Fundamental physics with trapped ions

Ion traps allow us to study single quantum systems, cooled to the lowest vibrational state of motion within confining electric and magnetic fields. In this article, we describe the working principles of ion traps and the methods used to cool the ions (principally laser cooling). We then discuss how these cold ions, held almost at rest in the trap, can be used to illuminate fundamental issues of quantum mechanics: quantum jumps, the quantum Zeno effect and the quantum statistics of photons scattered by the ions. Finally, we describe how several ions loaded in a trap are cooled into ordered quasi crystals.