Charge delocalisation and hopping in an ultra-cold atomic lattice

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Using lasers, a cloud of atoms can be cooled to within a millionth of a degree above absolute zero. Atoms that are this cold hardly move at all, and they can be trapped using laser beams to make a crystal of regularly spaced atoms held in place by light, known as an optical lattice. The aim of this project is to study what happens when we excite the electrons in these atoms to high energies using a pulse of laser light. Normally the electrons in an atom orbit close tothe nucleus, but if we give the outermost electron more and more energy, its orbit gets larger and larger. Eventually the electron orbits of neighbouring atoms in the lattice begin to overlap, and the electrons no longer belong to a particular atom and can spread out along the lattice. This concept is important in many areas of physics, for example in explaining how the electrons in a metal free themselves to form an electrical current. The advantage of studying this with ultra-cold atoms trapped in an optical lattice is that the properties of each of the atoms in thelattice can be controlled extremely precisely. For example using a laser beam an electron can be removed from just one atom in the lattice, leaving behind a single positively charged ion. An electron from a neighbouring atom can then ``hop'' across to replace the lost electron, leaving behind an ion in its turn. Studying how electrons hop from atom to atom is one of the goals of this proposal.