Physics, The University of Queensland
HEAD OF PHYSICS SCHOOL OF MATHEMATICS AND PHYSICS
THE UNIVERSITY OF QUEENSLAND
Physics at the University of Queensland (UQ) is one of the core disciplines in the Faculty of Science with a recognized national and international reputation for teaching and research. It was first established with the appointment of Thomas Parnell in 1911 who was to become the Chair of Physics in 1919. After its early focus on teaching, the department grew and developed research areas including ionospheric physics. In 1955, the University was relocated from the city to the new St Lucia campus, and Physics was housed in the Parnell Building, one of the sandstone buildings of the "Great Court" (Fig. 1). The department expanded further in the 1970s and 1980s with emerging research areas such as quantum optics and laser physics being added to existing areas that included astrophysics, microwave spectroscopy and geophysics. In 1999 Physics was merged with Mathematics and Earth Sciences to form the School of Physical Sciences. A further restructure in 2009 led to Mathematics and Physics becoming the School of Mathematics and Physics as it is today.
Fig.1: The Parnell Building at the University of Queensland. Inset: Thomas Parnell.
Physics has around 20 full time continuing Teaching and Research academics of which about 10 currently hold research fellowships including Australian Research Council Future Fellows, Australian Professorial Fellowships and Federation/Laureate Fellowships. There are around a further 25 research only staff who hold competitive research fellowships or who are employed on research grants.
The research undertaken by current staff members is wide and varied and is broadly categorized into the four areas of
• Biophotonics and Laser Science
• Condensed Matter Physics
• Quantum Science
Physics has an active undergraduate teaching program with majors in physics forming part of the Bachelor of Science degree and the more recently introduced Bachelor of Advanced Science degree. First year core courses generally have around 250 students enrolled. We also teach service courses for engineering and biology/biomedical science students with many hundreds of students each semester.
Physics staff supervise postgraduate students at honours, masters and PhD level. These students make an important contribution to the research output from Physics with many moving on to occupy senior positions within industry, the public sector and academia. Each year there are around 10-20 honours students and over 50 postgraduate students enrolled.
Physics has a very active research program and is ranked highly in the Australian Government's "State of University Research" 2015-16 Report. Astronomical and Space Sciences, Condensed Matter Physics and Quantum Physics are all recognized as being "well above world standard" (the highest rating) while Optical Physics is rated as "above world standard". Further information about each research area is given below.
UQ Astrophysics research is focused on origins at both large scales (cosmology) and smaller scales (the formation of stellar systems and galaxies). We host a node of the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO).
Fig.2: Galaxies and supernovae measured by the OzDES project. The most distant galaxies (top right) are 10 billion light years away. (Credit E. Macaulay.)
The biggest challenge for modern cosmology is to explain why the expansion of the universe is accelerating, contrary to our expectation that gravity should slow it down. We are using optical and radio telescopes to test models of this acceleration. UQ co-leads the Australian Dark Energy Survey (OzDES), which is doing the first deep time-lapse optical spectroscopic survey of the sky. By repeatedly observing the same patches of sky for five years we will detect more than 3,000 supernovae. These let us measure the distances to galaxies up to 10 billion light years away (Fig. 2). We will use the galaxy distances to test if Einstein's cosmological constant is the best explanation for the acceleration, or an alternative theory of gravity is better.
The 36-dish Australian Square Kilometre Array Pathfinder radio telescope will undertake the Evolutionary Map of the Universe survey. This will detect about 70 million radio sources, the signals from some of which were emitted as long ago as 500 million years after the Big Bang. We will use the radio sources to measure the structures that were created in the early universe, and how they have evolved over its history, testing the law of gravity on the largest scales.
Fig.3: The massive elliptical galaxy M 60 with its tiny ultra-compact dwarf galaxy neighbour M60-UCD1 (highlighted). We have discovered that M60-UCD1 contains a massive black hole. (Credit NASA/ESA.)
In recent years it has become clear that black holes play a key role in regulating the formation of galaxies. We are using supercomputers to model the dynamics of the stars around black holes and resolved spectroscopy on large telescopes to measure the stellar velocities. Our modelling recently helped discover the first massive black hole in a tiny ultra-compact dwarf galaxy (Fig. 3). This discovery proves our hypothesis that these objects form from the disruption of larger galaxies as massive black holes cannot form in such small systems.
Biophotonics and Laser Science
Research in the Biophotonics and Laser Science group encompasses a range of areas linked through the user of lasers, optics and spectroscopy to study applications ranging from single molecule nanoprobes through micron sized biological systems and up to large scale hypersonic flow.
The Optical Micromanipulation Group examines the interaction of light and matter at the micron scale. The group has developed and applied methods for trapping and spinning microscopic particles using optical light fields. Trapping is achieved using "Optical Tweezers" resulting from the gradient force generated by a focused laser beam. Such forces are small but sufficient to control transparent microscopic particles with applications in biology and biomedicine. The theory behind the trapping force has been developed and sophisticated numerical methods have been implemented to calculate the forces and understand the underlying physics (Fig. 4a).
