Quantum Coherence Survives in Ultracold Chemical Reaction

In the realm of the infinitesimal, where particles dance to the tune of quantum mechanics, a team of Harvard scientists has made a groundbreaking discovery. For the first time, they have demonstrated the survival of quantum coherence—the ability of particles to maintain phase relationships and exist in multiple states simultaneously—in a chemical reaction involving ultracold molecules. This finding, published in Science, opens the door to harnessing chemical reactions for future applications in quantum information science.

Led by Kang-Kuen Ni, Theodore William Richards Professor of Chemistry and Professor of Physics, the team delved into the intricate world of an atom-exchange chemical reaction involving 40K87Rb bialkali molecules. In this reaction, two potassium-rubidium (KRb) molecules combine to form potassium (K2) and rubidium (Rb2) products. By manipulating magnetic fields, the researchers prepared the initial nuclear spins in KRb molecules in an entangled state and then closely examined the outcome.

The Ultracold Frontier

To observe the quantum dynamics underlying the reaction process and outcome, the Ni Lab had to push the boundaries of temperature. Utilizing laser cooling and magnetic trapping, they cooled their molecules to a mere 500 nanoKelvin—just a fraction of a degree above Absolute Zero. In this ultracold environment, molecules slow down, enabling scientists to isolate, manipulate, and detect individual quantum states with remarkable precision.

This level of control allows for the observation of quantum effects such as superposition, entanglement, and coherence, which play fundamental roles in the behavior of molecules and chemical reactions. By employing sophisticated techniques, including coincidence detection, the researchers mapped and described the reaction products with unparalleled accuracy.

Quantum Coherence Preserved

The results revealed a surprising finding: quantum coherence was preserved within the nuclear spin degree of freedom throughout the reaction. This survival of coherence implied that the product molecules, K2 and Rb2, were in an entangled state, inheriting the entanglement from the reactants. Furthermore, by deliberately inducing decoherence in the reactants, the researchers demonstrated control over the reaction product distribution.

“I am extremely proud of our work investigating a very fundamental property of a chemical reaction where we really didn’t know what the result would be,” said Ni. “It was really gratifying to do an experiment to find out what Mother Nature tells us.”

First co-author and graduate student Lingbang Zhu sees the experiment as an opportunity to expand people’s understanding of chemical reactions in general. “We are probing phenomena that are possibly occurring in nature,” Zhu said. “We can try to broaden our concept to other chemical reactions. Although the electronic structure of KRb might be different, the idea of interference in reactions could be generalized to other chemical systems as well.”

Looking ahead, 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 happen in more warm and wet conditions,” Ni said. “That means there is a mechanism for chemical reactions that we just didn’t know about before.”

As scientists continue to unravel the mysteries of quantum mechanics in chemical reactions, this groundbreaking discovery by the Harvard team paves the way for a deeper understanding of the fundamental processes that govern our world and the potential for harnessing these phenomena in the realm of quantum information science.