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Physics Today

Controlling chemical reactions in the quantum regime

Paul W. Brumer, Moshe Shapiro

Published Dec 1, 2011• 3 citations
Publisher

Abstract

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.”11. M. Shapiro, P. Brumer, Principles of the Quantum Control of Molecular Processes, Wiley, New York (2003); Quantum Control of Molecular Processes, Wiley-VCH, Weinheim, Germany, in press. 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 11. M. Shapiro, P. Brumer, Principles of the Quantum Control of Molecular Processes, Wiley, New York (2003); Quantum Control of Molecular Processes, Wiley-VCH, Weinheim, Germany, in press. .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.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. M. Shapiro, P. Brumer, Principles of the Quantum Control of Molecular Processes, Wiley, New York (2003); Google ScholarQuantum Control of Molecular Processes, Wiley-VCH, Weinheim, Germany, in press. Google Scholar© 2011 American Institute of Physics.

DOI
10.1063/pt.3.1345

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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.”11. M. Shapiro, P. Brumer, Principles of the Quantum Control of Molecular Processes, Wiley, New York (2003); Quantum Control of Molecular Processes, Wiley-VCH, Weinheim, Germany, in press. 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 11. M. Shapiro, P. Brumer, Principles of the Quantum Control of Molecular Processes, Wiley, New York (2003); Quantum Control of Molecular Processes, Wiley-VCH, Weinheim, Germany, in press. .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.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. M. Shapiro, P. Brumer, Principles of the Quantum Control of Molecular Processes, Wiley, New York (2003); Google ScholarQuantum Control of Molecular Processes, Wiley-VCH, Weinheim, Germany, in press. Google Scholar© 2011 American Institute of Physics.
We consider ultracold, chemically reactive scattering collisions of the diatomic molecules KRb. When two such molecules collide in an ultracold gas, we find that they are energetically forbidden from reacting to form the trimer species K2Rb{\mathrm{K}}_{2}\mathrm{Rb}K2​Rb or Rb2K{\mathrm{Rb}}_{2}\mathrm{K}Rb2​K, and hence can only react via the bond-swapping reaction $2\mathrm{KRb}\ensuremath{\rightarrow}{\mathrm{K}}_{2}+{\mathrm{Rb}}_{2}$. Moreover, the tiny energy released in this reaction can in principle be set to zero by applying electric or microwave fields, implying a means of controlling the available reaction channels in a chemical reaction.
Ultracold polar molecular quantum gases promise to open new research directions ranging from the study of ultra-cold chemistry, precision measurements to novel quantum phase transitions. Based on the preparation of high-phase space density gases of polar KRb molecules [1–3], I will discuss the control of dipolar collisions and chemical reactions of polar molecules in a regime where quantum statistics, single scattering partial waves, and quantum threshold laws play a dominant role [4]. In particular, I will point out the crucial role of electric dipole-dipole interactions [5] and external confinement [6] in determining the chemical reaction rate. Finally, I will discuss prospects of reaching quantum degeneracy in bialkali samples of polar molecules and prospects for these systems as novel dipolar quantum many-body systems.
Studying ultracold chemical reactions with quantum state resolution reveals details of the reaction processes, especially with single quantum state preparation of the reactants and state-selective detection of reaction products. In this thesis, we investigate two main processes: KRb + KRb → K2 + Rb2 reactions, and Rb and KRb atom-molecule collisions. In the KRb bimolecular reaction, we demonstrate the preservation of quantum coherence in bimolecular reactions for the first time, which is surprising in a reaction considered to be largely chaotic. Additionally, we proposed a coherent control scheme to manipulate product yields across different product channels. These results represent a critical step toward probing quantum coherence and entanglement in the chemical reaction. In atom-molecule collisions between Rb and KRb, we discovered an exceptionally long-lived intermediate complex, KRb2, which can be photo-excited by trapping light. The observed complex lifetime was orders of magnitude longer than theoretical predictions, underscoring the limitations of the current theory. We further explored the dependence of complex lifetimes on the initial quantum states and external electric and magnetic fields, providing additional experimental