Meetings: Building a Molecule Atom by Atom

Physics 11, 60
Researchers demonstrated the simplest possible chemical reaction by using optical tweezers to bring together two atoms to form a molecule.
Y. Liu/Harvard University.
Researchers have built a single molecule by bringing together two atoms with optical tweezers.

Most of what we know about chemical reactions comes from experiments that mix together astronomical numbers of molecules. But while this approach gives ensemble averages of parameters like reaction rates, it misses certain details, such as whether the reaction dynamics differ for different quantum states of the same molecule. Researchers at Harvard University have now succeeded in stripping a chemical reaction down to its minimal components. The team built a single molecule from two atoms that they trapped and brought together with focused laser beams known as optical tweezers.

“With this scheme, we can control a chemical reaction at the level of single atoms, without depending on random encounters between the atoms,” said Lee Liu, a Ph.D. student in the Harvard group, who presented the result at the American Physical Society’s meeting of the Division of Atomic, Molecular and Optical Physics (DAMOP). The team, which is led by Kang-Kuen Ni, reported their findings in Science.

The group’s setup, which they dubbed an ultracold molecular assembler, allows them to trap laser-cooled atoms and molecules while precisely controlling the particles’ positions and quantum states. To make their molecule, the researchers first cooled atoms of sodium (Na) and cesium (Cs) to below 100 𝜇K—a temperature low enough to manipulate the atoms with optical tweezers. They then trapped exactly one atom of each species in separate tweezers. (The tweezers’ wavelengths can be chosen to selectively trap a certain type of atom.) Crossing the Cs tweezers with the Na tweezers, and turning off the latter, left exactly one atom of each species in the roughly micrometer-wide waist of the Cs tweezers. Finally, the team shined an infrared laser on the two trapped atoms in order to give them enough energy to form a NaCs molecule—a process known as photoassociation.

The team confirmed the formation of the molecule in two ways. First, they monitored the fluorescent light from the cesium and sodium atoms, which disappeared once the atoms had bonded together into a molecule. The second piece of evidence was provided by the detection of characteristic vibrational lines in the infrared spectrum of the NaCs molecule.

Jun Ye, who studies cold atoms and molecules at the National Institute of Standards and Technology and the University of Colorado, both in Boulder, described the researchers’ approach as unique. “This is the ultimate level of control of a chemical reaction,” he said. “[It] may allow us to develop a full quantum-mechanical description of chemical processes.”

Liu said he is intrigued by these possibilities in fundamental chemistry. But he raved more about using the method for quantum computing technologies. “This molecule has the potential to be an exceptional qubit,” he said. Liu envisions storing information in two of the molecule’s hyperfine states, which are long-lived and insensitive to environmental disturbances. This information could then, with the help of applied microwaves, be transferred to the molecule’s rotational states. NaCs is a dipolar molecule, and it can strongly interact with other dipolar molecules. This coupling could be used to transfer the information stored in an excited rotational state of NaCs to another molecule in its ground state, providing a way to perform logical operations.

Liu is eager to get to work pursuing this molecular qubit idea. He explained that the first challenge will be finding a way to put the molecule into a single quantum state. “We think we could soon do it with the help of additional lasers that coherently transfer the molecule to a specific vibrational level,” he said.

–Matteo Rini

Matteo Rini is the Deputy Editor of Physics.

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