Peter Barker

Myungshik Kim

Spin-probed matter-wave interformetry of levitated diamond nano-particles

Quantum mechanics is widely regarded as our most effective theory to date. Its accuracy and the insight it offers us are unprecedented and stunning. However, there remain serious problems with QM, and amongst them is the question of where it should break down and give way to classical mechanics. When we measure a quantum state we transition from unitary evolution governed by the Schrödinger equation to a probabilistic final outcome. However, what constitutes this measurement is not properly defined, other than a rough idea of scale. Unless we adopt a 'Many Worlds' interpretation, in which there is no wavefunction collapse; we must make a subjective distinction between a quantum system and a measurement device capable of collapsing superpositions into definite states. 

Collapse theories are one possible resolution to this problem, which is known as the measurement problem. By modifying the Schrödinger equation they promise a new, general mechanics; one which goes over to quantum mechanics in the limit of small masses, and goes over to classical mechanics in the limit of larger objects. Though various attempts have been made to resolve the measurement problem over the years, collapse theories are remarkable in that they are testable.

My work focuses on ways of testing collapse theories using optomechanical systems. Currently I am working on a scheme to use a levitated nanosphere, trapped inside an optical cavity, to probe the signature effects of the postulated noise field causing collapse. If such a field exists, it will interact with the sphere, acting on it like a Brownian noise source. In turn, this action on the position of the sphere will affect the light entering and leaving the cavity, and it is in the profile of the exiting light that we hope to look for evidence of collapse.