  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 which is free of driven motion and
combines advantages of optical trapping, such as state-dependent confinement
and nano-scale 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.
