A unique quantum situation has emerged in Colombia

Physicists in Columbia University They have taken molecules to a new, ultracold extreme and created a state of matter where quantum mechanics reigns supreme.

There's an exciting new BEC in town that has nothing to do with bacon, eggs and cheese. You won't find it in your local department store, but in the coldest place in New York: the laboratory of physicist Sebastian Weyl of Columbia University, whose experimental group specializes in pushing atoms and molecules to temperatures just fractions of a degree higher. Absolute zero.

Write in natureWill's lab, with support from theoretical collaborator Tijs Karman at Radboud University in the Netherlands, has succeeded in creating a unique quantum state called Bose-Einstein condensates (BEC) from molecules.

Breakthrough in Bose-Einstein condensates

Their BEC is cooled to just five nanokelvins, or about -459.66 degrees Fahrenheit, is stable for a remarkably long two seconds, and is made of sodium and cesium molecules. Like water molecules, these molecules are polar, meaning they carry both a positive charge and a negative charge. Weil noted that the unbalanced distribution of electric charge facilitates the long-range interactions that make up the most interesting physics.

The research the Weill Lab is excited to pursue with Bose-Einstein Molecular involves exploring a number of different quantum phenomena, including new types of superfluidity, a state of matter that flows without experiencing any friction. They also hope to turn their Bose-Einsteins into simulators that can recreate the enigmatic quantum properties of more complex materials, such as solid crystals.

With the help of microwaves, Columbia University physicists created a Bose-Einstein condensate, a unique state of matter, from sodium and cesium molecules. Image credit: Well Lab, Columbia University/Miles Marshall

“Molecular Bose-Einstein condensates open up completely new areas of research, from truly understanding fundamental physics to developing powerful quantum simulations,” he said. “This is an exciting accomplishment, but it's really just the beginning.”

It's a dream come true for Weill Lab, and one decades in the making for the larger ultracold research community.

Ultracold molecules, a century in the making

The science of BECs goes back a century to physicists Satyendra Nath Bose and Albert Einstein. In a series of papers published in 1924 and 1925, they predicted that a collection of particles cooled to near standstill would coalesce into a single, larger atom with common properties and behaviors dictated by the laws of quantum mechanics. If BECs could be created, they would provide researchers with an attractive platform to explore quantum mechanics on a more accessible scale than individual atoms or molecules.

It took about 70 years since those first theoretical predictions, but the first atomic BECs were created in 1995. This achievement was recognized by the Nobel Prize in Physics in 2001, around the time Weill started physics at the University of Mainz. In Germany. Laboratories now routinely manufacture Bose-Einstein atoms from several different types of atoms. These BECs have expanded our understanding of concepts such as the wave nature of matter and superfluids and have led to the development of technologies such as quantum gas microscopes and quantum simulators, to name a few.

From left to right: Associate Research Scientist Ian Stevenson; PhD student Niccolò Bigagli; PhD student Weijun Yuan; University student Boris Bulatovich; doctoral student Siwei Zhang; and lead investigator Sebastian Weil. Not shown: Tejce Kerman. Credit: Columbia University

But atoms, in the grand scheme of things, are relatively simple. They are round objects and usually do not contain interactions that might arise from polarity. Since the first atomic BECs were achieved, scientists have wanted to create more complex versions made of molecules. But even simple diatomic molecules made up of two atoms of different elements bonded together have proven difficult to cool below the temperature necessary to form a proper BEC.

The first breakthrough came in 2008 when Deborah Jin and Jun Yi, physicists at the Gila Institute in Boulder, Colorado, cooled a gas of potassium and rubidium molecules to about 350 nanokelvin. Such ultracold molecules have proven useful for performing quantum simulations, studying molecular collisions and quantum chemistry in recent years, but to cross the BEC threshold, lower temperatures were needed.

In 2023, he created Will's Lab The first extremely cold gas of the molecule they chose, sodium and cesium, using a combination of laser cooling and magnetic manipulation, similar to Jin Wei's approach. To make it cooler, they brought in microwaves.

