How ultracold, superdense atoms become invisible

An atom’s electrons are arranged in energy shells. Each electron occupies a single chair and cannot drop to a lower tier if all its chairs are occupied. This fundamental property of atomic physics is known as the Pauli exclusion principle or Pauli blocking.

When photons of light penetrate a cloud of atoms, the photons and atoms can ping off each other like billiard balls, scattering light in every direction to radiate light, and thus make the cloud visible.

Physicists observed that when atoms are supercooled and ultrasqueezed, the Pauli effect kicks in and the particles effectively have less room to scatter light. The photons instead stream through, without being scattered.

In their experiments, the physicists observed this effect in a cloud of Lithium atoms. As they were made colder and denser, the atoms scattered less light and became progressively dimmer. The researchers suspect that if they could push the conditions further, to temperatures of absolute zero, the cloud would become entirely invisible.

The team’s results, reported in science, represent the first observation of Pauli blocking’s effect on light-scattering by atoms. This effect was predicted 30 years ago but not observed until now.

In their study, they froze a cloud of Lithium atoms down to 20 microkelvins, (about 1/100,000 the temperature of interstellar space). 

They then used a tightly focused laser to squeeze the ultracold atoms to record densities, which reached about a quadrillion atoms per cubic cm.

The researchers then shone another laser beam into the cloud, which they carefully calibrated so that its photons would not heat up the ultracold atoms or alter their density as the light passed through.

At progressively colder temperatures and higher densities, the atoms scattered less and less light, the atoms were 38% dimmer, meaning they scattered 38% less light.

This is one way to suppress light scattering, and contributing to the general theme of controlling the atomic world.