Optical Trapping of Ion Coulomb Crystals

Abstract

The electronic and motional degrees of freedom of trapped ions can be controlled and coherently coupled on the level of individual quanta. Assembling complex quantum systems ion by ion while keeping this unique level of control remains a challenging task. For many applications, linear chains of ions in conventional traps are ideally suited to address this problem. However, driven motion due to the magnetic or radio-frequency electric trapping fields sometimes limits the performance in one dimension and severely affects the extension to higher-dimensional systems. Here, we report on the trapping of multiple barium ions in a single-beam optical dipole trap without radio-frequency or additional magnetic fields. We study the persistence of order in ensembles of up to six ions within the optical trap, measure their temperature, and conclude that the ions form a linear chain, commonly called a one-dimensional Coulomb crystal. As a proof-of-concept demonstration, we access the collective motion and perform spectrometry of the normal modes in the optical trap. Our system provides a platform that is free of driven motion and combines advantages of optical trapping, such as state-dependent confinement and nanoscale potentials, with the desirable properties of crystals of trapped ions, such as long-range interactions featuring collective motion. Starting with small numbers of ions, it has been proposed that these properties would allow the experimental study of many-body physics and the onset of structural quantum phase transitions between one-and two-dimensional crystals.

Multipath interference from large trapped ion chains

“…This is contrary to the equidistant case where scatterers have to be typically positioned independently by creation of many individual binding potentials. A feasible experimental way towards the utilization of the observed coherent scattering in the arrays of equidistant atomic scatterers could be represented by trapping of ions in optical lattices, where trapping potentials at multiples of trapping light wavelengths can be formed naturally [51][52][53]. These systems have already demonstrated the crucial control and tunability of two and threedimensional ion crystal structures [53].…”

Trapping Single Ions and Coulomb Crystals with Light Fields

“…This condition is met for a number of routinely employed ion species like Be + , Mg + , Ca + , Zn + , Sr + , Cd + , Ba + , Yb + , Lu + , Hg + which have been laser cooled in Paul traps. While the capability to sympathetically cool ions that are not resonant with any of the present optical fields with high efficiency by embedding them in a laser cooled Coulomb crystal is an advantage of ions over neutral atom systems, this kind of experiments requires a substantially more refined level of control that has only been demonstrated recently [16].…”

“…The effective potential is calculated at the position of the ions with i = 3 (red), i = 2, 4 (orange) and i = 1, 5 (blue). Taken from [16].…”

“…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 of optical traps for ions and provides an outlook for future applications building on the capabilities afforded by these novel tools.…”

“…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].…”