• Professor Danny Segal
  • Professor Richard Thompson

Physics of Ion Crystals in Traps

Ion traps can be used to trap atomic ions for long periods of time and laser cooling can be employed to cool the ions down to temperatures well below 1 K. A single atomic ion will reside at the centre of the trap and becomes essentially stationary. Small numbers of laser-cooled ions will form a crystal-like structure and the crystal will take on different configurations depending on the parameters of the trap.

One of the most interesting configurations is where the ions are all arranged in a line along the axis of the trap. This happens when the trapping potential along the trap symmetry axis is much weaker than in the transverse directions. If the strength of the axial confinement is gradually increased, a point comes when the minimum energy configuration of the crystal becomes a zig-zag rather than a line. At some point (which can be calculated) there is a phase transition between these two configurations. Moreover, the zig-zag configuration has two possible states, which are mirror images of each other, so the system effectively has a double well separated by a low (and adjustable) potential barrier.

It turns out that this phase transition can be used to study both the quantum mechanics of phase transitions and also quantum tunnelling. It is a good system for studying these effects because it can be well controlled and manipulated, and its state can be determined precisely using relatively non-invasive techniques. Aside from the study of a novel quantum system, there are also possibilities for using ion crystals as quantum simulators, which is a new and evolving area of quantum information science. Therefore, the experimental study of these phase transitions is of great interest at present.

This project will work forward to the design and construction of a new trap in which small crystals of ions will be prepared and laser-cooled. Then the trapping parameters will be manipulated so that the configuration of the crystal can be controlled at will. The state of the crystal will be detected by direct imaging and by the observation of its normal modes of oscillation. Then, when we have developed expertise in the control and determination of the crystal state, we will investigate quantum mechanical tunnelling between the two zig-zag states and also the quantum mechanical properties of the transition between linear and zig-zag configurations.