Creation of first molecular Bose-Einstein condensate opens doors to new science

April 28, 2021
A colorized lattice of tornado-like vortices within a spinning Bose-Einstein condensate of rubidium atoms, imaged by the National Institute of Standards and Technology in 2007. (NIST)

A colorized lattice of tornado-like vortices within a spinning Bose-Einstein condensate of rubidium atoms, imaged by the National Institute of Standards and Technology in 2007. (NIST)

For the first time, molecules have been put in an extreme quantum state of matter known as a Bose-Einstein condensate, an experimental result that sets the stage for applications such as new kinds of chemistry and quantum computing.

A research team led by University of Chicago physicists achieved this physics "holy grail" by bonding pairs of cesium atoms in a Bose-Einstein condensate, an ultracold quantum gas of intertwined particles. The findings were published Wednesday in Nature.

This work was able to avoid losing molecules too quickly from collisions in a process called three-body loss, a problem that has crippled previous attempts, according to Tin-Lun Ho, a professor of mathematical and physical sciences at The Ohio State University.

"The finding is both surprising and exciting," Ho, who was not involved in the study, said in a November commentary on a preprint version of the research. "The method discovered by the Chicago group to circumvent this loss is therefore much welcome news."

A Bose-Einstein condensate is a state of matter involving the effects of quantum mechanics, in which the states of individual particles blend together at extremely low temperatures into a single unified gas. The state develops more wave-like properties that are described by a single equation known as a wavefunction. It was first created in 1995 after decades of theoretical research, for which the lead scientists of two research groups were given the 2001 Nobel Prize in Physics.

Since then, physicists have wanted to go one step further and create a condensate of molecules, said Cheng Chin, a professor of physics at the University of Chicago and a co-lead author of the new paper. 

"That has been an essential holy grail in this community for a few decades," Chin said.

Cooling molecules enough to tap into quantum effects can turn them into Bose-Einstein condensates, but reactive collisions that occur between the condensed molecules can break them apart.

An alternate approach has been to take atoms in condensate form and bond them to form molecules, and there have been recent promising results in this freezing quantum space: In a 2019 paper, for instance, potassium-rubidium molecules were pushed into a degenerate Fermi gas, which forms under similar conditions as Bose-Einstein condensates but has different properties.

In the new study, the physicists prepared a Bose-Einstein condensate of 60,000 cesium atoms in a magnetic field and an optical trap, which used highly focused lasers to hold the atoms in place. The whole system was cooled to about 10 billionths of a degree Celsius above absolute zero and was given angular momentum — in other words, the atoms were spinning.

To create cesium-cesium molecules, the team decreased the magnetic field to a strength near a Feshbach resonance, which aligns the energy level of two free atoms with that of a molecule and allows a bond to form between atoms. Most of the cesium atoms were left unpaired and were ejected from the trap.

"Feshbach resonance is a key component in our experiment to pair atoms into molecules," Chin said. 

What remained was a two-dimensional Bose-Einstein condensate of spinning cesium molecules that were in thermal equilibrium. According to the researchers, between 30% and 50% of the molecules took the form of a superfluid, which can flow without any resistance. The spinning of the molecules is interesting for future applications, Chin said.

The individual molecules survived for about 30 milliseconds, roughly 10 times faster than the blink of an eye but plenty of time for them to interact with each other in elastic scattering events, demonstrating that can survive collisions. 

The researchers observed the molecular state by splitting the cesium pairings back into individual atoms by increasing in the magnetic field. The resulting atoms were then detected and lent information to the researchers about their former molecular form. 

Molecular Bose-Einstein condensates hold potential in many scientific and technological fields, according to the study's authors, such as a new way to control chemical reactions and to make precise measurements.

Chin noted that ultracold molecules in this state could also be used in quantum data storage or possibly quantum computing. Their increased complexity over atoms or other simple particles means they could be used to store more information, with each configuration corresponding to a piece of information.

Chin and his colleagues are exploring other ways to configure the molecules within a Bose-Einstein condensate, and they have already been able to prepare some molecules into different states of motion as a condensate, according to the professor. He expects that other molecules will be put into the quantum state as soon as the next couple of years.

"I'm quite confident this will not be one singular case that we can compose," Chin said, referring to the cesium molecules. "This will be a new era, a new direction."

The study, "Transition from an atomic to a molecular Bose–Einstein condensate," published April 28 in Nature, was authored by Zhendong Zhang, Kai-Xuan Yao and Cheng Chin, University of Chicago; and Cheng Chin, Shanxi University.

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