Chemical Physics Seminar

Quantum Phases in Chemical Reactions from Ultracold Chemistry

Ultracold chemistry, where exotic chemical species such as NaCs or KRb are assembled with precise quantum-state control, provides a new paradigm for studying chemical reactions. I will share three interconnected short stories. First, I introduce our effort to move beyond the usual paradigm of chemical reactions that proceed via stochastic encounters between reactants, toward a single, controlled reaction of exactly two atoms [Science 360, 900 (2018)]. These individual NaCs molecules, built in optical tweezers, have been entangled and used as the basis for a quantum logic gate [Nature 637, 821 (2025)].

Second, chemical reactions—processes that involve the breaking and forming of bonds—are inherently quantum dynamical. Can coherence be preserved during a chemical reaction and subsequently harnessed to produce entangled products? To gain insight, we investigate atom-exchange reactions (2KRb → K₂ + Rb₂) at a temperature of 500 nK and observe clear signatures of quantum interference [Science 384, 1117 (2024)].

Third, while nuclear spins are usually not thought to participate actively in chemical reactions, we now have an opportunity to examine this general assumption in the context of a new puzzle where certain reaction complexes live for timescales five orders of magnitude longer than expected. We speculate that quantum interference may be at play.

Kang-Kuen Ni is the T. W. Richards Professor of Chemistry and Professor of Physics at Harvard University. Her research group uses molecules as quantum building blocks of complex systems and harnesses chemical reactions as quantum resources. Notable achievements include developing techniques to bring selected molecules to a standstill and orchestrate them into quantum logic gates, as well as inventing new interrogation schemes to probe the quantum nature, including entanglement, in reactions.