Cold Radioactive Molecules Prepped and Readied for Physics Discoveries
For the first time, researchers have figured out a way to create chilled molecules containing the radioactive element radium. The resulting laboratory concoctions, generated in part from steps similar to those used to make candy, are poised to help researchers solve one of the biggest mysteries of our universe: How did matter in our early universe come to dominate over its antimatter counterpart?
Early in the universe, matter and antimatter were created in equal proportions. The negative electron, for example, has an antimatter twin called the positron, which is positively charged. An electron and positron can be created out of energy in perfect pairs, yet when the two meet, they annihilate each other back into pure energy. Just what happened to all the antimatter remains one of the biggest mysteries in physics. Some kind of difference, or asymmetry, between matter and antimatter must exist to explain why matter was favored during the creation of our universe.
A few years ago, researchers led by Nick Hutzler, professor of physics at Caltech, began investigating radium-containing molecules as a probe for studying this mystery. Their goal is to use lasers to look for subtle changes in the radium molecules that would indicate new particles and forces behind the matter/antimatter mystery. These new particles and forces would show up as particular energy shifts in the molecules that are not allowed by the known laws of physics.
Radium is ideal for these experiments because its nucleus is shaped like a pear.
"Pear-shaped nuclei are asymmetric and dramatically amplify the potential signals we are looking for to explain the asymmetry in matter and antimatter," Hutzler says. "Most nuclei are spherical like an orange or stretched out in one direction like an American football. Radium has the rare pear shape we want, it has been studied extensively by nuclear physicists, and it makes molecules that are ideal for laser-based quantum precision measurements."
The problem is that radium, which was discovered by Marie Curie in 1898, is radioactive, highly reactive, and comes in very small quantities, making it extremely difficult to work with.
The new research, published in Science, marks the first time the radium molecules have been successfully prepared and studied precisely with lasers in tabletop experiments. One of the requirements for these types of experiments is that the molecules be chilled to frosty temperatures, hence this is also the first time the radium molecules have been prepared in a cold state. What is more, the new method can be applied to other atoms besides radium to make similarly chilled molecules for physics experiments.
"How do you go from a fleck of radium to cold molecules that are ready for tabletop quantum experiments in the lab?" Hutzler says. "It took us years of trial and error to finally come up with a protocol for handling the radium, making the molecules, detecting them, and measuring their properties."
It's a Matter-Dominated World
Antimatter exists in our universe only in very small, fleeting quantities. In 1932, Caltech's Carl Anderson (BS '27, PhD '30) proved that antimatter exists by discovering the antimatter twin to the electron, the positron. But while physicists know antimatter exists in our universe, they do not understand why it largely disappeared.
Some groups of researchers use large particle accelerators to search for clues. Hutzler and his team are among a small number of researchers who are instead using smaller tabletop experiments. They use molecules as quantum sensors to search for new particles and forces that break symmetry rules. "The molecules act like antenna to amplify the properties we are looking for," Hutzler says. Because these experiments require studying the molecules down to the quantum level, they belong to a growing field called quantum precision measurement.
The team, which includes researchers at Johns Hopkins University and Michigan State University, experimented with different methods of working with microscopic quantities of radium in the lab. Because radium is radioactive and dangerous, it can only be safely handled in very small quantities.
Radium is also highly reactive, so the first step to working with it involved finding a way to keep it stable and transport it around. "We wanted to embed it in something we could handle, to basically put it in thick viscous goo."
The best goo, it turned out, came from a process similar to making candy. The radium was added to water and sugar, and the water was evaporated away to create the goo. But the process, which involved a lot of "bubbling, scorching, and caramelization," as Hutzler says, was finicky and hard to replicate when actual sugar was used. In the end, the team found that sugar-free sweetener xylitol worked best.
Once they had the goo ready, the team added it on top of a gold foil and installed it in a mini fridge–sized apparatus in the lab. They cooled the chamber down to around minus 450 degrees Fahrenheit using helium gas. To get the radium to react with the target molecules, they zapped the radium atoms to excite them into a chemically reactive state. Finally, the group showed that they could use yet another set of lasers to analyze and study the newly made molecules.
"Importantly, the molecules are now ready to be used in quantum precision measurements," Hutzler says.
In another recent paper, to appear soon in Physical Review X, Hutzler and colleagues developed a protocol for working with heavy molecules containing the element ytterbium. This protocol, referred to as "engineered molecular clocks," is a measurement approach developed by the Hutzler lab to search for signs of new particles and forces in these tabletop experiments once the molecules are prepared and ready for use. Usually, these types of measurements are very susceptible to external sources of noise and a phenomenon called decoherence, which destroys the quantum wave function of the molecules. In the new study, the researchers developed an approach to preparing the molecules that renders them far less sensitive to these effects and thus better probes for discovering new physics.
The engineered clock experiment is being used currently to search for signatures of new particles and forces in the ytterbium nucleus and will eventually be used to study the radium nucleus as well.
"Our goal is to create the best quantum tools out of these very complicated molecules," Hutzler says. "We are engineering molecules for precise quantum control."
The radium paper titled "Production and spectroscopy of cold radioactive molecules" was funded by the National Science Foundation, a De Logi Science and Technology grant, the Heising-Simons Foundation, an Alfred P. Sloan Research Fellowship, a Caltech Eddleman Graduate Fellowship, a Caltech Dominic Orr Graduate Fellowship, and a Thomas Lauritsen Caltech SURF Endowment. Other Caltech authors include Chandler Conn (PhD '26), Phelan Yu, Madison Howard, Yuxi Yang, Arian Jadbabaie (PhD '23), Aikaterini Gorou (BS '22), and Timothy Steimle. Additional authors include Chaoqun Zhang and Lan Cheng of Johns Hopkins University, and Alyssa Gaiser of Michigan State University.
Graduate students Yuxi Yang, Chandler Conn, Madison Howard, and Phelan Yu in the Hutzler lab.
Credit: Caltech
This artist's concept depicts a tabletop apparatus used in physics experiments to turn very hot atoms and molecules into cold ones, making their individual quantum states accessible with lasers.
Credit: AI-generated art by Nick Hutzler
Radium has a pear-shaped nucleus, which enhances its sensitivity to new particles and forces. The copper chamber below depicts a simplified version of the cryogenic cell used by the Hutzler lab to form radium-containing molecules.
Credit: Ella Maru Studio
