A hotel with a neon “no vacancy” sign is like a cloud of ultracold atoms.
If a motel guest wishes to change rooms, they will be unable to do so. There are no available accommodations, so you have no choice but to stay put. Similarly, atoms trapped in crowded settings have no possibility of changing their quantum states in future tests. Three teams of researchers write in the Nov. 19 issue of Science that because of this limitation, the atoms don’t disperse light as they typically would. This impact was predicted more than three decades ago and is now being observed for the first time.
Atoms interact with light rapidly under normal conditions. If you shine a light beam on a cloud of atoms, some of the light will disperse in all directions. This form of light scattering occurs often in the Earth’s atmosphere. “We view the sky as blue because of dispersed radiation from the sun,” explains MIT’s Yair Margalit, who was part of one of the studies’ team.
In ultracold, dense atom clouds, however, quantum physics comes to the fore. “The way they interact with light or scatter light is different,” explains Amita Deb, a physicist at the University of Otago in Dunedin, New Zealand, who was a coauthor of one of the research.
Atoms in the experiments can’t take on the same quantum state — that is, they can’t have the same momentum as another atom in the experiment — according to the Pauli exclusion principle (SN: 5/19/20). If atoms are crowded together in a thick cloud and chilled to near absolute zero, the lowest-energy quantum states will emerge. Those low-energy states will be completely occupied, similar to a hotel with no available rooms.
When an atom scatters light, it gains momentum and changes its quantum state by sending light in a different direction. However, if the atom is unable to change its state owing to the crowded surroundings, the light will not be scattered. The atom cloud thins out, allowing light to pass through rather of dispersing it.
Margalit and colleagues studied the effect by shining light into a cloud of lithium atoms and measured how much light was dispersed. The scientists then lowered the temperature to cause the atoms to fill up the lowest energy states, thus inhibiting light scattering. The atoms scattered 37% less light as the temperature fell, showing that many atoms were stopped from scattering light. (Some atoms can still scatter light, for example, if they are thrown into unoccupied higher-energy quantum states.)
A cloud of ultracold strontium atoms was analysed by physicist Christian Sanner of the JILA research centre in Boulder, Colo., and colleagues in another experiment. The researchers looked at how much light was dispersed at tiny angles, when the atoms are jostled less by the light and hence had a harder time finding an empty quantum state. The atoms dispersed half as much light at lower temperatures as they did at higher temperatures.
The third experiment, conducted by Deb and University of Otago physicist Niels Kjrgaard, observed a comparable decrease in scattering in an ultracold potassium atom cloud and a commensurate increase in the amount of light passed through the cloud.
The Pauli exclusion principle is responsible for the structure of atoms and matter as we know it because it regulates the behaviour of electrons, protons, and neutrons. According to Sanner, these new findings portray the broad concept in a fresh light. “It’s intriguing because it demonstrates a really basic natural principle at action.”
The findings also point to new techniques to manipulate light and atoms. “There may be a lot of fascinating applications,” says Peter Zoller, a theoretical physicist at the University of Innsbruck in Austria who was not involved in the study. Light scattering is particularly linked to spontaneous emission, a process in which an atom in a high-energy state decays to a lower energy by producing light. The findings show that degradation might be halted, extending the energetic state’s lifespan. For example, in a quantum computer, such a method might be beneficial over storing quantum information for longer periods of time than is generally achievable.
These uses are still speculative, according to Zoller. “It will be interesting to see how realistic they are in the future.”