# Homework Help: Bell's inequality

1. Jul 8, 2008

### Thrice

I'm trying to understand a part of the text where they prove QM doesn't satisfy Bell's inequality. I get how he derives the inequality. Apparently it's same as Sakuri (1985) and Townsend (2000). Problem is I lose him as soon as he starts the main part. Quoting almost directly,

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Consider a qubit oriented in an arbitrary direction. Consider a unit vector $$\vec{n} = \sin \theta \cos\phi \hat{x} + \sin \theta \sin\phi \hat{y} + \cos \theta \hat{z}$$. The eigenvectors of $$\sigma \cdot \vec{n}$$ are..

$$|+_n \rangle = \cos \frac{\theta }{2} |0 \rangle + e^{i\phi }\cos \frac{\theta }{2} |1 \rangle$$

$$|-_n \rangle = \cos \frac{\theta }{2} |0 \rangle - e^{i\phi }\cos \frac{\theta }{2} |1 \rangle$$

He then goes on to interpret $$|\langle 0|+_n\rangle |^2$$ as a probability & i can follow from there. I just don't get the bit I posted. The hint is to consider the x & y axes, eg to get the eigenvectors of $$\sigma \cdot \hat{x}$$ set $$\theta =\pi /2$$ & $$\phi =0$$

Thanks.

Last edited: Jul 8, 2008
2. Jul 11, 2008

The key is to understand what $$\sigma \cdot \vec{n}$$ means. This is a dot product of matrices (Pauli matrices) times scalars (components of $$\vec{n}$$. Let's just look at the first component. You will take the x Pauli matrix times by the x component of $$\vec{n}$$ to get $$\sigma_{x} n_{x}=\left(\begin{array}{cc}0 & 1 \\ 1 & 0\end{array} \right) \sin \theta \cos\phi=\left(\begin{array}{cc}0 & \sin\theta\cos\phi \\ \sin\theta\cos\phi & 0\end{array} \right)$$. You do the same for the y and z components and add them all up to get your matrix, $$\sigma \cdot \vec{n}$$. Find the eigenvectors of it and you should get what you gave from the book.