A curious observer’s guide to quantum mechanics, pt. 2: The particle melting pot - Ars Technica

Last week, we saw how particles move like waves and hit like particles and how a single particle takes multiple paths.

Instead of splitting the light from one laser into two paths, we can use two separate lasers to create the light coming into the final half-silvered mirror.

If you carefully measure the light from a normal laser, the color of the light and the phase of the wave (when the wave peaks occur) wander around.

Thanks to these, we can have two lasers both emitting photons of the same color with time-aligned wave crests?

The waves of particles produced by two different lasers are interacting.

We can turn the intensity of the two lasers down so low that we see the photons appear one at a time on the screen, like little paintballs.

If the rate is sufficiently low, only one photon will exist between the lasers and the screen at a time.

When we perform this experiment we will see the photons arrive at the screen one at a time; but when we look at the accumulated pointillism painting, we will see the same stripes we saw last week.

Nature does not care if one particle is interacting with itself or if two particles are interacting with each other—a wave is a wave, and particle waves act just like any other wave.

When we look at the screen we again see stripes, but now the stripes walk slowly sideways.

As we make the difference in color between the lasers larger, the speed of stripes increases.

A prism is usually used to split a single light beam and send each color in a different direction, but we can use it backwards and with careful alignment use the prism to combine the light from two lasers into a single beam.

Again we see the photons hit our detector one at a time like little paintballs.

It does not matter how low we turn the lasers—the photons can be so rare that they only show up one every 100 beats—but they will always arrive in time with the beats.

This pattern is even more interesting if we compare the arrival time of the photons in this experiment with the stripes we saw with our laser pointer last week.