Cold hybrid ion-atom systems (1708.07832v3)

Abstract: Hybrid systems of laser-cooled trapped ions and ultracold atoms combined in a single experimental setup have recently emerged as a new platform for fundamental research in quantum physics. This paper reviews the theoretical and experimental progress in research on cold hybrid ion-atom systems which aim to combine the best features of the two well-established fields. We provide a broad overview of the theoretical description of ion-atom mixtures and their applications, and report on advances in experiments with ions trapped in Paul or dipole traps overlapped with a cloud of cold atoms, and with ions directly produced in a Bose-Einstein condensate. We start with microscopic models describing the electronic structure, interactions, and collisional physics of ion-atom systems at low and ultralow temperatures, including radiative and non-radiative charge transfer processes and their control with magnetically tunable Feshbach resonances. Then we describe the relevant experimental techniques and the intrinsic properties of hybrid systems. In particular, we discuss the impact of the micromotion of ions in Paul traps on ion-atom hybrid systems. Next, we review recent proposals for using ions immersed in ultracold gases for studying cold collisions, chemistry, many-body physics, quantum simulation, and quantum computation and their experimental realizations. In the last part we focus on the formation of molecular ions via spontaneous radiative association, photoassociation, magnetoassociation, and sympathetic cooling. We discuss applications and prospects of cold molecular ions for cold controlled chemistry and precision spectroscopy.

Cold Hybrid Ion-Atom Systems

The field of cold hybrid ion-atom systems has garnered significant attention within quantum physics, serving as a robust platform that bridges traditional trapped ion systems with ultracold atomic gases. This compilation of theoretical and experimental insights delineates the progress, potential applications, and inherent challenges faced by such hybrid systems.

Theoretical and Experimental Foundations

The paper of cold hybrid ion-atom systems involves ion-atom mixtures where laser-cooled ions interact with a background of ultracold atoms. The theoretical framework for these systems extends classical scattering theories to account for interactions at the quantum level, focusing on electronic structure, collision dynamics, and charge transfer processes. This paper elaborates on microscopic models that describe ion-atom interactions at ultralow temperatures, emphasizing radiative and non-radiative charge transfer and their manipulation through magnetic Feshbach resonances.

Experimentally, two types of traps are commonly utilized: the Paul trap for ions and optical dipole traps for atoms. These setups allow precise control over the motional and internal states of ions. A critical aspect is the micromotion of ions within radio-frequency traps, which significantly impacts ion-atom collisions and resultant cooling processes. Experiments with various combinations, such as Ca$^+$ with Rb or Sr$^+$ with Li, elucidate fundamental principles driving collisional outcomes, including elastic collisions and charge exchange.

Research Highlights and Outcomes

An interesting outcome emphasized in the paper is the formation of molecular ions via radiative association, with rate constants evaluated for various ion-atom systems. For systems such as Yb$^+$ colliding with Li, this process leads to the production of relatively deeply bound molecular ions, providing a pathway toward cold controlled chemistry applications. The significant observation that radiative association can dominate over charge exchange reactions highlights the nuanced dynamics at play in these low-temperature environments.

Experimentally, the cooling of ion-atom mixtures is often limited by micromotion. The studies discussed suggest that mitigating micromotion through alternative trapping methods, such as optical traps, could allow for achieving lower collision energies and thus reach the $s$-wave regime. Furthermore, the exploration of hybrid systems without inducing micromotion, notably via optical methods or coupling with Rydberg states, is posited as a promising direction for achieving ultracold interactions.

Future Directions and Applications

One of the principal applications discussed is using these systems for quantum simulation. Ion-atom systems offer a novel platform for simulating complex many-body problems, leveraging ions’ long-range interactions alongside atoms' scalability. Proposals for quantum computation highlight the potential for utilizing ion-atom interactions as qubits, where ions serve as quantum gates to entangle atomic states—paving the way for more intricate quantum networks.

Moreover, the use of hybrid systems for probing quantum gases through precision measurements, leveraging the fine control over ions, has been proposed. Such experiments could elucidate the fundamental properties of quantum gases, contributing to the understanding of phenomena such as superfluidity and quantum phase transitions.

Implications and Theoretical Speculations

The confluence of ions and atoms in ultracold regimes also presents implications for cold chemistry, where state-to-state chemical reactions could be scrutinized with unprecedented precision. This setup allows for extracting reaction dynamics influenced by quantum statistics and external fields, propelling advances in fields like astrophysics and atmospheric chemistry.

In conclusion, the field of cold hybrid ion-atom systems stands at an exciting juncture, promising insights into quantum interactions that could revolutionize applications across quantum computation, simulation, and chemistry. Continued advancements in trap technology and a deeper theoretical understanding are expected to overcome current challenges, unlocking the full potential of these intriguingly complex systems.