Suppose we have a source that emits pairs of electrons with one electron sent to my destination A, and another to your destination B. We can arrange our source so that each emitted electron pair occupies quantum state called a spin singlet. This is a quantum superposition of two states: 1) electron A has spin pointing upward along the z-axis (+z) and electron B has spin pointing downward the z-axis (-z); 2) electron A has spin -z and electron B has spin +z. The electrons are thus said to be entangled. Now suppose you measure the spin along the z-axis at the destination B. There are two possible outcomes: +z and -z. But! If you obtain +z, I shall obtain -z at destination A with 100% probability. And vice versa,- if you obtain -z, I shall obtain +z at destination A with 100% probability. That means that if you do your measurement and tell me the result, I can be sure about the result of my measurement (the spin of my electron) without any measurement at all. Do you understand me? Having the result of measurement at destination B you need no measurement at destination A. This result of measurement at destination B that you tell me is so called "classical information" and the state of electron at destination A (that is not measured at all) is called "quantum information".
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timeasmymeasure
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Date: 2006-10-07 09:54 am (UTC)1) electron A has spin pointing upward along the z-axis (+z) and electron B has spin pointing downward the z-axis (-z);
2) electron A has spin -z and electron B has spin +z.
The electrons are thus said to be entangled.
Now suppose you measure the spin along the z-axis at the destination B. There are two possible outcomes: +z and -z. But! If you obtain +z, I shall obtain -z at destination A with 100% probability. And vice versa,- if you obtain -z, I shall obtain +z at destination A with 100% probability.
That means that if you do your measurement and tell me the result, I can be sure about the result of my measurement (the spin of my electron) without any measurement at all.
Do you understand me? Having the result of measurement at destination B you need no measurement at destination A.
This result of measurement at destination B that you tell me is so called "classical information" and the state of electron at destination A (that is not measured at all) is called "quantum information".