Desperately need someone to dumb this shit down for me.

So this beam of electrons is going through some analysers that measure its spin. In this particular set up because there is no measurement being made until the last analyser, all the electrons measure spin up on the last analyser. If you were to measure at the middle analyser as well, the results would be a 50:50 split because the system is disturbed.

So the probability of getting spin down in this case is zero:

$P_ - = | \langle - | \psi \rangle | ^2 \\<br /> <br /> P_ - = | \langle - | + \rangle \langle + | - \rangle + \langle - | - \rangle \langle - | + \rangle | ^2 \\<br />$

This gives four terms when expanded. Two are the normal probabilities of getting spin down by taking the two paths. The other two terms are "quantum interference".

$P_ - = P_ {top, -} + P_ {bottom, -} + interference \space terms$

Can someone explain what this interference is? Just an abstraction or does it have any physical meaning?