Helicity-dependent quantum phases

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Light is described by two characteristics - its direction of propagation and its polarisation. In everyday life the latter is usually of little consequence, an exception being for example the way the two frames of a 3D movie can be sent to each eye simultaneously glasses that filter out a given polarisation while leaving the other intact. The distinction between the two images is that one is left-circularly polarised, meaning that the light spirals like a left-handed corkscrew, while the other is right-handed, spiralling the other way. This is closely related to a property of light called the helicity, which will be used in this project to design a new example of a technological tool known as an optical lattice. These allow for the trapping and suspending a grid of atoms using only light. Optical lattices in general have a variety of uses, including making artificial molecules and acting as a component of a quantum computer.The optical lattices designed here will be made of light that has opposite helicity at neighbouring points in the grid (like a chessboard), so that helicity-sensitive atoms placed at each will feel a difference effect from the lattice. Using similar ideas it is also possible to design such lattices where the electric and magnetic fields that make up the light interact in different ways with the atoms placed in the lattice. Both of these ideas can be used to affect the degree to which the atoms interact with each other. In everyday materials, the strengths of such interactions are partly responsible for whether a particular material is a solid, liquid or gas - these are examples of phases of matter. When the interaction strength has some critical value, the phase changes (e.g. melting, boiling), this is known as a phase transition. Using helicity to vary the interaction strength in our optical lattice is expected to cause transitions between different phases describing the collective behaviour of the atoms in the lattice.