Polarization

Light is a Wave

Light travels as a wave. But unlike water waves that move up and down, light waves can oscillate in any direction perpendicular to their travel.

The direction of oscillation is called polarization.

Visualizing Polarization

Imagine light coming straight at you. The wave could be oscillating:

• Vertically ↑ (up and down)
• Horizontally → (left and right)
• Diagonally ↗ or ↘

Regular light from the sun or a bulb oscillates in all directions randomly. This is called “unpolarized” light.

Polarizing Filters

A polarizing filter only lets through light oscillating in one direction.

• Vertical filter: only vertically polarized light gets through
• Horizontal filter: only horizontally polarized light gets through

Sunglasses often use polarizing filters to block glare (which is mostly horizontally polarized).

The Quantum Part

Here’s where it gets interesting.

What happens if vertically polarized light hits a horizontal filter?

You might think: it gets blocked completely. And classically, that’s true.

But what about a diagonal filter?

If vertically polarized light (↑) hits a 45° diagonal filter:

• The photon has a 50% chance of getting through
• If it gets through, it’s now diagonally polarized
• The original vertical polarization is gone

Measuring polarization in the “wrong” orientation gives a random result and changes the photon.

Two Orientations

We can group polarizations into two orientations:

If you know the orientation, you can measure correctly. If you guess wrong, you get random results.

Why This Matters

This property is the foundation of quantum key distribution.

• You can encode information in polarization (↑ = 0, → = 1)
• Anyone measuring with the wrong orientation gets random garbage
• And they disturb the photon in the process

You cannot measure an unknown polarization without risking changing it.