A unique quantum state of matter arises in Columbia

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

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

To write Naturethe Will lab, supported by theoretical collaborator Tijs Karman from 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

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

Research that the Will lab is keen to pursue with their molecular BECs involves investigating 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 mimic the enigmatic quantum properties of more complex materials, such as solid crystals.

“Molecular Bose-Einstein condensates open up whole 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 one that’s been 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 near-standstill would merge into a single, larger superentity with shared properties and behaviors dictated by the laws of quantum mechanics. If BECs could be created, they would provide researchers with an attractive platform to investigate quantum mechanics on a more tractable scale than individual atoms or molecules.

It took about 70 years from those first theoretical predictions, but the first atomic BECs were created in 1995. This achievement was recognized with the Nobel Prize in Physics in 2001, just around the time Will got his start in physics at the University of Mainz. in Germany. Labs now routinely make atomic BECs from different types of atoms. These BECs have increased 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 arising from polarity. Ever since the first atomic BECs were realized, scientists have wanted to create more complicated versions of molecules. But even simple diatomic molecules made of two atoms of different elements bonded together proved difficult to cool 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 of potassium-rubidium molecules to about 350 nanoKelvin. Such ultracold molecules have proven useful in recent years for performing quantum simulations and studying molecular collisions and quantum chemistry, but exceeding the BEC threshold required even lower temperatures.

In 2023, the Will lab created the first ultracold gas from their favorite molecule, sodium-cesium, using a combination of laser cooling and magnetic manipulations, similar to Jin and Ye’s approach. To make it colder, they brought in microwaves.

Innovations with microwaves

Microwaves are a form of electromagnetic radiation with a long history in Columbia. In the 1930s, physicist Isidor Isaac Rabi, who would later receive the Nobel Prize in Physics, did pioneering work in microwaves, which 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 fits in with 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 make cooling down easier. Individual molecules tend to collide with each other and will therefore 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. Because the molecules are protected from lossy collisions, only the hottest ones can be preferentially removed from the sample – the same physics principle that cools your cup of coffee when you blow on it, explains 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 molecular BEC last fall in work published in Natural physics which 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 sodium-cesium eventually exceeded the BEC threshold—a goal the Will lab has been pursuing since it opened in Columbia in 2018.

“This was a fantastic ending for me,” says Bigagli, who received his PhD in physics this spring and co-founded the lab. “We have gone from not setting up a laboratory yet to these fantastic results.”

In addition to reducing collisions, the second microwave field can also manipulate the orientation of the molecules. That, in turn, is a way to determine how they interact 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,” says co-author and Columbia postdoc Ian Stevenson.

A new world is opening up for quantum physics

Ye, a pioneer of ultracold science based in Boulder, sees the results as 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 highly correlated quantum materials,” he said. “Will’s experiment provides precise control of molecular interactions to steer the system toward a desired outcome – a great achievement in quantum control technology.”

The Columbia team, meanwhile, is excited about a theoretical description of interactions between molecules that has been experimentally validated. “We have a really good picture of the interactions in this system, which is also crucial for the next steps, such as investigating dipolar many-body physics,” says Karman. “We devised schemes to control interactions, tested them in theory and implemented them in the experiment. It was truly an amazing experience to see these microwave shielding ideas realized in the laboratory.”

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

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

That includes dimensionality, said co-first author and PhD student Weijun Yuan. “We would like to use the 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 an important area of ​​research at Columbia; Having a model system made of molecular BECs could help Will and his condensed matter colleagues investigate quantum phenomena including superconductivity, superfluidity and more.

“It seems like it’s opening up a whole new world of possibilities,” says Will.

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, June 3, 2024, Nature.
DOI: 10.1038/s41586-024-07492-z