Quantum Information for Quantum Cats (15)

Bob's Result Depends on Eve's Action

So we now know that the answer to the quastion of the direction of the first arrow can be given only after the specification on the second arrow:

* If second arrow is not abserved, the first arrow is upward with 50% chance
* If second arrow is found upward, the first arrow is 100% downwards
* If second arrow is found dowmward, the first arrow is 100% upwards

Namely, even though Bob observes the first arrow, the result depends on whether or not the second arrow is observed or not by the third person Eve, and also the result of her observation. This is so even when Eve observe the second arrow in arbitrary faraway location from Bob. The secret is obviously in the construction of the state | S >. Such state, for which the state of a single arrow cannot be determined independent of the other arrow is called entangled state. There are Japanese terms "Karami joutai" or "Motsure joutai" which is invented as the translation, but these are not in wide usage. Practitioners call it just "Entangurudo suteeto".

EPR Paradox

This strange phenomenon involving entangled states has been first discussed by Einstein in his first ever article on the american journal Physical Review. (And this was the last too, since he was so mad at the negative referee report.) His interpretation of this result is very Einsteinian. Since this "instanteneous comunication" between faraway Alice and Bob is a direct result of the fundamental principle of quantum mechanics, and also this is against the local causality, which he believed was a fundamental priciple of physics, it could only be that either quantum physics or the interpretation of the standard quantum state must be wrong. This is the content of what is known as "Einstein-Podolsky-Rosen paradox" or EPR paradox, in short.

However, later advancement in the technology has made the experiment possible, and the result seem to go against Einstein. Such nonlocal effect of second arrow observation on the result of the first arrow observation has been proven to exist. The accuracy of the experiment has been improved dramatically since then along with the distance between the two observations which now stands beyond hundred km.

So we now have quantum metaphysics turned into a quantum wonder or magic.