Scientists demonstrate the survival of quantum coherence in a chemical reaction involving ultracold molecules

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If you zoom in on a chemical reaction at the quantum level, you will notice that particles behave like waves that can ripple and collide. Scientists have long tried to understand quantum coherence, the ability of particles to maintain phase relationships and exist in multiple states simultaneously; this is similar to synchronizing all parts of a wave. It was an open question whether quantum coherence could persist through a chemical reaction in which bonds break and form dynamically.

Now, for the first time, a team of Harvard scientists has demonstrated the persistence of quantum coherence in a chemical reaction involving ultracold molecules. These findings highlight the potential of harnessing chemical reactions for future applications in quantum information science.

“I am extremely proud of our work in investigating a very fundamental property of a chemical reaction where we really didn’t know what the outcome would be,” said senior co-author Kang-Kuen Ni, Theodore William Richards Professor of Chemistry and Professor of Chemistry . Physics. “It was really satisfying to do an experiment to find out what Mother Nature is telling us.”

In the newspaper, published in Sciencethe researchers describe how they studied a specific chemical reaction of atomic exchange in an ultra-cold environment 40K87Rb bialkali molecules, where two potassium rubidium (KRb) molecules react to form potassium (K2) and rubidium (Rb2) Products.

The team prepared the initial nuclear spins in KRb molecules in an entangled state by manipulating magnetic fields and then examined the outcome with specialized tools. In the ultracold environment, the Ni Lab was able to monitor the nuclear spin’s degrees of freedom and observe the intricate quantum dynamics underlying the reaction process and its outcome.

The work was performed by several members of Ni’s Lab, including Yi-Xiang Liu, Lingbang Zhu, Jeshurun ​​Luke, JJ Arfor Houwman, Mark C. Babin, and Ming-Guang Hu.

Using laser cooling and magnetic trapping, the team was able to cool their molecules to just a fraction of a degree above absolute zero. In this ultracold environment of just 500 nanoKelvin, molecules slow down, allowing scientists to isolate, manipulate and detect individual quantum states with remarkable precision. This control facilitates the observation of quantum effects such as superposition, entanglement and coherence, which play a fundamental role in the behavior of molecules and chemical reactions.

Using advanced techniques, including coincidence detection, which allows the researchers to pick out the exact pairs of reaction products from individual reaction events, the researchers were able to accurately map and describe the reaction products. Previously, they observed that the distribution of energy between the rotational and translational motion of the product molecules was chaotic. Therefore, it is surprising to find quantum order in the form of coherence in the same underlying reaction dynamics, this time in the degree of freedom of the nuclear spin.

The results showed that quantum coherence was maintained within the degree of freedom of the nuclear spin throughout the reaction. The persistence of coherence implied that the product molecules, K2 and Rb2, were in an entangled state and inherited the entanglement of the reactants. Furthermore, by deliberately inducing decoherence in the reactants, the researchers demonstrated control over the distribution of the reaction product.

In the future, Ni hopes to rigorously prove that the product molecules were entangled, and she is optimistic that quantum coherence can persist in non-ultracold environments.

“We believe the result is general and not necessarily limited to low temperatures and could occur in warmer and wetter conditions,” Ni said. “That means there is a mechanism for chemical reactions that we were not aware of before.”

First co-author and graduate student Lingbang Zhu sees the experiment as an opportunity to advance people’s understanding of chemical reactions in general.

“We are investigating phenomena that may occur in nature,” Zhu said. “We can try to extend our concept to other chemical reactions. Although the electronic structure of KRb may be different, the idea of ​​interference in reactions could also be generalized to other chemical systems.”

More information:
Yi-Xiang Liu et al., Quantum Interference in Atomic Exchange Reactions, Science (2024). DOI: 10.1126/science.adl6570.

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