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Young's double slit experiment: reconciling facts and theory 
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#1
May912, 03:47 AM

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I've read in many places that if, in a Young's double slit experiment, you can determine by whatever method through which slit your photon goes through (enhancing the "particle" behavior of light), then the interference pattern dissapears.
For this reason Zeilinger says that entangled photons do not produce interference patterns (because one could always use one photon of the pair to determine through which slit the other photon goes through). And related with this is the fact stated by Dirac that interference between different photons never occurs. BUT: a) A friend of me works at GSI with neutral pions. Neutral pions decay to two entangled photons going in opposite different directions. And she says that they work routinely with the interference of these photons. b) In 1963, Mandel and Magyar published an experiment in Nature (doi:10.1038/198255a0) where they produced interference fringes using two independent laser beams. The exit window of every laser can be considered as a single slit, so you have a double slit experiment which produces an interference pattern, where you know perfectly through which slit every photon has passed. How these facts reconcile with the above statements? 


#2
May912, 09:51 AM

Sci Advisor
P: 1,624

"When you read the first chapter of Dirac's famous textbook in quantum mechanics [8], however, you are confronted with a very clear statement that rings in everyone's memory. Dirac is talking about the intensity fringes in the Michelson interferometer, and he says, Every photon then interferes only with itself. Interference between two different photons never occurs. Now that simple statement, which has been treated as scripture, is absolute nonsense." A good discussion on that topic can be found in "Demonstration of the complementarity of one and twophoton interference" by A. F. Abouraddy et al., Phys. Rev. A 63, 063803 (2001). 


#3
May912, 10:34 AM

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PF Gold
P: 5,290

http://arxiv.org/abs/nuclth/0604021 Welcome to PhysicsForums! You can have interference but there will not be the kind of entanglement left to also get the perfect correlations you might otherwise expect. 


#4
May912, 10:38 AM

P: 2

Young's double slit experiment: reconciling facts and theory
Ok, I think almost everything is understood now.



#5
May912, 11:03 AM

Sci Advisor
P: 1,624

Welcome to these forums! Just to add on what I have written before, it is maybe a bit unfair to tackle Dirac's statement from the modern perspective like Glauber did. Dirac formulated it way before quantum optics came up and even many basic issues of qm were not understood properly in Dirac's time. Nevertheless the statement is so catchy that it routinely comes up again and again and often does more harm than good. 


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