# Spin behavior in entangled electron pair

Isn't it that in entangled electron pair, it's random which side will have
up and down spin... the two sides should produce opposite spin but the order is
random.. however I'm confused by the following statements:

"Recall that when you measure an electron's spin in one direction you
erase all information about the electron's spin in any other
direction. By this I mean the following: If I measure the spin in the
left/right direction I will always find it pointing exactly left or
exactly right. Say I measure it pointing left. If I measure it again
in the left/right direction then I will definitely see it pointing
left again. If I measure it in the up/down direction, however, then I
will have 50/50 odds of seeing it point either way. Once I do that
measurement, however, I will have fixed it to be pointing either up or
down, and a left/right measurement will once again yield 50/50 odds".

What is the above talking about? By using say up detector. You can
bias the pair to *always* produce up spin in that detector? But I
thought it's supposed to be random. Hope someone can clarify.

## Answers and Replies

The quoted statement is about a single electron only, not an entangled pair. The statement does apply to one electron in an entangled pair as well. For that one electron, if you measure it's spin about a particular axis, say "x" then you force it's spin about x to be up or down AND you eliminate any tendency for the electron to have a particular spin about y or z. This means a subsequent measurement of the spin about x will give the same result as the last, but a subsequent measurement on the spin about y or z will have a 50/50 chance of being up or down.

Your initial statement is also correct, if the spin of 2 electrons is entangled so that the net spin about x is 0 then the first time you measure their spin about x you will get opposite measurements on the 2 electrons (one up one down) but you do not know which will be up and which will be down until you measure.

I just humbly ask daezy to read the EPR pardox and the one which follows after it- Bell's theorem.In detail(my instructor saw and said so).