  Trapped ions are among the most promising systems for practical quantum
computing (QC). The basic requirements for universal QC have all been
demonstrated with ions and quantum algorithms using few-ion-qubit systems have
been implemented. We review the state of the field, covering the basics of how
trapped ions are used for QC and their strengths and limitations as qubits. In
addition, we discuss what is being done, and what may be required, to increase
the scale of trapped ion quantum computers while mitigating decoherence and
control errors. Finally, we explore the outlook for trapped-ion QC. In
particular, we discuss near-term applications, considerations impacting the
design of future systems of trapped ions, and experiments and demonstrations
that may further inform these considerations.
