape/size of optical photon, a natural approach is starting with approximations: like classical, semiclassical - you are welcomed to extend to complete quantum description (or complement the set of articles). Feynman diagrams are in another approximation: perturbative of QFT in momentum space. Their Dirac deltas in momentum space mean infinite size plane wave. To model finite size in position and momentum space there are e.g. used wavepackets ( https://en.wikipedia.org/wiki/Wave_packet ) : of bounded product of widths in position and momentum space in Heisenberg principle/Fourier transform. The question of size of single photon is e.g. about such width of wavepacket in position space. Even better would be experimental arguments I am especially asking for - like this minimal duration of ultrashort laser pulses.

The question is size in whatever view you want, like distortion of quantum position operator e.g. E[(x-E[x])^2]. I have posted at least 3 papers trying to do it from classical or semiclassical approximation of quantum mechanics, also started with and mostly ask for experimental suggestions. Instead, as for creationists, there is no discussion, only some nonsense excesses that we cannot even ask such holy/quantum questions. Being Dirac delta in standard Feynman diagrams: in momentum space, doesn't mean being perfect point but a plane wave: filling entire Universe.

But quantum mechanics is built on the classical one, e.g. through the quantization procedure. Classically we have single trajectory (or history of field) optimizing action, to take it to QM we e.g. take Feynman ensemble of such trajectories (field histories) instead (allowing to derive classical as approximation). In QM we have e.g. wavepackets - isn't single optical photon such a wavepacket? Photon is EM field - we should be able to say anything concrete about this EM field configuration/evolution - as wavepacket, or as quantum ensemble, or through distortion of position operator etc. Otherwise it is very similar to creationism: just blindly repeating "God created"/"It is quantum" as universal answer supposed to resolve every question.

"The size and shape of single photon" http://dx.doi.org/10.4236/oalib.1107179 has nice looking models of photon: The big question is how true they are??? Sadly, while many claim that physics is nearly solved, we know nearly nothing about such basic questions like EM field configuration and dynamics of photon (wavepacket) ...

As written e.g. E[(x-E[x])^2] - expected value of squared distance from the center of such wavepacket ... just any estimation of size photon, preferably experimental - e.g. minimal duration of laser pulse, weakening transmission through very narrow hole etc.

In quantum mechanics we have wavepackets - why shape of wavepacket from production of single photon is meaningless? Size is e.g. mean deviation from the center of particle (E[(x-E[x])^2] in position space) for its energy density.

Many nonlinear optics effect kind of split photons e.g. SPDC: https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion Regarding absorption of half of photon, I would say that it happens e.g. in spin echo: photon rotates spin by pi, here they are rotated by pi/2: https://en.wikipedia.org/wiki/Electron_paramagnetic_resonance#Pulsed_electron_paramagnetic_resonance SergUpstart, quatnum physics is built on classical e.g. through quantization, Feynman ensembles of classical scenarios. So we can ask what are dimensions of energy density of photons averaged over such ensemble.

EM field as consequence of electron dynamics travels with speed of light: ~0.3 nm per attosecond. Here is some paper trying to model emission of photon from hydrogen: https://link.springer.com/chapter/10.1007/0-306-48052-2_20

Sure such photon dynamics is extremely complex, but it doesn't mean we should just neglect this fundamental problem of understanding physics. For example while naively quantum processes are instant, reaching such measurement possibility, turns out e.g. that photoemission takes a few dozens of attoseconds: https://science.sciencemag.org/content/328/5986/1658 - there is some concrete hidden electron dynamics leading to EM field wave of the photon, we should at least try to understand. The ellipsoid view is probably only an approximation of energy density shape, to understand e.g. Mach-Zehnder there should be also some pilot/theta wave flying the second path, like from http://redshift.vif.com/JournalFiles/V16NO2PDF/V16N2CRO.pdf :

I have just found another - 2021 with more sophisticated models: "The size and shape of single photon" http://dx.doi.org/10.4236/oalib.1107179 Sure, these might be just the beginnings ... but asking for EM field configuration of photons is valid question, also from quantum perspective: as Feynman ensemble of classical ones - we can ask for e.g. dimensions averaged over such ensemble.

The basic difference between classical mechanics and quantum, is that in the former we have single trajectory optimizing action, while in the latter we have Feynman ensemble of trajectories, fields etc. So the question can be seen: what is mean size of such photon's energy density - averaged over quantum ensemble. QM cannot be used to completely neglect such a basic question of physics, and we have also experimental arguments like quoted above.

"Detecting" by who, what? This is very subjective question, while I am asking for objective EM field configuration - telling anything concrete about it. While this is a difficult question, there are at least these two articles, most importantly - providing experimental arguments (quoted in post above). What do you think about them?