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The well known Malus law predicts $\cos^2\theta$ for the probability of passing through a filter oriented with an angle $\theta$ w.r.t. the polarization direction of the incident photon. On the other hand the standard quantum treatment of spin $1/2$ particles gives for the same question the result $\cos^2\frac{\theta}{2}=\frac{1+\cos\theta}{2}$ (see for example http://www.lecture-notes.co.uk/susskind/quantum-entanglements/lecture-4/measurement/ ). Is this difference coming only fron the spin (1 vs. $1/2$) or am I comparing apples and oranges? My question is motivated by the fact that in many treatments of EPR experiments people speak of electron and photons while using always the simple Malus law.

Arnaldo Maccarone
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1 Answers1

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Maybe you are comparing different flavors of apples. Think about it experimentally. A polarizer is just a metal grid, so if you rotate it 180 degrees you have the same thing. For 1/2 particle you have a Stern Gerlach experiment, and if you turn this 180 degrees you have up and down switched. So the angle in the formula has to be divided by 2.

Apart from the Faktor of 2, I do not think there is a conceptual difference.

lalala
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