MyKell

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Wyz_sub10

A google search for "Quantum Decoherence" turned up 19,300 web sites, including one offering this definition:

http://www.aip.org/enews/physnews/19...t/pnu297-3.htm

Here is an abstract on the existence of quantum decoherence:

Is Quantum Decoherence Reality or Appearance

Lobstrosity

I actually think I heard about this experiment, or something very similar. If it's the one I heard of then here is a very simplified version of what they found:

First of all, when you send photons through a double-slit aperture and onto a screen, you find that an interference pattern forms (you'll get a series of light and dark spots on the screen where the light constructively and destructively interferes, respectively). Interestingly, however, you can also get such an interference pattern sending matter particles (in this case rubidium atoms) through a double-slit. Their quantum-mechanical probability wave functions can interfere just like light waves do. The weirdest thing is that each particles is actually interfering with itself. Using the Feynman Path Integral outlook, you can say that each particle actually goes through both slits and the two possible paths quantum-mechanically interfere with each other. This is definitely not a classical view. But here's where it gets even wackier: if you place a detector on each slit to physically measure which one each particle passes through (perhaps you're a skeptical person and you just don't buy into the fact that each particle is actually going through both slits), the interference pattern vanishes. The interference only happens when you don't "look" until the very end. If you cheat and check where each particle is going, you just get a standard classical distribution on the screen.

The reason for this difference in results is that you are causing quantum decoherence when you place the detector at the slits. Initially, the particles are in a quantum superposition of states and is this superposition that allows for the interference. The detector collapses the superposition into one definite state, and as such prevents any interference from being able to occur. Thus this experiment actually demonstrates the principle of quantum decoherence at work. Theoretically, however, quantum decoherence is an integral part of quantum mechanics and has been for a long time. States are very often a quantum superposition of eigenstates. Making a measurement on the system will always collapse the superposition to one specific allowed eigenstate (with probabilities of getting each result well known) that corresponds with the result of the measurement.

For example, the position of particle is given by a probability wave function that allows for it to be found anywhere over a region (usually this region extends to infinity with probability dropping to zero asymptotocally as you get farther away). When you actually measure the position, its wave function will collapse to represent the location at which you found the particle. If your measurement is exact, the wave function will now be a Dirac delta function. Then, over time, the collapsed position wave function will spread out again.

This is what I interpreted quantum decoherence to mean (I never learned anything about that specific term), but if I'm mistaken please feel free to ignore everything I just wrote and call me stupid.

Addendum: I happened to find the full article in case you wanted to read through it yourself. I very well may have been thinking of a slightly different experiment, but I think the principle illustrated is the same.

MyKell

Lobstrosity,
Thanks alot for taking the time to write all that. I understood most of what you said