Quantum-State Controlled Chemical Reactions of Ultracold KRb Molecules (0912.3854v1)

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.

• The paper reveals that p-wave scattering dominates in ultracold KRb molecules due to angular momentum barriers, aligning with universal loss predictions.
• It uses precise optical trapping and cooling techniques to prepare KRb molecules, validating quantum threshold laws and long-range interaction effects.
• The study finds that controlling internal quantum states increases reaction rates by 10–100 times, with implications for quantum simulation and precision measurements.

Quantum-State Controlled Chemical Reactions of Ultracold KRb Molecules

The paper investigates chemical reactions controlled at the quantum state level, focusing on ultracold polar $^{40}$K$^{87}$Rb molecules. Through meticulous experimentation, the authors explore how chemical reactions unfold under conditions where traditional thermal energies are negligible, and quantum mechanical principles such as quantum statistics and threshold laws dominate the interaction dynamics.

The experimental setup involves preparing a gaseous medium of $^{40}$K$^{87}$Rb molecules in their absolute ground state under ultra-cold conditions (a few hundreds of nanoKelvins) within an optical trap. Such a condition is conducive to studying novel molecular reactivity dynamics characterized predominantly by quantum mechanical interactions and angular momentum barriers. The molecules, being fermionic, exhibit distinctive collisional characteristics governed by symmetry considerations, notably leading to $p$-wave-dominated scattering when they are prepared in a homogeneous quantum state.

Key findings include the observation of $p$-wave scattering dynamics, where molecules must tunnel through a centrifugal barrier, generating a near-unity probability of short-range chemical reactions. At these temperatures, allowing the molecules to reside in multiple internal states boosts reaction rates by orders of magnitude (10 to 100 times) through $s$-wave collisions devoid of centrifugal barriers. The experimental results align well with theoretical predictions, precisely with universal loss rates predicted, based on the van der Waals length, demonstrating the significant influence of long-range interactions on ultracold chemical reaction rates.

Implications of these findings are profound both theoretically and practically:

1. Theoretical Insight: The paper extends the understanding of chemical reactions into a regime traditionally unexplored, where quantum mechanical effects are not merely perturbative influences but the primary drivers of molecular reactivity. It compellingly demonstrates the applicability of quantum threshold laws and the Bethe-Wigner model to chemical reactions in ultracold regimes.
2. Practical Applications: The implications for future technologies are substantial. Controlling molecular interactions at the quantum level opens prospects in quantum simulation and computation, as ultracold molecules could be harnessed to implement models of quantum many-body systems. Moreover, these findings could inform the development of new quantum sensors or precision measurement techniques where controlled reactivity and coherence are critical.
3. Universal Collision Dynamics: The research identifies universal characteristics in ultracold reactions, suggesting that similar mechanisms could apply to other molecular systems and open new exploration avenues for studying collisions and interactions under exotic conditions.
4. Experimental Methodology: The complexity involved in achieving precise control over such low-temperature systems could propel advancements in laser cooling, optical trapping, and molecule manipulation techniques, all of which are essential for further quantum technology advancements.

In summary, this paper contributes significantly to molecular physics by elucidating the dynamics of chemical reactions from a quantum mechanical perspective and establishing a framework for future experimentation and technology development at the confluence of chemistry and quantum physics. These findings lay the groundwork for exploring and unlocking new phases of matter and molecular processes in the quantum degenerate regime.