Physicists take molecules to new ultracold limit, creating a state of matter where quantum mechanics reigns

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

Write in Naturethe Will lab, supported by theoretical collaborator Tijs Karman from Radboud University in the Netherlands, succeeded in creating a unique quantum state of matter called Bose-Einstein condensate (BEC) from molecules.

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

The research that the Will lab is excited to pursue with their molecular BECs includes the exploration of a number of different quantum phenomena, including new types of superfluidity, a state of matter that flows without experiencing friction. They also hope to turn their BECs into simulators capable of recreating the enigmatic quantum properties of more complex materials, such as solid crystals.

“Bose-Einstein molecular condensates open up entirely new areas of research, from understanding truly 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 ultracold temperature research community as a whole.

To make it colder, add microwaves

Microwaves are a form of electromagnetic radiation with a long history in Colombia. In the 1930s, physicist Isidor Isaac Rabi, who would go on to receive the Nobel Prize in Physics, carried out pioneering work on 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 in microwave research,” Will said. “Our work is part of this 90-year-old tradition.”

While you may be familiar with the role of microwaves in reheating your food, it turns out they can also make cooling easier. Individual molecules tend to collide with each other and thus form larger complexes which disappear from the samples. Microwaves can create small shields around each molecule that prevent them from colliding, an idea proposed by Karman, their collaborator in the Netherlands.

Because the molecules are protected from lossy collisions, only the hottest ones can be preferentially removed from the sample – the same physical principle that cools your cup of coffee when you blow on it, explained author Niccolò Bigagli. The remaining molecules will be colder and the overall temperature of the sample will drop.

The team came close to creating a molecular BEC last fall in work published in Natural physics who introduced the microwave shielding method. But another experimental twist was needed. When they added a second microwave field, the cooling became even more efficient and the sodium-cesium finally crossed the BEC threshold – a goal the Will lab had harbored since opening at Columbia in 2018.

“It was fantastic closure for me,” said Bigagli, who graduated with his Ph.D. in physics this spring and was a founding member of the lab. “We went from no lab to these fantastic results.”

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

A new world for quantum physics opens up

Ye, a Boulder-based pioneer of ultracold science, calls the results a fine piece of science. “This work will have significant impacts on a number of scientific fields, including the study of quantum chemistry and the exploration of highly correlated quantum materials,” he commented. “Will’s experiment involves precise control of molecular interactions to steer the system toward a desired outcome: a marvelous achievement in quantum control technology.”

The Columbia team, for its part, is delighted to have a theoretical description of interactions between molecules that has been validated experimentally. “We really have a good idea of ​​the interactions in this system, which is also essential for next steps, like exploring many-body dipolar physics,” Karman said. “We developed schemes to control the interactions, tested them in theory, and implemented them in the experiment. It was truly an incredible experience to see these ideas for microwave ‘shielding’ come to fruition in laboratory.”

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

One idea is to create artificial crystals with the BECs trapped in an optical array made of lasers. This would enable powerful quantum simulations mimicking interactions in natural crystals, Will noted, which is an area of ​​interest in condensed matter physics.

Quantum simulators are typically made with atoms, but atoms have short-range interactions (they practically have to be on top of each other), which limits their ability to model more complex materials. “The molecular BEC will introduce more flavor,” Will said.

This includes dimensionality, said the co-first author and doctoral student. student Weijun Yuan. “We would like to use BECs in a 2D system. When you go from three dimensions to two dimensions, 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 seems like a whole new world of possibilities is opening up,” Will said.