A camera is used to collect fermionic atoms. The ensemble's temperature is lowered as fewer atoms scatter light. Credit: Niels Kjrgaard.
The electrons in an atom are arranged in energy shells. If all the chairs are occupied, each electron can't go to a lower tier. The shell structure of atoms, the diversity of the periodic table of elements, and the stability of the material universe are explained by the Pauli exclusion principle.
MIT physicists have observed the Pauli exclusion principle in a completely new way and found that it can suppress how a cloud of atoms scatter light.
When light hits a cloud of atoms, the particles can ping off each other like billiard balls, scattering light in every direction to make the cloud visible. The MIT team observed that when atoms are supercooled and ultrasqueezed, the Pauli effect kicks in and the particles have less room to scatter light. Without being scattered, the photons stream through.
Physicists observed this effect in a cloud of lithium atoms. The atoms became dimmer as they were made colder and denser. The researchers think that if the conditions were pushed further, the cloud would become completely invisible.
The first observation of Pauli blocking's effect on light-scattering by atoms was reported by the team. 30 years ago, this effect was predicted but not observed now.
Wolfgang Ketterle, the John D. Arthur Professor of Physics at MIT, says that Pauli blocking is essential for the stability of the world around us. Pauli blocking is a form of blocking that prevents an atom from being 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- This is the first clear observation that this effect exists, and it shows a new phenomenon in physics.
The lead author is Yair Margalit, a former MIT postdoc, and the other co-authors are graduate student Yukun Lu and Furkan Top. The team is affiliated with the MIT physics department, the MIT-Harvard Center for Ultracold Atoms, and the RLE.
A kick.
When Ketterle came to MIT as a graduate student, his mentor, David Pritchard, the Cecil and Ida Green Professor of Physics, made a prediction that Pauli blocking would suppress the way certain atoms scatter light.
His idea was that if atoms were frozen to a near standstill and squeezed into a tight space, they would behave like electrons in a packed energy shell. The atoms wouldn't be able to light up if the light was to stream in.
"An atom can only scatter a photon if it can move to another chair and absorb the force of its kick," says Ketterle. The ability to absorb the kick and scatter the photon has been lost if all other chairs are occupied. The atom becomes transparent.
People were not able to generate clouds that were cold and dense enough to cause this phenomenon.
"Controlling the atomic world"
Physicists in Ketterle's group have developed magnetic and laser-based techniques to bring atoms down to ultracold temperatures. He says the limiting factor was density.
An atom can scatter light by jumping over a few chairs if the density is not high. That was the biggest problem.
In their new study, he and his colleagues used techniques they developed previously to first freeze a cloud of fermions. The temperature of the space is about 1/100,000, so they froze the cloud down to 20 microkelvins.
The densities were reached by squeezing the ultracold atoms with a tightly focused laser.
The researchers carefully adjusted the light's intensity so that it wouldn't change the density of the atoms in the cloud. They used a lens and camera to count the photons that flew away.
"We're actually counting a few hundred photons, which is really amazing," says Margalit. The equipment we use is so sensitive that we can see a small blob of light on the camera.
The atoms scattered less and less light at higher densities. 38 percent dimmer atoms mean they scattered 38 percent less light than less dense atoms.
"This regime of ultracold and very dense clouds has other effects that could possibly deceive us." We spent a few months sifting through the effects to get the clearest measurement.
Ketterle says that the team's knowledge of Pauli blocking may be used to develop light-suppressing materials for use in quantum computers.
Light scattering is a problem when we control the quantum world like in quantum computers. This is one way to suppress light scattering, and we are contributing to the general theme of controlling the atomic world.
The University of Colorado team's work appears in the same issue of Science.
Science has more information about Yair Margalit and Pauli blocking light scattering in fermions. www.science.org/doi/10.1126/science.abi6153
Science journal information.
How ultracold, superdense atoms become invisible was retrieved 18 November 2021.
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