Innovations with the microwave

Microwaves are a form of electromagnetic radiation and have a long history in Colombia. In the 1930s, physicist Isidore Isaac Rabi, who later won the Nobel Prize in Physics, did pioneering work in microwaves that led to the development of airborne radar systems. “Rabe was one of the first to master quantum states of molecules and was a pioneer in microwave research,” Weil said. “Our business follows this 90-year tradition.”

Although you may be familiar with the role of microwaves in heating your food, it turns out that they can also facilitate the cooling process. Individual molecules tend to collide with each other and, as a result, form larger complexes that disappear from the samples. Microwaves can create tiny shields around each molecule, preventing them from colliding, an idea proposed by Karman, their colleague in the Netherlands. With molecules protected from missed collisions, only the hottest molecules can be preferentially removed from the sample, which is the same physical principle that cools your coffee cup when you blow on the top of it, explains author Niccolò Bigagli. Those remaining molecules will be colder, and the overall temperature of the sample will decrease.

The team came close to creating a molecular BEC last fall in work published in Nature physics Which introduced the microwave shielding method. But another experimental development was needed. When they added a second microwave field, the cooling became more efficient, and the cesium sodium finally crossed the BEC threshold, a goal the Weil Lab has achieved since opening in Columbia in 2018.

“This was a great finale for me,” said Bigagli, who graduated with a doctorate in physics this spring and was a founding member of the lab. “We went from not having a laboratory yet to these amazing results.”

In addition to reducing collisions, the second microwave field can also control the orientation of molecules. This, in turn, is a way to control how they interact, which the lab is currently exploring. “By controlling these dipole interactions, we hope to create new quantum states and phases of matter,” said Ian Stevenson, co-author and postdoctoral researcher at Columbia University.

A new world of quantum physics is opening up

Yi, a Boulder-based pioneer of ultracold science, considers the results a beautiful piece of science. “The work will have important implications for a number of scientific fields, including the study of quantum chemistry and the exploration of strongly coupled quantum materials,” he commented. “Weill's experiment features precise control of molecular interactions to direct the system toward a desired outcome, a remarkable achievement in quantum control technology.”

Meanwhile, the Columbia team is excited to have a theoretical description of intermolecular interactions validated experimentally. “We already have a good idea of ​​the interactions in this system, which is also crucial for next steps, such as exploring the physics of multipolar bodies,” Kerman said. “We came up with schemes to control the reactions, tested them theoretically, and implemented them in experiment. It was a really cool experience to see these ideas of microwave 'protection' realized in the laboratory.”

There are dozens of theoretical predictions that can now be tested experimentally using molecular BECs, which co-first author and doctoral student Siwei Zhang points out are quite stable. Most ultracold experiments are performed within one second, some are as short as a few milliseconds, but BEC molecular reactions in the laboratory last more than two seconds. “This will allow us to investigate open questions in quantum physics,” he said.

One idea is to create artificial Bose-Einstein crystals trapped in an optical lattice made of lasers. This would enable powerful quantum simulations that mimic interactions in natural crystals, Weil noted, and is a focus area in condensed matter physics. Quantum simulators are routinely made using atoms, but atoms have short-range interactions — where they must practically be on top of each other — which limits the extent to which they can model more complex materials. “Molecular BEC will deliver more flavor,” Weil said.

This includes dimensions, said co-first author and doctoral student Weijun Yuan. “We would like to use BECs in a 2D system. When you move from 3D to 2D, you can always expect new physics to emerge. 2D materials are a major area of ​​research at Columbia University; having a model system made of molecular BECs could help Weil and his colleagues in the matter Intensive on the exploration of quantum phenomena including superconductivity, superfluidity, and more.

“It seems like a whole new world of possibilities is opening up,” Will said.

Reference: “Observation of Bose-Einstein condensates of dipole particles” by Niccolò Bigagli, Weijun Yuan, Siwei Zhang, Boris Bulatovic, Tess Carman, Ian Stevenson, and Sebastian Weyl, 3 June 2024, nature.
doi: 10.1038/s41586-024-07492-z