Oscillating patterns of light aren’t strictly quantum but can help with quantum information.

Up and down or side to side? Polarization is all about the shape of waves, and it can determine if a wave gets blocked or is allowed to pass (kind of like the letter and the mail slot above).

Polarization describes the shape of a wave, regardless of whether it is traveling on water or as a vibration on a violin string. It answers a question like: Is the wave moving up and down or side to side? A buoy bobbing up and down in the wake of a boat is one signature of a wave that’s polarized up and down. And since “up” and “down” are connected by a straight line, this particular wave has linear polarization. Some waves can have more complicated shapes, too. For instance, light waves can travel through space like a corkscrew, which is an example of circular polarization.

Waves can be linearly polarized (up and down, side to side, back to front) and circularly polarized (like a corkscrew), among many other choices.

Polarization usually depends on a wave’s origin story. The sunlight streaming toward Earth, which comes from the random fusion of nuclei in the sun’s core, is blasted out every which way in a mixture of polarizations. And the journey through the atmosphere doesn’t do anything to unscramble this mess. Scientists call this kind of disorganized light unpolarized. Energy-conserving LED lights also send out unpolarized light. Lasers, on the other hand, are a human-made source of polarized light.

No matter where light comes from, its polarization isn’t usually permanent. When light bounces off or passes through stuff, its polarization can meander, rotating a little or even flipping directions. For example, certain materials can block light with one polarization while letting another polarization through. If you’ve ever turned your head and caught the glare off a lake, you’ve seen a natural polarizer at work: The water reflects light that is polarized parallel to its surface, and when you turn your head in that precise direction, your eyes catch the glare. Ouch! Grab your polarized sunglasses to filter it out.

Some smartphones emit polarized light, which you can discover by taking a pair of polarized sunglasses and rotating them to see if they block out the screen.

There’s nothing particularly quantum about polarization. However, light itself has a quantum side because in addition to being composed of waves, it can be described as indivisible bundles of energy called photons. These blips of light can be visualized as little waves that don’t extend over large distances. Each individual photon also has a polarization oriented this way or that.

Scientists can make devices that emit photons one at a time, all with the same polarization. They can also use a combination of lenses (similar to those in eyeglasses), mirrors and crystals to manipulate and measure photon polarization. It turns out that polarization is one of the simplest properties of light to work with and so it is commonly used for storing a quantum bit, entangling photons and processing quantum information.