Multi-qubit entanglement in an ion based quantum processor

Quantum computation and quantum information science are one of the most exciting and active research areas because of their potential for efficiently solving some computational problems for which no efficient solution exists on a classical computer. The most important examples of such problems are large number factorization and the simulation of quantum systems. The former enables the decoding of the encryption schemes most widely used today, the latter promises to aid in the development of novel materials such as high-temperature superconductors. Both examples illustrate that the realization of a large-scale quantum computer would have profound impacts on modern society. Moreover, the study of entanglement, the quantum property that allows a quantum computer to function, promises a deeper understanding of the transition from quantum mechanics to the classical behaviour of macroscopic systems. Individual trapped ions, acting as `quantum bits` and manipulated and read out by laser light, are a promising candidate for the implementation of such a quantum computer, and recent experimental progress has been remarkable. Not only have all necessary ingredients of quantum computation been demonstrated, but more advanced quantum algorithms such as quantum teleportation have been achieved in the recent past. Despite the brilliant developments of ion traps some essential steps towards practical quantum computer have not yet been achieved. The main limitation is the difficulty to enlarge the number of quantum bits while retaining entanglement for a time sufficient to perform the desired computations. Thus this work is dedicated to studying the entanglement properties of a growing number of ions, as well as to developing computational techniques that enable scalable quantum computation. One example of such techniques is entanglement swapping, which enables the exchange of quantum information between different locations within a large ion trap structure. Another is the exploration of `decoherence-free subspaces`, where a quantum bit is encoded in two physical ions in such a way that it becomes much less susceptible to the decay of entanglement. Along with this work will go technical improvements of the state-of-the-art quantum processor available in Innsbruck.