A unique quantum state of matter appears in Columbia

Physicists in Columbia University they pushed 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, eggs and cheese. You won’t find it at your local wine bar, but at the coldest place in New York: the lab of Colombian physicist Sebastian Will, whose experimental group specializes in pushing atoms and molecules to temperatures just fractions of a degree higher. absolute zero.

Recording Naturethe Will 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.

Breakthrough in Bose-Einstein condensates

Cooled to just five nanoKelvins, or about -459.66°F, and stable for a surprisingly long two seconds, their BEC is made of sodium and 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 up the most interesting physics, Will noted.

The research that Will’s lab is enthusiastically pursuing with its molecular BECs involves exploring a number of different quantum phenomena, including new types of superfluidity, a state of matter that flows without any friction. They also hope to turn their BECs into simulators that can recreate the mysterious quantum properties of more complex materials such as solid crystals.

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

It’s a dream come true for the Will lab, and one that’s been decades in the making for the larger ultracold research community.

Ultracold molecules, a century in the making

The science of BEC 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 near rest 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 an enticing platform to explore quantum mechanics on a more manageable scale than individual atoms or molecules.

It took about 70 years from the first theoretical predictions, but the first atomic BECs were created in 1995. This achievement was awarded 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 have led to the development of technologies such as quantum gas microscopes and quantum simulators, to name a few.

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

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

In 2023, Will’s lab created the first ultracold gas of its choice, sodium-cesium, using a combination of laser cooling and magnetic manipulations, similar to Jin and Ye. They brought microwaves to keep it cool.

Innovation with microwaves

Microwaves are a form of electromagnetic radiation with a long history in Columbia. In the 1930s, physicist Isidor Isaac Rabi, who later received the Nobel Prize in Physics, did 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 pioneered microwave research,” Will said. “Our work continues this 90-year tradition.”

While you may be familiar with the role of microwaves in heating food, it turns out they can also facilitate cooling. Individual molecules tend to bump into each other and as a result will form larger complexes that disappear from the samples. Microwaves can create small shields around each molecule to prevent them from colliding, an idea proposed by Karman, their collaborator in the Netherlands. With molecules protected against loss collisions, only the hottest ones can be preferentially removed from the sample – the same physical principle that cools your coffee cup when you blow on it, explained author Niccolò Bigagli. The molecules that remain will be cooler 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 which introduced the microwave shielding method. But another experimental twist was necessary. When they added a second microwave array, the cooling became even more efficient, and sodium-cesium finally crossed the BEC threshold—a goal the Will lab has championed since opening at Columbia in 2018.

“It was fantastic closure 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 lab set up 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 their interaction with each other, which the lab is currently investigating. “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 is opening up for quantum physics

Ye, a Boulder-based ultracold science pioneer, sees the results as a beautiful piece of science. “The work will have a significant impact on a number of scientific fields, including the study of quantum chemistry and the investigation of strongly correlated quantum materials,” he said. “Will’s experiment represents the precise control of molecular interactions to direct a system to a desired outcome—a stunning achievement in quantum control technology.”

Meanwhile, the Columbia team is excited to have a theoretical description of the interactions between molecules that has been verified experimentally. “We have a really good understanding of the interactions in this system, which is also important for next steps, such as investigating many-body dipolar physics,” Karman said. “We came up with interaction control schemes, tested them theoretically and implemented them in an experiment. It was truly an amazing experience to see these ideas for microwave ‘shielding’ being realized in the laboratory.”

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

One idea is to create artificial crystals with BECs trapped in an optical grating made of lasers. This would enable powerful quantum simulations that mimic the interactions in natural crystals, Will noted, an area of ​​interest in condensed matter physics. Quantum simulators are commonly 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 complex materials. “Molecular BEC will introduce 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 seems 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, 03 Jun 2024, Nature.
DOI: 10.1038/s41586-024-07492-z