Exploring the unknown: A unique quantum state of matter appears in Columbia

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

There’s a hot new BEC in town that has nothing to do with bacon, egg and cheese. You won’t find it in your local bodega, but in the coldest place in New York: the lab of Columbia physicist Sebastian Will, whose experimental group specializes in pushing atoms and molecules to temperatures just fractions of a degree above absolute zero.

Writing in NatureWill lab, supported by theoretical collaborator Tijs Karman at Radboud University in the Netherlands, has successfully created a unique quantum state of matter called a Bose-Einstein condensate (BEC) from molecules.

Progress in Bose-Einstein Condensates

Their BEC, cooled to just five nanoKelvin, or about -459.66 °F, and lasting for two incredibly long seconds, is made of sodium-cesium molecules. Like water molecules, these molecules are polar, meaning they carry both a positive and negative charge. The unbalanced distribution of electric charge facilitates the long-range interactions that make physics more interesting, Will noted.

The research that the Will lab is excited to pursue with their molecular BECs 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 BECs into simulators that can recreate the enigmatic quantum properties of more complex materials, such as solid crystals.

“Molecular Bose-Einstein condensates open up entirely new fields of research, from understanding really fundamental physics to advancing powerful quantum simulations,” he said. “This is an exciting achievement, but it’s really just the beginning.”

It’s a dream come true for the Will lab, and a dream 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 group of particles cooled to a near stop would coalesce into a single, larger superentity with common properties and behavior dictated by the laws of quantum mechanics. If BECs could be created, they would offer researchers a tantalizing platform to explore quantum mechanics on a scale more mobile than individual atoms or molecules.

It took about 70 years from those first theoretical predictions, but the first atomic BECs were created in 1995. The achievement was recognized with the Nobel Prize in Physics in 2001, just as Will was starting physics at the University of Mainz. in Germany. Laboratories now routinely make atomic BECs from several different types of atoms. These BECs have expanded our understanding of concepts such as the wave nature of matter and superfluids and led to the development of technologies such as quantum gas microscopes and quantum simulators, to name a few.

But atoms are, in the grand scheme of things, relatively simple. They are round objects and usually do not exhibit interactions that can arise from polarity. Ever since the first atomic BECs were made, scientists have wanted to create more complicated versions made of molecules. But even simple diatomic molecules composed of two atoms of different elements bonded together had proved tricky to cool below the temperature needed to form a proper BEC.

The first discovery came in 2008 when Deborah Jin and Jun Ye, physicists at JILA in Boulder, Colorado, cooled a gas of potassium-rubidium molecules to about 350 nanoKelvin. Such ultracold molecules have proven useful for performing quantum simulations and studying molecular collisions and quantum chemistry in recent years, but to break the BEC threshold, even lower temperatures were needed.

In 2023, the Will lab created the first ultracold gas of their molecule of choice, sodium-cesium, using a combination of laser cooling and magnetic manipulation, similar to Jin and Ye’s approach. To make it cooler, they brought a microwave.

Microwave Innovations

Microwaves are a form of electromagnetic radiation with a long history in Colombia. In the 1930s, physicist Isidor Isaac Rabi, who would receive the Nobel Prize in Physics, did pioneering work in microwaves that led to the development of airborne radar systems. “Rabi was one of the first to control the quantum states of molecules and was a pioneer of microwave research,” Will said. “Our work follows that 90-year tradition.”

While you may be familiar with the role of microwaves in heating up your food, it turns out that they can also facilitate cooling. Individual molecules have a tendency to collide with each other and, as a result, will form larger complexes that disappear from the samples. Microwaves can create tiny shields around each molecule that prevent them from colliding, an idea proposed by Karman, their collaborator in the Netherlands. With molecules protected against lossy collisions, only the hottest ones can be preferentially removed from the sample—the same physical principle that cools your coffee cup when you blow across its top, explained author Niccolò Bigagli. Those molecules that remain will be cooler and the overall temperature of the sample will drop.

The team came close to creating the molecular BEC last fall in work published in Nature Physics who introduced the microwave shielding method. But another experimental twist was necessary. When they added a second microwave field, the cooling became even more efficient, and the sodium-cesium finally passed the BEC threshold—a goal the Will lab had held since it opened in Columbia in 2018.

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

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

A new world for quantum physics opens up

You, a Boulder-based ultracold science pioneer, considers the results a beautiful piece of science. “The work will have significant impacts on a number of scientific fields, including the study of quantum chemistry and the exploration of strongly bound quantum materials,” he commented. “Will’s experiment features precise control of molecular interactions to steer 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 the interactions between molecules that has been validated experimentally. “We have a really good idea of ​​the interactions in this system, which is also critical for next steps like exploring many-body dipolar physics,” Karman said. “We have come up with schemes to control interactions, tested them in theory and implemented them in experiment. It was truly an amazing experience to see these microwave ‘shielding’ ideas being realized in the lab.”

There are dozens of theoretical predictions that can now be tested experimentally with molecular BECs, which co-first author and doctoral student Siwei Zhang noted are quite robust. Most ultracold experiments take place within a second—some as short as a few milliseconds—but the lab’s molecular BECs take more than two seconds. “This will really allow us to investigate open questions in quantum physics,” he said.

One idea is to create artificial crystals with BECs trapped in an optical lattice made by lasers. This would enable powerful quantum simulations that mimic the interactions in natural crystals, Will noted, which is an area of ​​focus in condensed matter physics. Quantum simulators are routinely made with atoms, but the atoms have short-range interactions—they practically have to be on top of each other—which limits how well they can model more complicated materials. “Molecular BEC will bring more flavor,” Will said.

This includes dimensionality, said co-first author and doctoral student Weijun Yuan. “We would like to use BEC in a 2D system. When you go from three dimensions to two, you can always expect new physics to emerge,” he said. 2D materials are a major area of ​​research at Columbia; Having a model system made of molecular BECs could help Will and his condensed matter colleagues explore quantum phenomena including superconductivity, superfluidity and more.

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

Reference: “Observation of Bose-Einstein Condensation of Dipolar Molecules” by Niccolò Bigagli, Weijun Yuan, Siwei Zhang, Boris Bulatovic, Tijs Karman, Ian Stevenson and Sebastian Will, 3 June 2024, Nature.
DOI: 10.1038/s41586-024-07492-z