Quantum Entanglement: Self-interference of Photons

Shown here is the interference pattern produced by a green 532nm laser beam passing around a wire (0.254mm in diameter) at a distance of about 4.5m from the wall acting as a screen.

The interference pattern is created by individual photons interfering with themselves. The interference pattern occurs even when the intensity of the light is so small that only one photon leaves the laser at a time.

From en.wikipedia.org/wiki/Photon_polarization#The_nature_of_p...:

"Some time before the discovery of quantum mechanics people realized that the connection between light waves and photons must be of a statistical character. What they did not clearly realize, however, was that the wave function gives information about the probability of one photon being in a particular place and not the probable number of photons in that place. The importance of the distinction can be made clear in the following way. Suppose we have a beam of light consisting of a large number of photons split up into two components of equal intensity. On the assumption that the beam is connected with the probable number of photons in it, we should have half the total number going into each component. If the two components are now made to interfere, we should require a photon in one component to be able to interfere with one in the other. Sometimes these two photons would have to annihilate one another and other times they would have to produce four photons. This would contradict the conservation of energy. The new theory, which connects the wave function with probabilities for one photon gets over the difficulty by making each photon go partly into each of the two components. Each photon then interferes only with itself. Interference between two different photons never occurs.
—Paul Dirac, The Principles of Quantum Mechanics, Fourth Edition, Chapter 1"

It's as if the photon "splits" and both partners become entangled (with each other and with the experiment which "measures" the states of the system). Both partners are in a superposition of polarization states. If one were to allow the system to measure the polarization of one partner, the polarization of the other partner would "instantly" become definite, no longer being in a superposition of states. This would result in the destruction of the interference pattern.

This apparent communication between the partners would happen "instantly" even if the partners were light years apart.


See:
en.wikipedia.org/wiki/Photon_entanglement
en.wikipedia.org/wiki/Double-slit_experiment
and
www.arturekert.org/miscellaneous/quantum-eraser.pdf


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