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arXiv:1807.11608 · Physical Review Letters

Observation of Quantum Interference and Coherent Control in a Photochemical Reaction

David Blasing, Jesús Pérez‐Ríos, Yangqian Yan, Sourav Dutta, Chuan-Hsun Li, Qi Zhou, Yong P. Chen

Published Aug 16, 2018• 18 citations
arXivHTMLPDF

Abstract

Coherent control of reactants remains a long-standing challenge in quantum chemistry. In particular, we have studied laser-induced molecular formation (photoassociation) in a Raman-dressed spin-orbit-coupled ^{87}Rb Bose-Einstein condensate, whose spin quantum state is a superposition of multiple bare spin components. In contrast to the notably different photoassociation-induced fractional atom losses observed for the bare spin components of a statistical mixture, a superposition state with a comparable spin composition displays the same fractional loss on every spin component. We interpret this as the superposition state itself undergoing photoassociation. For superposition states induced by a large Raman coupling and zero Raman detuning, we observe a nearly complete suppression of the photoassociation rate. This suppression is consistent with a model based upon quantum destructive interference between two photoassociation pathways for colliding atoms with different spin combinations. This model also explains the measured dependence of the photoassociation rate on the Raman detuning at a moderate Raman coupling. Our work thus suggests that preparing atoms in quantum superpositions may represent a powerful new technique to coherently control photochemical reactions.

DOI
10.1103/physrevlett.121.073202

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Coherent control of reactants remains a long-standing challenge in quantum chemistry. In particular, we have studied laser-induced molecular formation (photoassociation) in a Raman-dressed spin-orbit-coupled ^{87}Rb Bose-Einstein condensate, whose spin quantum state is a superposition of multiple bare spin components. In contrast to the notably different photoassociation-induced fractional atom losses observed for the bare spin components of a statistical mixture, a superposition state with a comparable spin composition displays the same fractional loss on every spin component. We interpret this as the superposition state itself undergoing photoassociation. For superposition states induced by a large Raman coupling and zero Raman detuning, we observe a nearly complete suppression of the photoassociation rate. This suppression is consistent with a model based upon quantum destructive interference between two photoassociation pathways for colliding atoms with different spin combinations. This model also explains the measured dependence of the photoassociation rate on the Raman detuning at a moderate Raman coupling. Our work thus suggests that preparing atoms in quantum superpositions may represent a powerful new technique to coherently control 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 87 Rb 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.
Constructive Quantum Interference in a Photo-Chemical Reaction of 87^{87}87Rb Bose Einstein Condensate
Interferences emerge when multiple pathways coexist together leading towards the same result. A Recent study by Blasing and coworkers [PRL 121(7):073202] showed that in a photo-association reaction of Raman dressed spin orbit coupled 87^{87}87Rb Bose Einstein Condensate when the reactant spin state is prepared in a coherent superposition of multiple bare spin states it leads to a destructive interference between reaction pathways. Here we report a theoretical study for a reaction scheme which leads to constructive quantum interference. This is achieved by changing the reactive scattering channel in the reaction. As the origin of coherent control comes from the spin part of the wavefunction it is sufficient to use radio frequency coupling to achieve the superposition state. Our results show that interferences can be used as a resource for the coherent control of photo-chemical reactions. The approach is general and can be employed to study a wide spectra of chemical reactions in the ultracold regime.
We show that the photoassociation of an atomic Bose-Einstein condensate to form condensed molecules is a chemical process which not only does not obey the Arrhenius rules for chemical reactions, but that it can also depend on the quantum statistics of the reactants. Comparing the predictions of a truncated Wigner representation for different initial quantum states, we find that, even when the quantum prediction for an initial coherent state is close to the Gross-Pitaevskii prediction, other quantum states may result in very different dynamics.
This thesis describes the results of several experiments that studied the photoassociation of an ultracold atomic Rb gas. In the first experiment, we produced ultracold diatomic molecules from an atomic gas via single-color photoassociation. The molecules were detected with resonance-enhanced multiphoton ionization. Trapping of these molecules in a quadrupole magnetic trap, with lifetimes up to 20 seconds, was also demonstrated. In addition, the rate constant for inelastic collisions between the trapped molecules and atoms was determined from measurements of the atomic density dependence of the decay rate of the trapped molecules. In another experiment, stimulated Raman photoassociation of Rb atoms in a Mott insulator state was studied. A Bose-Einstein condensate (BEC) of 87Rb atoms was loaded into a three-dimensional optical lattice formed by the interference pattern of three orthogonal standing wave laser fields. This system constitutes a very good realization of the Bose-Hubbard model, which predicts a quantum phase transition between a superfluid state and a Mott insulator state at a particular lattice height. A time-of-flight imaging method was used to study the state of the atomic gas, and the quantum phase transition was observed at the predicted lattice height. The signature of the phase transition was the disappearance and reappearance of peaks in the image that arose from the interference of atoms originating from different lattice sites. Two coherent laser fields were applied to the gas in its Mott insulating state, and tuned close to a Raman photoassociation resonance, and this resulted in an observable loss of atoms due to the formation of molecules. This transition exhibited a double-peaked spectrum, with one of the peaks arising from photoassociation of atoms in sites containing only two atoms, and the other from sites containing three atoms. Also, the loss of atoms vs. the duration of the Raman photoassociation period was studied, with the lasers tuned to the peak corresponding to two atoms per site. It was found that a central core of the gas, containing about 40 percent of the atoms, exhibited a coherent oscillation between an atomic and molecular quantum gas.
Summary form only given. We consider the quantum theory of two atomic fields interacting coherently to form a molecular field via Raman photoassociation. This can be applied to treat interactions in either atomic Bose condensates or quantum Fermi gases, describing coherent molecule formation together with s-wave scattering This new type of super-chemical reaction (or nonlinear atom optics) has never been observed on Earth-and temperature even in space would be far too hot to observe it naturally. Thus, it would represent a breakthrough into a completely new type of low-temperature physics, only accessible at the temperatures that have been recently obtained experimentally. The theory required is also new, yet surprisingly relatively simple.
We model the formation of stable heteronuclear molecules via pulsed Raman photoassociation of a two-component Bose-Einstein condensate near a strong Feshbach resonance, for both counterintuitive and intuitive pulse sequencing. Compared to lasers alone, weak Raman photoassociation is enhanced by as much as a factor of ten (five) for a counterintuitive (intuitive) pulse sequence, whereas strong Raman photoassociation is barely enhanced at all---regardless of pulse sequence. Stronger intra-atom, molecule, or atom-molecule collisions lead to an expected decrease in conversion efficiency, but stronger ambient inter-atom collisions lead to an unexpected increase in the efficiency of stable molecule production. Numerical results agree reasonably with an analytical approximation.