Tunable scattering resonances are crucial for controlling atomic and molecular systems. However, their use has so far been limited to ultracold temperatures. These conditions remain hard to achieve for most hybrid trapped ion-atom systems—a prospective platform for quantum technologies and fundamental research. Here, we measure inelastic collision probabilities for Sr + + Rb and use them to calibrate a comprehensive theoretical model of ion-atom collisions. Our theoretical results, compared with experimental observations, confirm that quantum interference effects persist to the multiple-partial-wave regime, leading to the pronounced state and mass dependence of the collision rates. Using our model, we go beyond interference and identify a rich spectrum of Feshbach resonances at moderate magnetic fields with the Rb atom in its lower ( f = 1) hyperfine state, which persist at temperatures as high as 1 millikelvin. Future observation of these predicted resonances should allow precise control of the short-range dynamics in Sr + + Rb collisions under unprecedentedly warm conditions.
Tunable scattering resonances are crucial for controlling atomic and molecular systems. However, their use has so far been limited to ultracold temperatures. These conditions remain hard to achieve for most hybrid trapped ion-atom systems—a prospective platform for quantum technologies and fundamental research. Here, we measure inelastic collision probabilities for Sr + + Rb and use them to calibrate a comprehensive theoretical model of ion-atom collisions. Our theoretical results, compared with experimental observations, confirm that quantum interference effects persist to the multiple-partial-wave regime, leading to the pronounced state and mass dependence of the collision rates. Using our model, we go beyond interference and identify a rich spectrum of Feshbach resonances at moderate magnetic fields with the Rb atom in its lower ( f = 1) hyperfine state, which persist at temperatures as high as 1 millikelvin. Future observation of these predicted resonances should allow precise control of the short-range dynamics in Sr + + Rb collisions under unprecedentedly warm conditions.
Hybrid systems of single, trapped ions embedded in quantum gases are a promising platform for quantum simulations and the study of long-range interactions in the ultracold regime. Feshbach resonances allow for experimental control over the character and strength of the atom-ion interaction. However, the complexity of atom-ion Feshbach spectra, e.g., due to second-order spin-orbit coupling, requires a detailed experimental understanding of the resonance properties—such as the contributing open-channel partial waves. In this work, we immerse a single barium ( Ba + ) ion in a bath of lithium (Li) atoms spin polarized in their hyperfine ground state to investigate the collision energy dependence of magnetically tunable atom-ion Feshbach resonances. We demonstrate fine control over the kinetic energy of the Ba + ion and employ it to explore three-body recombination in the transition from the many- to the few-partial wave regime, marked by a sudden increase of resonant loss. In a dense spectrum—with on average 0.58(1) resonances per Gauss—we select a narrow, isolated feature and characterize it as an s -wave resonance. We introduce a quantum recombination model that allows us to distinguish it from higher-partial-wave resonances. Furthermore, in a magnetic field range with no significant loss at the lowest collision energies, we identify a higher-partial-wave resonance that appears and peaks only when we increase the energy to around the s -wave limit. Our results demonstrate that hybrid atom-ion traps can reach collision energies well in the ultracold regime and that the ion’s kinetic energy can be employed to tune the collisional complex to resonance, paving the way for fast control over the interaction in settings where magnetic field variations are detrimental to coherence.
Abstract Quantum effects in chemical reactions are most pronounced at ultracold temperatures, where only a few partial waves contribute. While interference among many partial waves is theoretically expected to persist at higher temperatures, direct evidence for such quantum effects in reactive processes has been lacking. Here, we report signatures of quantum interference suppressing a chemical reaction in the multi-partial-wave regime: resonant charge exchange between a single 87 Rb + ion and its parent atom 87 Rb. Using quantum-logic detection on a single atom-ion pair and a calibrated in-situ measurement of Langevin collision probabilities, we benchmark the thermally averaged reaction rate against both classical and quantum predictions. We find that the reaction rate is suppressed by over an order of magnitude relative to the classical expectation, despite occurring in the millikelvin temperature regime (more than three orders of magnitude above the s -wave threshold), where more than a dozen partial waves contribute. These results suggest quantum interference as a key mechanism in chemical reactivity beyond the ultracold limit and offer a platform for probing coherent quantum effects in atom-ion reactions where ab initio methods remain intractable.