Fig.4: Optical Micromanipulation. (a) Calculation of the light field around a 2µm particle; (b) Scanning electron microscope image of an "optical paddle wheel".
Laser light has also been used to rotate trapped particles. For example, small calcite particles have been rotated at rates of hundreds of hertz using light fields with spin angular momentum. This work has evolved to the development of sophisticated micromachines such as in the image shown in Fig. 4(b). Novel optical fields have also been generated with orbital angular momentum about a central optical vortex, also capable of spinning microscopic particles.
Fig.5: Holographic Interferometry of high enthalpy flow. (a) Flow of air over a re-entry capsule travelling at 10 km/s; (b) Interferogram of flow on the intake of a supersonic combustion ramjet engine; (c) Density field obtained from the interferogram in (b).
The Laser Diagnostic Group uses light to probe conditions in high enthalpy flows generated in experimental facilities at the Centre for Hypersonics. The group aims to develop optical techniques that can be used for flow visualisation and quantitative measurements. Emission spectroscopy has been used to study the species present in the high temperature flows, and to characterise absolute radiances generated by the flow. Interferometric techniques have been developed to image and measure density variations in the flow (Fig. 5a). In combusting flows, relevant for the development of hypersonic aircraft, interferometry and planar laser-induced fluorescence have been applied to visualise the flow, the latter technique allowing the identification of locations where the OH molecule has been generated indicating ignition. Raw and processed interferograms are shown in Fig. 5(b) and Fig. 5(c).
Condensed Matter Physics
The Condensed Matter Physics program at UQ is big question focused. We operate at the boundary between physics, chemistry and biology. We address key questions in specific systems such as organic superconductors, biological macromolecules, and organic electronics. Our approach is based upon combining theory with experiment across a range of disciplines such as quantum many body theory, quantum chemistry, synthetic organic chemistry, molecular biophysics, spectroscopy and experimental solid state physics. We encompass both hard and soft matter systems.
In the materials found in your laptop or mobile phone the interactions between the electrons are relatively small; this greatly simplifies the task of describing their behaviour. Technically, one says that the electrons in these materials are 'weakly correlated'. Over the past few decades many new materials have been found which cannot be described within the weakly correlated paradigm. These materials, which are referred to as 'strongly correlated' and include cuprates, manganites and organic charge transfer salts, have a wide variety of potential applications including catalysis, energy applications, and display technologies. However, our lack of fundamental understanding of strongly correlated materials greatly hampers materials development for such applications.
Qualitatively new behaviours emerge as one increases the complexity of a system. For example, when one grows crystals of a single atomic species one can observe insulating, semiconducting, metallic and superconducting states - none of these behaviours are seen in individual atoms. Strongly correlated materials show yet more novel effects such as colossal magnetoresistance in the manganites, high temperature superconductivity in the cuprates and heavy fermion behaviour in many rare earth compounds. If we further increase the complexity of the systems we reach organic chemistry and, eventually, biology. Organic charge transfer salts represent an important rung on this ladder of complexity as they sit between the most complicated inorganic crystalline materials, such as transition metal oxides, and the simplest biomolecular systems. Organic charge transfer salts display a fascinating range of behaviours, many of which are not yet well understood. Thus, organic charge transfer salts are model systems for the study of strongly correlated electrons in chemically complex systems.
Fig.6: A family of phosphorescent dendrimers under investigation for use in flat panel displays.
OLEDs are an exciting new technology for flat panel displays that may even lead to roll up television screens. Light-emitting dendrimers consist of surface groups, dendrons (branched units) and cores. Dendrimers have a number of potential advantages over existing OLED materials (small molecules and polymers) including a modular approach to synthesis leading to dendrimer libraries and disconnection of the electronic and processing properties. Fig. 6 illustrates an example of a family of highly efficient phosphorescent dendrimers that are currently exciting international academic and industrial interest.
The melanins are a class of functional bio-macromolecule found throughout nature. In humans they serve as our primary photoprotectants and pigment. As a biomolecule class they possess a number of intriguing physico-chemical properties including: broad band monotonic absorbance in the UV and visible; condensed phase electrical and photoconductivity; extremely efficient non-radiative relaxation of photoexcited electronic states (melanins have radiative quantum yields <1%); and free radical scavenging and anti-oxidant behaviour. We focus our attentions primarily on eumelanin (the most prevalent form in humans) which are macromolecules of indolequinones. Our work is motivated by the general desire to better understand how these systems are put together and why they behave like they do, and also by the intriguing suggestion that we may be able to create a new class of bio-inspired high tech organic material.
