# Bell experiment: Rotate measurement device 180°

Hi,

I was just writing another thread when I stumbled upon something strange:
Hi,

I'm trying get a better understanding of Bell's inequality in the form
$$\left|E\left(\bf{a},\bf{b}\right) -E\left(\bf{a},\bf{c}\right)\right|\leq 1+E\left(\bf{b},\bf{c}\right)\enspace.$$
I'm considering the Bell state
$$\left|\psi\right\rangle= \frac{1}{\sqrt{2}}\left(\left|+\right\rangle_1\left|-\right\rangle_2- \left|-\right\rangle_1\left|+\right\rangle_2\right)\enspace.$$
and the expectation value ##E^{qt}## of the product of the result of a measurement of spin 1 in direction ##\bf{a}## and spin 2 in direction ##\bf{b}##
$$E^{qt}\left(\bf{a},\bf{b}\right)= \left\langle\psi\right|\vec{\sigma_1} \cdot\bf{a}\otimes \vec{\sigma_2} \cdot\bf{b} \left|\psi\right\rangle=-\bf{a}\cdot\bf{b}$$
which is a straigthforward calculation. I'm using the notation from 1.5.1 in
http://www.uibk.ac.at/exphys/photonik/people/gwdiss.pdf
which is, however, in German.

We can violate the inequality by choosing e.g. ##\bf{a}=e_x##, ##\bf{b}=\left(e_x+e_z\right)/\sqrt{2}##, ##\bf{c}=e_z## which yields
$$\left|-\frac{1}{\sqrt{2}}-0\right|=\frac{1}{\sqrt{2}}\leq 1-\frac{1}{\sqrt{2}}$$
which is obviously wrong. I think this choice of vectors also maximally violates the inequality.

What if I now make a slight change and replace ##\bf{b}\rightarrow-\bf{b}##? The expectation values containing ##\bf{b}## change signs and I get
$$\left|\frac{1}{\sqrt{2}}-0\right|=\frac{1}{\sqrt{2}}\leq 1+\frac{1}{\sqrt{2}}$$
which is correct! But I've only turned my measurement device 180°, i.e. exchanged the (+1) and (-1) results, or more mathematically, I've permuted my measurement basis. How can this lead to such a profound change, i.e. this would allow a local-realistic description for these measurement angles? Surely there has to be something wrong.

I don't see something wrong, but Bell's inequality is violated only by some cases of the measurement configuration by QM.

Yes I know that, but in this case I'm not even really changing the configuration but only swapping the up/down-outcomes. The measurement basis is the same as before (only permuted), that confuses me.

no your b vector is rotated by 180 degrees so the angles are not the same any more. Usually Bell inequalities are violated when the angles between them is 45 degrees.

You don't change +/- outcome, those are always given by probability 1/2 1/2 by QM.

But it's not because you can simulate the result by hidden variables that nature will do it. How will it know the configuration permits to do that ?

Last edited:
DrChinese