Quantum-State Controlled Chemical Reactions of Ultracold KRb Molecules

TL;DR

This study experimentally investigates ultracold KRb molecules, demonstrating quantum state-controlled chemical reactions at nanoKelvin temperatures, with reaction rates aligning with quantum threshold laws and universal loss predictions.

Contribution

It provides the first experimental evidence linking quantum mechanical rules to chemical reactivity in ultracold molecules, highlighting the role of quantum statistics and scattering partial waves.

Findings

Reactions are dominated by p-wave quantum threshold collisions in single quantum states.

Reaction rates increase significantly when molecules are in different internal states or mixed with atoms.

Measured rates match theoretical universal loss rate predictions based on van der Waals interactions.

Abstract

How does a chemical reaction proceed at ultralow temperatures? Can simple quantum mechanical rules such as quantum statistics, single scattering partial waves, and quantum threshold laws provide a clear understanding for the molecular reactivity under a vanishing collision energy? Starting with an optically trapped near quantum degenerate gas of polar $^{40}$K$^{87}$Rb molecules prepared in their absolute ground state, we report experimental evidence for exothermic atom-exchange chemical reactions. When these fermionic molecules are prepared in a single quantum state at a temperature of a few hundreds of nanoKelvins, we observe p-wave-dominated quantum threshold collisions arising from tunneling through an angular momentum barrier followed by a near-unity probability short-range chemical reaction. When these molecules are prepared in two different internal states or when molecules and atoms are brought together, the reaction rates are enhanced by a factor of 10 to 100 due to s-wave scattering, which does not have a centrifugal barrier. The measured rates agree with predicted universal loss rates related to the two-body van der Waals length.