Quantum interference in cold collisions has been predicted to dominate chemical dynamics at low temperatures. However, experimental observations have been largely confined to light-atom systems due to technical challenges. Here, we report direct observation of scattering oscillations in cold ionizing collisions between metastable krypton and rubidium atoms. By combining laser cooling with velocity-mapped ion imaging, we precisely controlled the collision energy from 15 millikelvin to 8 kelvin and detected the ionization products. Below 1 kelvin, pronounced quantum interference between partial waves emerges as the de Broglie wavelength approaches the interaction range. The angular oscillation period scales as [Formula: see text], consistent with a Langevin capture model for van der Waals-dominated collisions. Moreover, interference patterns in the angular distribution-collision temperature spectrum are highly sensitive to the long-range potential shape and reveal subtle shape resonances. These observations were corroborated by theoretical calculations. Our results demonstrate the quantum-classical crossover in heavy-atom systems. This study establishes a versatile platform for studying a wide range of laser-coolable species.
Ultracold chemistry operates in a regime where thermal motion is suppressed to the microkelvin scale, allowing quantum mechanics to dictate every aspect of molecular interaction. This paper reviews the theoretical foundations and experimental breakthroughs that now make coherent control of ultracold reactions possible. It first outlines the separation of short- and long-range physics through quantum defect theory, emphasizing why short-range dynamics remain intractable and why universal loss emerges naturally. It then examines how long-range potentials, resonances, and quantum interference enable tunable scattering behavior. Two major milestones anchor this discussion: phase-coherent atom–atom–molecule coupling, which confirms the transfer and control of reaction phase, and the experimental resolution of long-range atom–molecule Feshbach resonances using a Fabry–P´erot interferometer model. Finally, it highlights the recent observation of a magnetically tunable molecule–molecule Feshbach resonance in NaLi, overturning long-held assumptions about the impossibility of resonance control in complex molecular systems. Together, these results demonstrate that ultracold molecules can be engineered to undergo fully coherent, interference-driven reactions, establishing a framework for quantum-controlled chemistry and laying the groundwork for applications in quantum simulation and computation.
The control of the ultracold collisions between neutral atoms is an extensive and successful field of study. The tools developed allow for ultracold chemical reactions to be managed using magnetic fields, light fields and spin-state manipulation of the colliding particles among other methods. The control of chemical reactions in ultracold atom-ion collisions is a young and growing field of research. Recently, the collision energy and the ion electronic state were used to control atom-ion interactions. Here, we demonstrate spin-controlled atom-ion inelastic processes. In our experiment, both spin-exchange and charge-exchange reactions are controlled in an ultracold Rb-Sr+ mixture by the atomic spin state. We prepare a cloud of atoms in a single hyperfine spin-state. Spin-exchange collisions between atoms and ion subsequently polarize the ion spin. Electron transfer is only allowed for (RbSr)+ colliding in the singlet manifold. Initializing the atoms in various spin states affects the overlap of the collision wavefunction with the singlet molecular manifold and therefore also the reaction rate. We experimentally show that by preparing the atoms in different spin states one can vary the charge-exchange rate in agreement with theoretical predictions.
We explore collisions of atomic particles in two different regimes. In the first part of this work, we study atom-atom collisions of alkali metals at ultracold temperatures and conditions typically found in modern experiments. Feshbach resonances induced by external magnetic fields present a way to change collisional properties and exhibit control over cold collisions. We perform a fully quantum coupled-channel calculation in molecular orbital formalism to calculate collisional properties and characterize Feshbach resonances in ultracold Li+Na and Li+Rb systems. Furthermore, we use the results of these calculations as a foundation to propose a novel scheme for formation of stable ultracold molecules in their lowest rovibrational level. The formation scheme relies on the fact that the photoassociation rate becomes greatly enhanced in the vicinity of a Feshbach resonance, resulting in a large increase of the number of produced excited molecules. Efficient production of stable ultracold molecules is a prerequisite for realization of various proposed platforms for quantum computing with neutral atoms and molecules. In addition, we suggest a way to exploit specifics of the photoassociation rate behavior near its minimum for a precision measurement experiment to detect hypothetical variation of the electron-proton mass ratio in time. ^ The second part of this study examines ion-atom collisions in our solar system. Highly charged ions present in the solar wind collide with neutrals, capture one or more electrons into high excited states and deexcite, radiating energetic X-ray and UV photons. In particular, we study X-ray emissions charge-exchange collisions between fully stripped C6+, N7+ and O8+ solar wind ions and heliospheric hydrogen. We analytically solve the two-center Schrödinger equation and construct electronic potential curves for the aforementioned molecular ions. Subsequently, we compute polarization of the X-rays emitted in a single-step deexcitation following charge-exchange collisions and construct a polarization map for the ecliptic plane, as observed from Earth. Our analysis indicates that the heliospheric charge-exchange X-rays are slightly polarized, with the polarization P close to ten percent for the optimal line-of-sight. ^