Revolutionary Experiment Confirms Incompleteness of Quantum Mechanics

[colorSpace]

It would seem to me that according to this view, there is never a true which-way-information. Would this mean that using the existence of which-way-information to explain the presence or absence of interference patterns doesn't work in the first place? Wouldn't such a view confirm Afshar's view? Or if not, then how specifically is this experiment different from other situations?

 

[rkastner]

A good question. Whenever you have a source permitted to go through 2 open slits, a photon emanating from the source *will* go through both slits. The closer your "which-way" detector (i.e., something that physically interacts with the source beam) is to those slits, the more meaningful it is to say that you "caught" the photon going through a particular slit. But once you have allowed interference, then obviously the photon went through both. Afshar is mistaken because he wants to claim that he has shown interference and also that the detected particle went through a particular slit (which it didn't--it went through both). If you look at the discussion in the Feynman Lectures, which shows detectors right behind the slits, that setup allows you to say that you "caught" the particle going through that slit--to the extent that further downstream interference is destroyed. But if the Schrödinger wave goes through two slits, then the photon goes through two slits. My recent arxiv.org paper discusses this, as well as my (2005), which discusses the significance of this for "delayed choice" experiments.

 

[lightarrow]

A good question. Whenever you have a source permitted to go through 2 open slits, a photon emanating from the source *will* go through both slits. The closer your "which-way" detector (i.e., something that physically interacts with the source beam) is to those slits, the more meaningful it is to say that you "caught" the photon going through a particular slit. But once you have allowed interference, then obviously the photon went through both. Afshar is mistaken because he wants to claim that he has shown interference and also that the detected particle went through a particular slit (which it didn't--it went through both). If you look at the discussion in the Feynman Lectures, which shows detectors right behind the slits, that setup allows you to say that you "caught" the particle going through that slit--to the extent that further downstream interference is destroyed. But if the Schrödinger wave goes through two slits, then the photon goes through two slits. My recent arxiv.org paper discusses this, as well as my (2005), which discusses the significance of this for "delayed choice" experiments.

Correct me if I'm wrong, the Afshar's experiment with both slits open, when a photon is found in detector A or detector B, is no much different from reducing the multiple-bands interference pattern in two bands only (in A and B), where one detects the photon in one or in another, statistically, so no proving which-way slit at all.

 

[rkastner]

I'm not sure I'd call it "reducing" an interference pattern--there isn't really interference at the final detection plane, but there is still a superposition of two beams. Interference and superposition are two different things--you can have one without the other. More precisely, you can have superposition of two quantum states without interference. For example, let a source emit into two separated collimated beams which don't physically interact. The two beams are in a superposition but there is no interference between them.

 

[gptejms]

There was a discussion on Afsar 3 years back-may like to see the link below
https://www.physicsforums.com/showthread.php?t=62460&highlight=afshar

This is what I wrote(I hope it's of some help):-

...given a field distribution f(x,y) on one plane,you can find the distribution on another parallel to it at a distance d by a convolving f(x,y) with exp(-ia(x^2+y^2) where a =k/2d.The effect of a lens is to multiply the incident phase distribution by exp(ik(x^2+y^2)/f),where f is the focal length.Using the above one can easily show that given some field distribution g(x,y)in the front focal plane of the lens,the distribution on the back focal plane is simply the Fourier tranform of g(x,y).Now in Afshar's experiment,neither the wire grid nor the slits are at a focal distance from the lens.Besides what you obtain on the image plane is not simply the image of the wire grid----you have to distinguish between the case when one slit is open from the case when both are open.So one could follow the following approach:-a single slit gives a wavefunction of the kind \psi(x)=constt. over the slit width,0 elsewhere.Fourier transforming this you get \psi(p)=constt. sin(ap)/p(where a=slit width) in the momentum space.Now the wire grid and lens are some kind of momentum filters.One should work out the whole thing in this manner for the cases of one slit open and both open.I haven't done this but I guess what Afshar gets from simple ray optics should come out---but his conclusions are contestable

 

[Count Iblis]

http://arxiv.org/abs/0807.5079"

V. Jacques, N. D. Lai, A. Dreau, D. Zheng, D. Chauvat, F. Treussart, P. Grangier, J-F Roch

(Submitted on 31 Jul 2008)


Abstract: A recent experiment performed by S. S. Afshar et al. has been interpreted as a violation of Bohr's complementarity principle between interference visibility and which-path information in a two-path interferometer. We have reproduced this experiment, using true single-photon pulses propagating in a two-path wavefront- splitting interferometer realized with a Fresnel's biprism, and followed by a grating with adjustable transmitting slits. The measured values of interference visibility V and which-path information, characterized by the distinguishability parameter D, are found to obey the complementarity relation V^2+D^2=<1. This result demonstrates that the experiment can be perfectly explained by the Copenhagen interpretation of quantum mechanics.