The quantum theory of light and atoms provides a fundamental understanding of most of the physical processes we observe in the world. It is an area strongly driven by the experimental realisation of ultracold quantum gases, and the continuing advances in quantum information and quantum optics. A typical temperature for ultracold atoms is in the micro-Kelvin range, which means that thermal fluctuations are suppressed, and coherence and quantum correlations can emerge. Experimental measurement and manipulation can be carried out with extraordinary precision, and the striking quantum phenomena observed in experiments have stimulated the development of new theories capable of describing equilibrium and dynamical properties. Advances in the quantum theory of light-matter interactions enable a deeper understanding of the quantum world, and new tools for atomic manipulation.
My research concerns the theory of ultra-cold atomic gases - systems of quantum degenerate matter waves - and associated computational physics techniques. My current focus is on two systems:
1. Quantum Dipolar Gases: Equilibrium and dynamical properties of ultra-cold atomic gases with dipole-dipole interactions. A significant interest going forward is the emergence of rotonic excitations in single and multi-layed systems, including: finite temperature (dynamical) calculations of dipolar gases with rotons; number and density fluctuations in systems with rotons; probing rotons using spectroscopic techniques; aiding the experimental hunt to find rotons.
2. Spinor Quantum Gases: The properties of warm spinor Bose gases (particularly with spin-1 atoms). My work aims to characterize how the normal component interacts with the spinor condensate, the dynamics of spin oscillations, and quench dynamics.
Superfluids are a large-scale manifestation of quantum mechanics. Able to flow without any of the viscous damping affecting normal fluids, superfluids allow a current to persist indefinitely. Supefluids form through a basic quantum mechanical phenomena - the strong tendency of Bosonic particles to collect into a single quantum state. In experiments, this ideal is never attained, and perfect superfluidity is fundamentally altered by various sources of dissipation.
One way a superfluid can dissipate is through the nucleation of quantum vortices, as seen here caused by rapid flow past an obstacle. These tiny quantum whirlpools carry rotation in discrete chunks, give rise to effective superfluid damping, and ultimately to the chaotic dynamics of quantum turbulence. This emergent quantum phenomena offers a simplified realization of the turbulent maelstrom, and a new way to understand the complexities of fluid turbulence.
At very low temperatures many materials become superconducting, i.e. an electrical current can flow through them without resistance. This quantum state of matter is a superfluid of so-called Cooper pairs, a pair of electrons bound together by attractive interactions in the material. A detailed understanding of most superconductors was provided by the Nobel prize-winning 1957 theory of Bardeen, Cooper, and Schrieffer. Over the last thirty years, however, many superconductors have been discovered with properties that are inconsistent with this theory. These so-called "unconventional superconductors" remain very poorly understood. My research focuses on a number of important open questions in this field: What types of unconventional superconductor are possible? What are the interactions which give rise to unconventional superconductivity? How do they differ from conventional superconductors?
I undertake theoretical research in the broad area of quantum physics.
Current research topics include:
Disorder in ultracold atomic gases
Path-Integral Quantum Monte Carlo studies of ultra-cold gases
Finite-temperature theories of ultra-cold gases
Connections between quantum physics and number theory
The Centre for Quantum Science is a University of Otago Research Centre hosted by the Department of Physics.
ashton.bradley [at] otago.ac.nz
niels.kjaergaard [at] otago.ac.nz