Cold hybrid ion-atom systems

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.

Effect of ion-trap parameters on energy distributions of ultra-cold atom–ion mixtures

“…In addition, it is also a promising platform for performing quantum computations [4], quantum simulations [5] and for studying out-of-equilibrium dynamics [6]. In the last decade these hybrid systems were realized in several experiments, for reviews see [7][8][9][10]. The interaction of trappedions with ultra-cold thermal clouds [11] and quantum degenerate gases of neutral atoms [12] was studied.…”

Vibrational Quenching of Weakly Bound Cold Molecular Ions Immersed in Their Parent Gas

“…Once produced, the ion will undergo collisions with the surrounding atoms. The ion-atom interaction at long range is governed by the −C 4 /r 4 term (leading induction coefficient in the multipole expansion) [1], with C 4 = 1 2 q 2 α. Here, q is the ionic charge, α the atomic polarizability, and r denotes the ion-atom distance.…”

“…The characteristic energy E in the case of Rb is h × 1640 Hz, or equivalently k B × 78.7 nK [1]. Elastic collisions between a molecular ion and an atom do not fundamentally differ from atomic ion -neutral atom collisions, which have been widely studied already [1,25]. At collision energies E high enough to involve multiple partial waves, the elastic and reactive collision cross sections can be well approximated by quasiclassical formulas derived in the framework of the Langevin capture model, yielding σ el = π…”

“…Hybrid ion-neutral systems have been the subject of intense research [1], offering the possibility to prepare highly refined molecular systems, study their dynamics in the quantum regime and possibly utilize them for quantum technological applications. As charged particles are easy to manipulate and to address with external fields, an especially promising research direction is the study of cold quantum chemistry involving ions, granting the opportunity to prepare the reactants in a well defined quantum state as well as the access to some of the reaction products.…”

“…The simplest idea relies on sympathetic cooling of the ion placed in an external trap by already ultracold atoms, which requires sufficiently large ion-to-atom mass ratio to be fully effective. Multiple experiments have been performed using this technique for an ion in a time-dependent Paul trap (see [1] and references therein), allowing for valuable insight in the collisional dynamics of ions in a buffer gas, including inelastic three-body recombination [2]. A related study used the ion trap to capture the reaction products of the recombination of three neutral atoms [3].…”