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I'm a Software engineering bachelor student and hence i don't have a strong physical background. i've been recently studying material related to quantum computing and its underlying physics and i came up with this rather mystifying quantum entanglement phenomena. what i don't understand is the significance of it. imagine the following thought experiment :

I have 20 pegs and input them to a machine which randomly splits them in two groups of X and Y pegs (which obviously X + Y = 20) with equal probability for each possible outcome (probably any other assumption about the splitting would do) and puts them in two perfectly identical boxes and outputs the boxes. (imagine the pegs are solidly glued to the box so you can't just guess the count of pegs in each box by simply shaking it and analyzing the sound it produces). lets name the boxes A and B. we take A to London and send B with a super freaky spaceship to some distant planet name Endor in galaxy far far away (assuming relativity works and i'm immoral so the time it takes to get to Endor won't be an issue. also neither the pegs nor the boxes would decay or ...). we don't know the exact number of pegs in each box, but each has some probability of containing a certain number of pegs, hence we could conclude the boxes are in a superposition of all the possible states (just like the electrons with unknown spin values). now when i make a measurement, which means i open one of the boxes and count the number of pegs in it, the superposition will instantly collapse into a single unique state, cause when i know the number of pegs in A then i will immediately know that there are 20 - |A| pegs in B.

is this an example of quantum entanglement ? cause if it is, then at least in my opinion, quantum entanglement would be a subtle event happening on a daily basis with no real significance or wow factor. or perhaps i'm just being too careless and short sighted regarding the concept ?

i would be grateful if someone could provide a clarification on the matter.

Paghillect
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  • If you think that you can determine the number by shaking the box and listening to the sound, then you are dealing with a classical theory. So this is not an example of quantum entanglement and quantum entanglement does not hold here. Entanglement a purely quantum phenomenon. It is the inability to describe a particle independently of another. –  Feb 14 '16 at 21:45

2 Answers2

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What you describe is a classical correlation, not entanglement. The whole point of quantum entanglement is that there is no definite individual state before you measure one part of the entangled system, while for your boxes, there always was a definite number of pegs in them, you just didn't know it.

The significance of quantum entanglement is precisely that the quantum situation has no classical equivalent. In classical physics, when you have a system that is composed of several subsystems, a state of the system corresponds uniquely to single, definite states of the subsystems. Entanglement is the phenomenon that this is not true in quantum physics - an entangled state is exactly one that cannot be expressed by single, definite states of the subsystems.

ACuriousMind
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  • i don't understand it when you say "there is no definite individual state before measurement" , don't we already know the relation between the values even before conducting the measurement ? – Paghillect Feb 16 '16 at 16:37
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Your question does miss the significance of quantum entanglement. A sketch of the problem runs as follows. Suppose that you have two electrons with entangled spin. For each electron you can measure the spin along the X, Y or Z direction. If you measure X on both electrons, then you get opposite values, likewise for measuring Y or Z on both electrons. If you measure X on one electron and Y or Z on the other, then you have a 50% probability of a match. The crucial issue is that whether you find a correlation when you do the comparison depends on whether you measure the same quantity on each electron.

Bell's theorem just explains that the extent of this correlation is greater than a local theory would allow if the measured quantities were represented by stochastic variables (i.e. - numbers picked out of a hat).

This fact is often misrepresented as implying that quantum mechanics is non-local. But in quantum mechanics, systems are not characterised by stochastic variables, but, rather, by Hermitian operators. For an explanation of how the correlations arise, see

http://arxiv.org/abs/quant-ph/9906007

and

http://arxiv.org/abs/1109.6223.

alanf
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