Collective Phenomena at the Interface Between the Quantum and Classical Worlds

On the atomic scale, the world is governed by very different laws from those that operate on the human scale; namely the laws of quantum mechanics. The way in which the behaviour that we experience on the human scale emerges from this underlying quantum world presents science with some of its deepest questions. As well as their fundamental import, these questions have profound technological implications: as silicon chips are made which contain more and more components, they approach a scale where quantum mechanical effects begin to dominate. The advent of nano-technology will bring similar challenges.The work described in this proposal seeks to extend knowledge of the transition between the atomic scale/quantum mechanical behaviour and the human scale/classical behaviour in a number of ways.In certain systems - known as quantum-critical - many quantum constituents interact with one-another in such a way that the group behaviour is part way between quantum and classical. These systems have the additional useful property that their group behaviour is largely independent of small scale details (a property known as universality). This property, above all, has allowed progress to be made in understanding these systems.In this proposed research, I intend to explore the limits of quantum-critical behaviour; how it responds to being driven strongly out of equilibrium and how it may be replaced by alternative behaviour in certain circumstances.