Trapping ions

The primary focus of this group since it was set-up 15 years ago has been researching physics by use of trapped ions. By this we mean confining ions to a small region of space, typically of the order of millimeters. We use either time varying electric fields (the RF or Paul trap) or a combination of electric and magnetic fields (the Penning trap) to achieve this. Earnshaw's theorem states that it is not possible to form a three dimensional potential well in which to trap particles by static electric or magnetic fields alone.

The trapping is done in UHV (ultra-high vacuum) conditions which affords a very clean system where the ions are unaffected by collisions with other particles or the walls of the container. This is ideal for testing the fundamental predictions of quantum mechanics. Historically, ion traps have also been used extensively for other important objectives such as setting time and frequency standards and precision mass measurements.

More information on ion trapping at an undergraduate level.

Laser cooling ions

By use of lasers it is possible to cool the ions to millikelvin temperatures. Basically these lasers are detuned to the red side of an electronic transition, using the Doppler shift of the moving ions to effect selective absorption of the laser light. In this way ions always absorb the laser light when they are moving towards the laser, so they gradually slow down. For a very basic and clear account of laser cooling (with some interesting applets) try Physics 2000 at the University of Boulder, Colorado.

Cooling slows the ions down by removing kinetic energy. This serves two main purposes: making the ions more stable in the trap (ie. increasing the duration of the confinement) and removing unwanted random movement from the ions. When ions are confined in a trap, they are effectively in a quantum well, and as such will only exist in quantised energy states. By judicious use of lasers, energy can be removed from the ions placing them in the ground state of the trap. This is very important for most of the envisioned realisations a quantum computer.

More information on laser cooling and interaction of laser light with trapped ions.

Quantum information processing and the quantum computer

This is one of the most exciting areas of modern physics and one that has enjoyed massive growth through the past few years. Much of the work in this area has been done during the past five years. Major areas of research include quantum teleportation and quantum computation.

Presently we are involved in the QUBITS collaboration to investigate quantum computation. A quantum computer works in a radically different way from a classical computer, utilising the quantum property of superposition and the resource of entanglement. One example of where a quantum computer may be exponentially quicker than a classical computer is factoring large numbers. This is important since the current standard for encrypting information relies on the fundamental difficulty of factoring such large numbers quickly.

Building any such quantum computer, however is a considerable challenge and much physics will have to be done before it is possible to even perform simple tasks. The current state of the art in the area is to build one CNOT gate (a quantum equivalent of an XOR gate) using trapped ions. The fundamental difficulty in realising a quantum computer has been identified as decoherence. This is the process by which quantum information 'decays' and is dissipated throughout the environment.

Small numbers of laser cooled trapped ions form one of the most promising realizations of a quantum computer and much of the work performed so far in this area has been with trapped ions. This continues to be an attractive system and much of the QUBITS programme is focused on trapped ion studies. More detailed description of the basics of quantum information.