How atoms interact with light reflects some of the most basic principles in physics. On a quantum level, how atoms and light interact has been a topic of interest in the worldwide scientific community for many years. Light scattering is a process where incoming light excites an atom to a higher-lying energy state from which it subsequently decays back to its ground state by reemitting a quantum of light. In the quantum realm, there are many factors that affect light scattering. In a new paper published in Science, JILA and NIST Fellow Jun Ye and his laboratory members report on how light scattering is affected by the quantum nature of the atoms, more specifically, thequantum statistical rule such as the Pauli Exclusion Principle.
The Pauli Exclusion Principle
Within quantum mechanics, there are rules that dictate how indistinguishable quantum objects, e.g., atoms of the same kind, interact with each other. One of these rules is the Pauli Exclusion Principle. This rule states that identical particles obeying Fermi statistics can't occupy the same internal and external quantum states in a quantum system simultaneously. This means that fermionic atoms, also called fermions, instinctively arrange to occupy different states when they are forced together. Electrons, protons, and neutrons are all fermions, and the Pauli Exclusion Principle is ultimately responsible for the stability of atoms which are built from these elementary particles.
Ye and his team wanted to study how the Pauli Exclusion Principle affected the scattering of light on fermionic atoms. Their study was focused on Rayleigh scattering, where the incoming and scattered light quanta (photons) have the same energy. In order to experimentally observe the implications of the Exclusion Principle one has to cool the atoms down to ultralow temperatures, below a threshold of the so-called quantum degeneracy, a process pioneered first by the late physicist, JILA and NIST Fellow Deborah Jin. This is something the Ye lab does routinely now in order to load their 3D optical lattice clock with fermionic strontium atoms. Ultimately, the gas of strontium atoms is so cold that it reaches the quantum degenerate regime where essentially all states up to the so-called Fermi energy are occupied by a strontium atom. This dense gas of cold fermions is also called a Fermi sea. “You need a high density of fermions to enter the regime where light scattering effects are expected, and there are many parasitic effects we had to overcome to reveal the signature we were looking for,” first author Christian Sanner said.
Using their cold gas, Sanner and the team used a laser with low intensity, as a probe, to test how light would scatter from the gas. They found that at the gas’s lowest temperatures and highest densities the measured scattering rates were smaller, by a factor of two, than expected for a normal gas. “The atoms undergoing a scattering process acquire a small momentum kick and to return to their ground state they need to settle into a quantum state that is not yet occupied by another fermion. If no free quantum state is available the light scattering process will be suppressed,” Sanner explained. The team probed different regions of the inhomogeneous gas cloud and confirmed the theoretically expected scattering behavior with the strongest suppression in the center of the gas cloud. The fermions on the outside of the gas had more open slots to move to in order to avoid breaking the Pauli Exclusion Principle.
Atomic Musical Chairs
The results of this experiment were important, as Ye and his team were the first to experimentally validate an idea over thirty years old—that quantum statistics of atoms can affect how light scatters off of them. “The atoms kind of play this game of musical chairs,” said Sanner. “We play with them and excite them, so they move and have to find a new quantum state to settle into.” Sanner highlighted that these findings are important in studying and engineering atom-light interactions. He pointed out that the process of spontaneous emission should be affected similarly: An excited atom surrounded by a deep Fermi sea cannot decay back to the ground state, it will stay in the excited state beyond its natural lifetime. The team has also explored this process and has found some preliminary evidence consistent with this idea. This paper was actually published with two other similar papers in Science. “Two other groups, one at MIT and one at the University of Otago in New Zealand, were independently working on this,” Sanner said. “In hindsight it kind of was a race to publish first, except that we didn't really know about our competitors until the very end and all of us came through the finish line more or less at the same time. All three papers give similar findings, just using different probes.” It was exciting for the researchers to see their results confirmed by two other groups.
The Next Steps
After this fundamental finding, the team is hoping to apply their experiment to a properly engineered 2D Fermi gas and directly observe a substantially prolonged excited state lifetime. They are also collaborating with JILA and NIST Fellow Ana Maria Rey to theoretically model light scattering dynamics, and study how “parasitic” collective scattering effects impact this process. According to Sanner, they're hoping in the long term to exert better control over atom-light interactions. “This experiment shows that nature has given us a great tool set to control and engineer atom-photon processes at the quantum level. There is a lot more to work on,” Sanner added.
Written by Kenna Castlebery, JILA Science Communicator