Trapping Single Ions and Coulomb Crystals with Light Fields

Abstract

ii Preface The scope of this book is on providing insight into the recently emerged field of optical trapping of ions. Ever since the ground-breaking introduction of light fields as tools for exerting trapping forces on matter in 1970 by Ashkin [1], optical dipole traps have been used with remarkable success in several fields of research, most notably in atomic physics, where they have enabled an unprecedented level of control over neutral atoms and molecules both at the level of quantum ensembles [2] as well as individual particles [3,4].Another tremendously influential development in Physics was enabled by the invention of radiofrequency traps pioneered by Paul [5] and Dehmelt, allowing for confinement of single atomic ions demonstrated by Wineland [6] decades before this was realized on the level of individual neutral atoms in optical traps. Ions provide a number of properties that render them ideal for state-of-the-art applications in several cutting-edge areas of contemporary research ranging from metrology [7], quantum simulation and computation [8] to ultracold chemistry [9].However, it was found recently that in some situations it is highly advantageous to confine ions without employing any radiofrequency-based Paul traps or strong external magnetic fields in Penning traps, e.g. when investigating the interaction of neutral atoms and ions in the regime of ultralow interaction energies [10]. Adapting optical traps for ions [11] is a promising way to approach such scenarios and the focus of this work is to present a comprehensive overview of the background and concepts behind this technique as well as to discuss the currently achievable level of control, encountered limitations and perspectives for future applications.iii iv PREFACE Chapter 1 is intended to provide a context of the described experiments within the field of atomic physics. It highlights the advantages of the currently used techniques applied for trapping and manipulating ions in comparison with the features granted by optical traps for neutral atoms.Chapter 2 lays out the general concepts, requirements and methodology for realizations of optical trapping of ions.Chapter 3 describes how the tools and methods presented in the previous chapter can be used to confine single atomic ions in optical traps. Individual developments achieved in the last years are highlighted by presenting and discussing milestone experiments [12][13][14][15]. The aim of this chapter is to provide an understanding of the current performance granted by this approach and its prospective possibilities for applications in the near future. It concludes with a discussion of the identified limitations and strategies for future improvement.Chapter 4 is focused on experiments reaching beyond the established optical traps for single ions. It provides an in-depth view on the recently demonstrated extension of the presented methods to the case of ion Coulomb crystals [16].Chapter 5 highlights the key features of the concepts discussed in this book, summarizes the main features o...

Optical Traps for Sympathetic Cooling of Ions with Ultracold Neutral Atoms

“…Subsequently, the ions are laser cooled to the Doppler limit of T Ba + D ≈ 370 µK. We compensate stray electric fields to the level of E str ≤ 7 mV m −1 [30,32], and align two axially overlapped counter-propagating dipole trap laser beams (VIS and NIR) with a Ba + confined at the center of the RF trap (z = 0 m) [27,28]. The two beams are operated at wavelengths λ V IS = 532 nm and λ N IR = 1064 nm while the beam waist radii of w N IR 0 ≈ w V IS 0 = 3.7 ± 0.05 µm are approximately matched at z = 0.…”

Interactions of Ions and Ultracold Neutral Atom Ensembles in Composite Optical Dipole Traps: Developments and Perspectives

“…Optical traps provide an extremely versatile and powerful tool-set that has lead to several breakthroughs in trapping and manipulating neutral atom ensembles over the coarse of several decades [38,39]. In the case of ions however, Coulomb interactions with residual stray electric fields or with other ions typically dominate over the much smaller optical dipole forces [30,31]. Nonetheless, it was shown that with adapted methods for preparing ions, such as optimized geometries and improved compensation of stray electric fields, essentially the same methods can be applied to realize optical trapping of single ions [27], Coulomb crystals [29], long lifetimes [28] and stateselective potentials [40].…”

“…The preparation of the atomic sample follows standard techniques such as magnetooptical trapping, transferring the atoms to an optical dipole trap and evaporative cooling therein. Optical trapping of ions is achieved by employing previously developed techniques [27][28][29][30][31]42]. Trapping probabilities p opt ≈ 1 on timescales of ms relevant to the work discussed here are routinely achieved even if additional techniques for extending the lifetime, e.g., repumping from electronic states that experience a repulsive optical force [28], are omitted in favour of reducing the number of optical fields that might interact with the neutral atom ensemble.…”

“…Here, we discuss a recently demonstrated realization of a trap allowing to combine atomic ions and neutral atoms in absence of rf fields, based on confinement provided by optical potentials [27][28][29][30][31][32][33]. In principle, this approach is applicable to any combination of ions and atoms with a finite polarizability, as is the case for all commonly used ionic and atomic species including, for example, Yb + , Ba + , Ca + , Mg + , Be + ions and Li, K, Na, Rb, Yb, Er atoms [31].…”

“…Here, we discuss a recently demonstrated realization of a trap allowing to combine atomic ions and neutral atoms in absence of rf fields, based on confinement provided by optical potentials [27][28][29][30][31][32][33]. In principle, this approach is applicable to any combination of ions and atoms with a finite polarizability, as is the case for all commonly used ionic and atomic species including, for example, Yb + , Ba + , Ca + , Mg + , Be + ions and Li, K, Na, Rb, Yb, Er atoms [31]. For Ba + overlapped with Rb, a system that has been studied in great detail using conventional rf-based ion traps [34][35][36], rf-free trapping was shown to enable efficient sympathetic cooling in the ultracold regime well below the Doppler limit by avoiding micromotion-induced heating.…”