Chemical reactions, in which 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 → K 2 + Rb 2 reaction at 500 nanokelvins, focusing on the nuclear spin degrees of freedom. We prepared the initial nuclear spins in KRb (potassium-rubidium) in an entangled state by lowering the magnetic field to where the spin-spin interaction dominates and characterized the preserved coherence in nuclear spin wave function 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.
Chemical reactions, in which 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 → K 2 + Rb 2 reaction at 500 nanokelvins, focusing on the nuclear spin degrees of freedom. We prepared the initial nuclear spins in KRb (potassium-rubidium) in an entangled state by lowering the magnetic field to where the spin-spin interaction dominates and characterized the preserved coherence in nuclear spin wave function 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.
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.
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.
Chemical reactions in a statistical ensemble are conventionally regarded as incoherent processes driven by thermodynamics. In the classical regime, reactions are expected to proceed faster at high temperatures and slow to a halt in the zero-temperature limit according to Arrhenius kinetics. In the quantum degenerate regime, where bosonic atoms and molecules form coherent matter waves, reactions are instead described by nonlinear mixing of matter-wave fields, leading to collectively-enhanced reaction rates and coherent amplification. This raises the question: how does the matter-wave phase evolve during a reaction, and how can it influence reaction dynamics? In the quantum regime, the matter-wave phases of the reactants and products are expected to match through wave mixing, analogous to phase matching of photonic fields in nonlinear optics.\par We report 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 reactants and products and observe phase doubling when atomic waves combine into molecular waves - the matter-wave analogue of second-harmonic generation. The diffraction patterns also reveal distinct signatures of entanglement generated between reacting particles as the reaction is initiated. Our observations establish phase coherence and entanglement as two essential features of "quantum many-body chemistry" where macroscopic quantum fields govern the reaction dynamics.
Chemical reactions in a statistical ensemble are conventionally regarded as incoherent processes driven by thermodynamics. In the classical regime, reactions are expected to proceed faster at high temperatures and slow to a halt in the zero-temperature limit according to Arrhenius kinetics. In the quantum degenerate regime, where bosonic atoms and molecules form coherent matter waves, reactions are instead described by nonlinear mixing of matter-wave fields, leading to collectively-enhanced reaction rates and coherent amplification. This raises the question: how does the matter-wave phase evolve during a reaction, and how can it influence reaction dynamics? In the quantum regime, the matter-wave phases of the reactants and products are expected to match through wave mixing, analogous to phase matching of photonic fields in nonlinear optics.\par We report 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 reactants and products and observe phase doubling when atomic waves combine into molecular waves - the matter-wave analogue of second-harmonic generation. The diffraction patterns also reveal distinct signatures of entanglement generated between reacting particles as the reaction is initiated. Our observations establish phase coherence and entanglement as two essential features of "quantum many-body chemistry" where macroscopic quantum fields govern the reaction dynamics.
Quantum control of reactive systems has enabled microscopic probes of underlying interaction potentials, the opening of novel reaction pathways, and the alteration of reaction rates using quantum statistics. However, extending such control to the quantum states of reaction outcomes remains challenging. In this work, we realize this goal through the nuclear spin degree of freedom, a result which relies on the conservation of nuclear spins throughout the reaction. Using resonance-enhanced multiphoton ionization spectroscopy to investigate the products formed in bimolecular reactions between ultracold KRb molecules, we find that the system retains a near-perfect memory of the reactants nuclear spins, manifested as a strong parity preference for the rotational states of the products. We leverage this effect to alter the occupation of these product states by changing the superposition of initial nuclear spin states with an external magnetic field. The techniques demonstrated here open up the possibility to study ultracold reaction dynamics at the state-to-state level as well as quantum entanglement between reaction products.
Chemical reactions in a statistical ensemble are conventionally regarded as incoherent processes driven by thermodynamics. In the classical regime, reactions are expected to proceed faster at high temperatures and slow to a halt in the zero-temperature limit according to Arrhenius kinetics. In the quantum degenerate regime, where bosonic atoms and molecules form coherent matter waves, reactions are instead described by nonlinear mixing of matter-wave fields, leading to collectively-enhanced reaction rates and coherent amplification. This raises the question: how does the matter-wave phase evolve during a reaction, and how can it influence reaction dynamics? In the quantum regime, the matter-wave phases of the reactants and products are expected to match through wave mixing, analogous to phase matching of photonic fields in nonlinear optics.\par We report 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 reactants and products and observe phase doubling when atomic waves combine into molecular waves - the matter-wave analogue of second-harmonic generation. The diffraction patterns also reveal distinct signatures of entanglement generated between reacting particles as the reaction is initiated. Our observations establish phase coherence and entanglement as two essential features of "quantum many-body chemistry" where macroscopic quantum fields govern the reaction dynamics.