Observation of Quantum Interference and Coherent Control in a Photochemical Reaction

Weak-Field Coherent Control of Ultrafast Molecule Making

Coherent control of ultrafast molecule making from colliding reactants is crucial for realizing coherent control of binary photoreactions (CCBP). To handle diverse excitation scenarios, feasibility with both weak and strong fields is essential. We experimentally demonstrate here the weak-field feasibility, achieving it even under thermally hot conditions typical of chemical reactions. The making of KAr complexes from hot pairs of colliding K and Ar atoms via resonance-mediated two-photon excitation is controlled by weak linearly chirped femtosecond pulses. Negative chirps enhance the yield. Our experimental and ab initio theoretical results are in excellent agreement. New routes to CCBP are opened.

Reaction interferometry with ultracold molecules

We propose to coherently control the ultracold 2KRb → K2 + Rb2 reaction product state distribution via quantum interference. By leveraging that the nuclear spin degrees of freedom in the reaction maintain coherence, which was demonstrated in Liu, Zhu et al., arXiv, 2023, arXiv:2310.07620, https://doi.org/10.48550/arXiv.2310.07620, we explore the concept of a "reaction interferometer". Such an interferometer involves splitting one KRb molecular cloud into two, imprinting a well-defined relative phase between them, recombining the clouds for reactions, and measuring the product state distribution. We show that the interference patterns provide a mechanism to coherently control the product states, and specific product channels also serve as an entanglement witness of the atoms in the reactant KRb molecule.

Quantum interference in atom-exchange reactions

Chemical reactions, where bonds break and form, are highly dynamic quantum processes. A fundamental question is whether coherence can be preserved in chemical reactions and then harnessed to generate entangled products. Here we investigated this question by studying the 2KRb → K2 + Rb2 reaction at 500 nK, focusing on the nuclear spin degrees of freedom. We prepared the initial nuclear spins in KRb in an entangled state by lowering the magnetic field to where the spin-spin interaction dominates and characterized the preserved coherence in nuclear spin wavefunction after the reaction. We observed an interference pattern that is consistent with full coherence at the end of the reaction, suggesting that entanglement prepared within the reactants could be redistributed through the atom-exchange process.

Constructive Quantum Interference in Photochemical Reactions

Interferences emerge when multiple pathways coexist together, leading toward the same result. Here, we report a theoretical study for a reaction scheme that leads to constructive quantum interference in a photoassociation (PA) reaction of a ⁸⁷Rb Bose-Einstein condensate where the reactant spin state is prepared in a coherent superposition of multiple bare spin states. This is achieved by changing the reactive scattering channel in the PA reaction. As the origin of coherent control comes from the spin part of the wavefunction, we show that it is sufficient to use radio frequency (RF) coupling to achieve the superposition state. We simulate the RF coupling on a quantum processor (IBMQ Lima), and our results show that interferences can be used as a resource for the coherent control of photochemical reactions. The approach is general and can be employed to study a wide spectrum of chemical reactions in the ultracold regime.

The diatomic molecular spectroscopy database

Motivation

The spectroscopy of diatomic molecules is an important research area in chemical physics due to its relevance in astrochemistry, combustion chemistry, and ultracold physics. However, there is currently no database where the user can easily retrieve, in a useful format, the spectroscopic constants of a given molecule. A similar situation appears concerning the vibrational Franck–Condon factors for diatomic molecules, a crucial parameter to infer laser cooling prospects for molecules. To address this problem, and inspired by the idea that data should be open and freely accessible, we have developed a user-friendly website (https://rios.mp.fhi.mpg.de) where the user can retrieve spectroscopic constants and Franck–Condon factors in useful formats.

Implementation

In this database, the spectroscopic constants of the ground states and first excited states of the diatomic molecules are accessible from the website and can be retrieved in readable formats. The website is implemented within the LAMP web service stacks. In particular, using Linux as the operative system, Apache as the HTTP Server, MySQL as the database management system, and PHP as the programming language for the web. Furthermore, the user can register and upload new data. This project is licensed under the Free-Libre/Open Source Software (FLOSS) license Apache License 2.0 which allows free and open access to the codes as well as efficient collaboration in the maintenance of the software.

Conclusions and impact

The present data-driven website presents essential information in a user-friendly manner and may help the chemical physics community to identify molecules that should be explored through spectroscopic techniques.

Spin-momentum entanglement in a Bose–Einstein condensate

Mechanisms including two types of Raman laser coupling (Ω₁ & Ω₂) and rf field coupling (Ω_rf) are applied to drive transitions between different hyperfine spin states. We investigated the entanglement between the spin and momentum degrees of freedom.