I was having a discussion about quantum physics with a friend, and we came to realise that we perceived the same situation in two very different ways. After searching online, we still weren't able to come to a conclusion on who is correct. Here is the situation:
Lets say we have two electrons that are quantum entangled, such that the spin of electron A is opposite to the spin of electron B.
You then measure the spin of electron A, and find it to be up spin.
My interpretation:
- By observing electron A, you improve upon your knowledge about the second one. To someone who knows the spin of A, they can deduce the spin of B. To someone who doesn't know the spin of A, they have a 50-50 chance of guessing correctly. The entire time, ever since they were entangled, electron A was spin up and B was spin down. Simply no one knew it yet.
Their interpretation:
- By observing electron A, you physically change the state of electron B. When you observed that A was spin up, electron B physically changed from being undetermined to become spin down, whereas before neither was individually a definite state, but in some combined state that exists in the tensor product space of A and B.
So my overall question: Whose interpretation is correct?
Are the electrons in an absolute state of having either up or down spin from the moment of being quantum entangled and we simply do not yet have enough information to know (until we observe A), or does observing electron A's spin physically change the spin of electron B, such that the results may have been different if electron B was measured first?
Best Answer
In my view, both interpretations are incorrect (contradictory with the usual interpretations of QM). Before measurement the system of two electrons is in a superposition of $|\uparrow\downarrow\rangle$ and $|\downarrow\uparrow\rangle$. At that point, it doesn't really make sense to say that "electron $B$ is in a state", even undefinite. Measurement doesn't act on electron $B$, it acts on the entangled system of $2$ particles. After the measurement, the state of the $2$-particle system is un-entangled, so it makes sense to say "electron $A/B$ is in the state up/down".
The first interpretation is incorrect : the spin of $A$ is not defined before measurement. The act of measurement forces the system into a state in which the spin of $A$ has a definite value.
The second interpretation is incorrect as well : When you observe $A$ in the state "up", you acted physically on the whole system of $2$ particles. You didn't "change the state of $B$", since the state of $B$ was not defined before measurement