, 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.
, 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.
Coherent superpositions and entanglement are hallmarks of quantum mechanics, but they are fragile and can easily be perturbed by their environment. Selected isolated physical systems can maintain coherence and generate entanglement using well-controlled interactions. 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 investigate this question by studying the 2KRb → K2 + Rb2 reaction at 500 nK, focusing on the the nuclear spin degrees of freedom. We prepare the initial nuclear spins in KRb in an entangled state and characterize the preserved coherence in nuclear spin wavefunction after the reaction. The data are consistent with full coherence at the end of the reaction. This suggests that entanglement can be prepared within the reactants, followed by a chemical reaction that produces separate, entangled molecules. We additionally demonstrate control of the reaction product state distribution by deliberately decohering the reactants.
Entanglement is a crucial resource for achieving quantum advantages in quantum computation, quantum sensing, and quantum communication. As shown in this Letter, entanglement is also a valuable resource for the coherent control of the large class of bimolecular chemical reactions. We utilize an entanglement-enhanced coherent control scheme, in which the initial preparation of the superposition state is divided into two steps: the first entangles the reactants, and the second is responsible for coherent control. This approach can overcome the limitations of traditional coherent control of scattering caused by noninterfering pathways, known as satellite terms. By tuning the amount of entanglement between reactants, the visibility of coherent control in chemical reactions can be modulated and optimized. Significantly, there exists an optimal amount of entanglement, which ensures complete indistinguishability of the reaction pathways, maximizing the extent of control. This entanglement-enhanced coherent control scheme is computationally illustrated using the ultracold KRb + KRb reaction, where perfect control over the parity of the product rotational states is achieved.
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.
Rapid progress in atomic, molecular, and optical (AMO) physics techniques enabled the creation of ultracold samples of molecular species and opened opportunities to explore chemistry in the ultralow temperature regime. In particular, both the external and internal quantum degrees of freedom of the reactant atoms and molecules are controlled, allowing studies that explored the role of the long-range potential in ultracold reactions. The kinetics of these reactions have typically been determined using the loss of reactants as proxies. To extend such studies into the short-range, we developed an experimental apparatus that combines the production of quantum-state-selected ultracold KRb molecules with ion mass and kinetic energy spectrometry, and directly observed KRb + KRb reaction intermediates and products [M.-G. Hu and Y. Liu, et al., Science, 2019, 366, 1111]. Here, we present the apparatus in detail. For future studies that aim for detecting the quantum states of the reaction products, we demonstrate a photodissociation based scheme to calibrate the ion kinetic energy spectrometer at low energies.
Advances in quantum manipulation of molecules bring unique opportunities, including the use of molecules to search for new physics, harnessing molecular resources for quantum engineering, and exploring chemical reactions in the ultra-low temperature regime. In this talk, I will focus on the latter two topics. First, I will introduce our effort on building single ultracold molecules with full internal and motional state control in optical tweezers for future quantum simulators and computers. This work allows us to go beyond the usual paradigm of chemical reactions that proceed via stochastic encounters between reactants, to a single, controlled reaction of exactly two atoms. Second, I will present our work giving a detailed microscopic picture of molecules transforming from one species to another. We develop full quantum state mapping of chemical reaction product-pairs from single events, which we use to precisely benchmark statistical theory.
Chemical reactions in a statistical ensemble are conventionally regarded as incoherent processes driven by thermodynamics. In the quantum degenerate regime, where atoms and molecules form coherent matter waves, reactions are theoretically described by nonlinear mixing of matter-wave fields. In this scenario, we expect phase matching between reactants and products, analogous to the mixing of photonic fields in nonlinear optics. Here we report on the observation of phase coherent reaction dynamics of Bose-condensed atoms and molecules near a Feshbach resonance. Using matter-wave diffraction with optical lattices, we verify spatial coherence of both atoms and molecules and observe phase doubling when atomic waves combine into molecular waves, the matter-wave analogue of optical frequency doubling. The diffraction patterns further reveal two-atom entanglement generated during the reaction. Our observations establish phase coherence and entanglement generation as two essential features of "quantum many-body chemistry". Moreover, our work opens a pathway to control of reaction dynamics by manipulation of matter-wave phases.