benchmarks to guide future theoretical work. Separately, we found that inelastic collisions between hyperfine-excited Rb atoms and KRb molecules can lead to rotation excitation of KRb post-collision. By probing the product state distribution, the result suggests that mechanical angular momentum is coupled to the spins. Such couplings are too weak in the current theory models to explain the result. Moreover, our result contradicts state-of-the-art coupled-channel calculations. These suggest that some subtle effects, such as molecular vibration and conical intersections, play a critical role in the reaction dynamics. The final piece of the thesis describes the observation of resonant interaction between a Rydberg atom and an ensemble of polar molecules. Such hybrid quantum systems have been proposed for a wide range of applications. Our work provides an experimental demonstration of resonant dipolar interactions between Rydberg atoms and ultracold polar molecules, paving the way for the realization of hybrid systems for quantum computation and simulation.
The creation of a gas of ultracold polar molecules with a high phase space density brings new possibilities beyond experiments with ultracold atomic gases. In particular, long-range, anisotropic, and tunable dipole-dipole interactions open the way for novel quantum gases, with applications including strongly correlated many-body systems, and ultracold chemistry. This thesis will present the final steps to complete control over both internal and external degrees of freedom of the molecule which allows us to control, and even completely suppress, the chemical reactions between molecules. First, the control over internal states has been achieved through coherent state transfer to the ro-vibronic ground state and coherent manipulations of the hyperfine and rotational states with microwave radiation. Second, external degrees of freedom are controlled by loading the gas into an optical lattice. With the molecules loaded into a one-dimensional lattice, the orientation of the molecular collisions is controlled by manipulating both internal (hyperfine states) and external (motional states in the direction of tight confinement) degrees of freedom. Most striking is that by preparing the molecules all in the lowest band of the lattice in the same internal state, the molecular collisions can only occur in a side-by-side" orientation, where the chemical reaction rate is suppressed by the repulsive dipole-dipole interactions. The chemical reaction can be suppressed completely by further constraining the motion in the trap in a strong 3D lattice. Here we see lifetimes longer than 20 s, limited by o-resonant light scattering. Finally, the ac polarizability of the molecules is explored and controlled. The different rotational states of the molecule have different polarizabilities and will experience a different trapping force in both the optical dipole trap or lattice. We show that there is a magic angle" between the quantization axis and the polarization of the trapping laser at which the polarizabilities of two different rotational states can be matched, eliminating dephasing and allowing for coherent manipulations between rotational states.
Fundamental entanglement related challenges have prevented quantum-interference-based control (i.e., coherent control) of collisional cross sections from being implemented in the laboratory. Here, differential cross sections for reactive scattering at low temperatures are shown to provide a unique opportunity to display such interference-based control by forming coherent superpositions of degenerate rotational states of reactant molecules $|jm\ensuremath{\rangle}$ with different mmm. In particular, we identify and quantify a unique signature of coherent control in reactive scattering with applications to $\mathrm{F}+ {\mathrm{H}}_{2}\ensuremath{\rightarrow}$ H +HF+\mathrm{HF}+HF and HF +++ D $\ensuremath{\leftarrow}$ F $+\mathrm{HD} \ensuremath{\rightarrow}\mathrm{HD} +$ F at 11K11\phantom{\rule{4pt}{0ex}}\mathrm{K}11K. Control is shown to be extensive.
Ultracold molecules offer unprecedented opportunities for the controlled interrogation of molecular events, including chemical reactivity in the ultimate quantum regime. The proliferation of methods to create, cool, and confine them has allowed the investigation of a diverse array of molecular systems and chemical reactions at temperatures where only a single partial wave contributes. Here we present a brief account of recent progress on the experimental and theoretical fronts on cold and ultracold molecules and the opportunities and challenges they provide for a fundamental understanding of bimolecular chemical reaction dynamics.