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I recently read

Photons act like they go through two paths, even when we know which they took, at Ars Technica,

which reports on the paper

Wave-particle dualism and complementarity unraveled by a different mode. R. Menzel et al. PNAS 109 (2012), 9314.

Please refer the Ars Technica link and its conclusion.

I am an Engineer. What I infer from this is :-

  1. This proves ERP.
  2. Einstein Wins.
  3. This basically proves that quantum mechanics is incomplete/incorrect.
  4. There is a requirement for an extension for QM.

What this does is :

  1. "Declaration of completeness of quantum mechanics" by Heisenberg needs to be pulled down officially
  2. Other theories needs to be thought about, like Bohm's.

Can somebody confirm my understanding ? Any help is appreciated.

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1 : the original paper. – user1813004 Feb 19 '13 at 11:13
No, that is not true. There is interference in another channel. Relevant quote: "We employ the obtain by a coincidence measurement of the two photons which-slit information about the signal photon without ever touching it....we observe in a separate coincidence experiment interference fringes in the signal photon...." (continues below) – Michael Brown Feb 19 '13 at 11:28
"The explanation of this puzzling experimental observation springs from the transverse mode structure of the pump with two intensity maxima that creates a superposition of two macroscopically distinguishable wave vectors of the signal photon. In the case of a Gaussian pump with a single maximum no such superposition arises." In other words - they were clever enough to make an experiment which obeys ordinary quantum mechanics perfectly well, but violates the commonly used slogans and shows them to be too sloppy and inaccurate. – Michael Brown Feb 19 '13 at 11:29
No, I'm not saying it was sloppy at all (though I'm not qualified to comment, really, being a theoretician). I'm saying they cleverly designed the experiment to have more than one interference channel. This shows that the naive slogans about complementarity don't completely capture the reality of quantum interference. This is no surprise to someone who has used quantum scattering theory, but it often gets passed over in popular treatments. – Michael Brown Feb 19 '13 at 11:44
@user1813004 No, the experimentalists methodology was not sloppy, it is the science journalists and the slogans they use that are sloppy. There is probably a deeper and more important point for you to take on board, which is that EPR's argument has already been proven wrong beyond all reasonable doubt. Reason being, EPR assumed local realism, which was shown by Bell and Aspect to be a false assumption, see Wiki. – Mark Mitchison Feb 19 '13 at 11:50

In my answer to wave-particle duality I expain the key to this misunderstanding in popular expositions on quantum mechanics.

The basic problem is that people think that the wave nature of the elementary particle entity, is exhibited in its mass distribution. They think that the entity is spread out in space according to the wave function of the quantum mechanical solution. It is the probability of finding the particle at (x,y,z,t) that is predicted by the quantum mechanical solutions.

When one looks at a probability curve, for example the life expectancy as a function of current age, one does not see oneself spreading out in fractions or continually until 100 years. One knows that it is just a probability distribution derived by the census bureau. It is the same with the quantum mechanical probability distributions ( the square of the wave function). The experiment you link to is just a smarter experiment than the traditional "check the slit" experiments which introduced large distortion to the measurements and created the artifact of a spread particle magically gathering itself up.

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