Reaction Interferometry to Probe Nuclear Spin Dynamics in Ultracold Reactions

With support from the Chemical Structure and Dynamics (CSD) program in the Chemistry Section, Professor Kang-Kuen Ni of Harvard University is investigating coherent control in reactions of extremely cold molecules where the quantum state of the reactants can be carefully prepared and reaction outcomes accurately measured. So far, researchers have shown that certain properties, like nuclear spin and quantum coherence, are preserved during reactions that start from very specific quantum conditions. However, it’s still unknown whether this holds true when the molecules begin in more complex or arbitrary quantum states. To find out, Professor Ni and her team will design special experiments—"reaction interferometer"—to test what happens when molecules start from a variety of quantum states, including those with deliberately added phase controls. This will help them understand when quantum coherence is maintained or lost, and what factors control the behavior of nuclear spins during the reaction. The studies could also include the possibilities of engineering chemical entangled pairs for quantum communication or as a new mechanism of chemical reactions for biological signaling and regulation. Students and postdoctoral researchers working on this project will gain comprehensive training in quantum science knowledge and techniques. Professor Ni’s research group also gives undergraduate students a chance to be involved in cutting-edge scientific work. Coherent control of reactions at the quantum state level has long been a goal in chemistry. Recent advances in ultracold molecule techniques now enable quantum state-selective preparation of reactants and detection of products in unconventional species such as bi-alkalis. Remarkably, nuclear spin coherence can survive the atom-exchange reaction 2 KRb → K₂ + Rb₂, making this system a promising platform to demonstrate coherent control and probe the role of quantum phases in reactions. Professor Ni and her students will build a reaction interferometer that splits a cloud of reactant molecules, imprints a relative phase via microwave driving, and then recombines the clouds to measure collision outcomes. By preparing nuclear spin superposition states with varying phases, the experiments will reveal how coherence and quantum phases influence reactions. This work offers a new window into quantum coherence, nuclear spin dynamics, and the potential to control reaction outcomes through phase manipulation. The project bridges atomic physics and physical chemistry, encouraging cross-disciplinary collaboration. It will also train early-career researchers, preparing them to contribute to the workforce and advance the frontiers of quantum science. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria. NSF Award ID: 2517644 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Kang-Kuen Ni | Institution: Harvard University, CAMBRIDGE, MA | Award Amount: $648,877 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2517644 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2517644.html