Suppose you measure the spin of an electron with a sensor oriented in the +z direction and find that the spin is up (aligned with the sensor). Now if you immediately measure the spin of the electon with a sensor oriented in the -z direction, you are guaranteed that it will be down (oppositely aligned). It takes a 180-degree rotation of the sensor to guarantee a measurement of opposite spin. Now do the same experiment with photons. If the first measurment of photon spin in the +z direction is up, then as I understand it, it will take a 90-degree rotation of the sensor (in the +x direction) to get a guaranteed measurement of down. Is this correct? As I understand it (though I could be wrong), photon spin is related to polarization. For a photon traveling in the +y direction, the opposite of polarization in the z direction is polarization in the x direction; hence the 90-degree rotation. Is this related to the fact that electrons are fermions (having spin 1/2), while photons are bosons (having spin 1)?