Chapter 6 – Entanglement

The Bell test

In 1964 John Bell used the way polarization rotates a wave function to test entanglement.

Start by sending two entangled particles with opposite but unknown spin axes to aligned polarizers. Each has 50% probability of passing through the polarizers. This is expected!

Now slightly rotate the axis of one polarizer while particles are traveling. If the particles were independent, they would still have 50% chance of passing through the polarizers. The odds change with entanglement because the first that passes through its polarizer changes the wave function for the other.

Bell used this to create an experiment where multiple tests are run to see the probability of the results at the two polarizers matching. If the orientation of the polarizer are set after the particles leave the source but before they reach the polarizer, he was able to develop a test that confirmed no hidden variables can explain the entangled result.

These tests were run with increasing complexity between 1972 and 2010 by Alain Aspect, John Clauser, and Anton Zeilinger. The testing of Bell’s theorem proved nature cannot be explained by local hidden variables

Most physicists interpret this result as confirming quantum particles do not possess specific, predefined properties (like a definite spin or position) prior to measurement. Instead, they exist in a probabilistic state defined by a wave function.

There really is “spooky action at a distance” but Einstein didn’t live to see this confirmed.