Controlling chemical reactions in the quantum regime

Debbie Jin and Jun Ye, in “Polar molecules in the quantum regime” (PHYSICS TODAY, May 2011, page 27 ), provide an enlightening overview of their recent work on cold molecular gases; they note a significant opportunity to study and manipulate chemical reactions in a regime in which quantum effects are important. In the case of their potassium–rubidium system, they regard the observed reaction KRb + KRb → K₂ + Rb₂ as being of great interest but also as being a significant impediment to the goal of preparing a quantum gas of oriented KRb molecules.

The cold gas that Jin and Ye have created does indeed offer a unique environment to study chemical reactions that are strongly affected by quantum mechanics. One such opportunity, not noted in their article, is the ability to manipulate reaction cross sections by “coherent control.” ¹ In that approach one creates an initial superposition of scattering states that allows control over reaction cross sections through quantum interference.

Two coherent control scenarios are worth examining toward the goal of controlling or reducing the KRb + KRb reactive cross section. The first was designed to control cross sections in the scattering of identical diatomic molecules, and the second is a method capable of suppressing reactive scattering by suitable preparation of the initial scattering state. Both are discussed in reference .

Demonstrating coherent control of collisional processes in the cold KRb gas would contribute greatly to understanding and manipulating chemical reactions on a fundamental quantum level. The use of such control to suppress the KRb + KRb reaction, if successful, would be an added technological benefit on the way to producing the desired quantum gas of oriented molecules.