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Physical Chemistry

Optical tweezers trap two atoms in arranged marriage

Fundamental advance could provide quantum-level insight into chemical reactivity

by Stu Borman
April 12, 2018 | A version of this story appeared in Volume 96, Issue 16

Schematic shows combining of two atoms in separate traps into one trap and zapping them with light, inducing them to form a single molecule.
Credit: Lee R. Liu
To build a single molecule from one Na (orange) atom and one Cs (blue) atom, researchers isolated them in optical traps (black parabolas), merged them into a single trap, and excited them vibrationally by zapping them with infrared light (yellow).

Harvard researchers have made the remarkable discovery that if you bring two separate atoms together, they can react to form something called a molecule.

Put that way, the achievement may not sound too amazing. Expert opinion, however, considers the single-molecule study to be truly groundbreaking.

Most chemical reactions between two atoms involve chance encounters in large ensembles of atomic reactants that result in productive addition reactions. But Kang-Kuen Ni and her coworkers didn’t wish to leave anything to chance.

They used optical tweezers—laser beam devices that exert forces on atoms that prevent them from moving—to trap two supercooled atoms, sodium and cesium, in separate compartments. When they then merged the separated atoms into one of the compartments and excited them vibrationally with infrared radiation, the atoms combined into an excited-state species, NaCs*(Science 2018, DOI: 10.1126/science.aar7797). It lasts about 30 nanoseconds and then decays to an electronic ground state. It’s an arranged marriage; each of the participants gets a chance to meet only one partner.

The work is a fundamental advance that reduces chemical reaction dynamics to an ultimate level of simplicity. However, the technique could also lead to a better understanding of the detailed steps by which molecular complexity arises from atomic building blocks. And single molecules trapped in optical tweezers could be useful as qubits, information units for quantum computing.

Jun Ye, an expert on quantum light-matter interactions at JILA and the University of Colorado, Boulder, comments that this is the first time scientists have demonstrated a process that produces an individual molecule from two separate optically trapped atoms. “Future work along this line could provide important insights into the complex process of a chemical reaction as a fully controlled quantum process,” Ye says.

Ni says her group now hopes to gain complete quantum control over the internal vibrations and external motion of single molecules—to help reveal the quantum nature of chemical reactions and develop single molecules as qubits.


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