Title:

Macroscopic superpositions using BoseEinstein condensates

The differences between classical and quantum mechanics were highlighted early in the development of quantum mechanics when Schrodinger proposed the thought experiment of a cat in a superposition of alive and dead. In this thesis I try to understand these differences by considering superpositions of large objects at a single particle level. Research in the field of superconductors has provided evidence for macroscopic quantum superpositions (or cat states) of currents in superconducting loops. BoseEinstein condensates of ultracold atoms provide another promising system for experimentally producing similar results. I begin by describing two straightforward schemes that make macroscopic superpositions of superfluid flow states of BoseEinstein condensates trapped in optical lattice rings. The first scheme achieves a superposition of three flow states by nonadiabatically evolving the barrier heights between the sites. The second scheme produces a superposition of two flow states by applying a 7f phase around the ring. This could be experimentally achieved by physically rotating the sites or imparting angular momentum from two copropagating lasers. The next part of the thesis investigates why it is difficult to produce macroscopic superpositions. By treating the interaction strength between the atoms as a perturbation I show three reasons, other than decoherence, why macroscopic superpositions are hard to make. Firstly, the energy of the two distinct flow states must be sufficiently close. Secondly, coupling between the two states must be sufficiently strong, and thirdly, other states must be well separated from those two flow states. To make larger superpositions I look at a Josephson junction coupled to a superfluid loop. This shows that making superpositions depends on the number of atoms in the junction rather than the whole system. Finally I propose ways of developing the work. This concentrates on how the systems could experimentally create macroscopic superpositions and how we could measure signatures of these states. I then suggest ways of using the systems, such as quantum information and precision measurement schemes.
