Chapter 6 – Entanglement

Revisiting polarization

Before we can look at how entanglement was confirmed, we need to take a digression and return to polarization.

Polarizing filters only let light through with a certain direction of polarization. This experiment can be done with polarized sunglasses. Two filters with the same orientation let light through and they block light with perpendicular orientation.

An unexpected result is seen if a third filter with 45 degree orientation is placed between the other two. Adding this filter somehow increases the light that makes it through! How does this happen?

We need to look more closely at what a polarizing filter does. Remember back in the section on spin, we covered how polarization is represented by spin in photons and spin is always measured as +1 or -1. That means that the polarizing filter always measures light as parallel or perpendicular to its orientation – light coming out of it cannot have any other orientation.

This shows the result of light with different orientations when it goes through a polarizing filter:

The four examples are:

• 100% of light aligned with the filter goes through.
• 0% of light perpendicular to the filter gets through
• If the incoming light is at 45 degrees to the filter, half gets through and half is blocked. However, the half that gets through is now aligned with the filter.
• Other angles change the percentage that gets through but the result is the same – all light coming out of the filter is aligned with the filter.

What this means is adding the middle filter rotates the orientation of the light that gets through with them allows some to go through the last filter.

The key point here is that a polarizing filter modifies the wave function. This will be important in tests on entanglement.