Quantum Science has been a recognized research area of strength at the University of Queensland for over two decades. There are experimental and theoretical research groups investigating quantum atom-optics, quantum nanomechanics, quantum superconducting circuits, and quantum photonics. Since 2000, Physics at UQ has been an intrinsic part of five Australian Research Council Centres focussed on quantum research. UQ is the only university in Australia to host multi-investigator nodes for both current (2011-2017) quantum Centres of Excellence - Engineered Quantum Systems (EQuS) and Quantum Computing and Communication Technology (CQC2T).
In quantum optomechanics optical fields are used to control and manipulate the quantum behaviour of a micro- or nano-mechanical oscillator. Such research has prospects for not only fundamental tests of quantum mechanics at size scales inaccessible to other approaches, but allow applications in precision sensing, metrology, and information technology. The group is currently working across a range of areas including developing and applying new quantum control techniques that enable non-classical states of mechanical oscillators to be generated and studied, developing optomechanical systems with very high light-mechanics coupling strengths, and applying quantum control to enhance sensing applications of micromechanical systems.
Photonic-crystal based integrated optical systems have been used for a broad range of sensing applications with great success. This has been motivated by several advantages such as high sensitivity, miniaturization, remote sensing, selectivity and stability. Many photonic crystal sensors have been proposed with various fabrication designs that result in improved optical properties. We developed a proposal for a novel multipurpose sensor architecture that can be used for force, refractive index and possibly local temperature detection. In this scheme, two coupled cavities behave as an effective beam splitter. The sensor works based on fourth order interference — the Hong-Ou-Mandel effect — a uniquely quantum phenomenon.
Ultracold atom experiments over the past decade have demonstrated a high degree of precision and control over a number of system parameters, such as confinement geometries, system dimension, and engineering a range of different interparticle interactions. Recently, the state of the art has been to achieve single atom imaging resolution in a single component degenerate gas held in an optical lattice. This impressive technology has enabled a number of experimental demonstrations of quantum simulations/emulations using ultracold atoms. We have embarked on a similar route in the Atom Optics Laboratory, but with an additional innovation: we are developing an experiment with similar imaging capability, but for a two-species bosonic quantum gas, consisting of 87Rb and 41K (see Fig. 7). With their large symmetry groups, such multicomponent gases exhibit a wide range of phases and non-trivial dynamics. In particular, these two species can be experimentally driven far from equilibrium by utilising a magnetic resonance.
Fig.7: Absorption images of atom distributions created using a digital-micromirror device patterned optical trap.
Single electrons individually trapped and manipulated in semiconductors are one of the most promising avenues for engineered quantum systems. This project investigates the ways in which the magnetic moment, or spin, of these electrons can be controlled, either electrically or through applying microwaves, and how acoustic vibrations, or phonons, affect this control. These results highlight the role of the phononic environment in understanding the driven dynamics of coherent quantum systems and provide a path for transducing quantum information between photons, phonons, spins and charge.
Superconducting qubits are one of the leading platforms for quantum computation. The three lowest energy states of a superconducting artificial atom constitute the most logical realization of a qutrit: a system with almost equidistant energy levels. However, due to the latter property, realization of a projective measurement on a particular state without disrupting quantum coherence in two other states poses a substantial challenge to test the Kochen-Specker (KS) inequality with superconducting qutrits. Using 3D superconducting qutrit of the transmon type incorporated into microwave cavity, we engineered the dispersive shifts of the cavity frequency for the first and second excited states to be identical. As a result, an observer cannot distinguish between these two states by measuring transmission of microwave radiation through the cavity. We experimentally tested that our scheme realizes the strong projective measurement on the ground state of a qutrit by measuring quantum coherence between different levels of the qutrit, a prerequisite for testing of contextuality. In the next step, we are going to use this property and our capabilities for quantum manipulation of the state of the qutrit for measurement of correlation for different pairs of observables in order to violate KS inequality.
Quantum photonics can be used to experimentally emulate natural and engineered quantum systems. Quantum mechanics is an outstandingly successful theory of nature at the small scale. Yet we are still unable to apply it exactly to situations more complicated than, say, 4 or 5 atoms — let alone a caffeine or cholesterol molecule — as the number of equations grows exponentially with the number of particles. Instead, a host of approximate methods have been developed to use quantum mechanics in fields such as biology, chemistry, and materials science, but this approach raises the concern that natural behaviours are being missed, and limits the development of new technologies. Nearly thirty years ago, Nobel Laureate Richard Feynman proposed a better solution: model quantum systems with technology that is itself quantum mechanical. Recent years have seen two approaches to this: simulation, where digital outcomes yield the desired physical quantities, e.g. using a quantum computer; and emulation, where physical measurements yield the physical quantities, e.g. spatial probabilities of a quantum walk.
Quantum information theory has been used for the design of a quantum information network using quantum repeater nodes — which enable "flying qubits" (photons) to be converted to stationary qubits for storage and potentially for deterministic multi-qubit processing — and to design an interface between photons and stationary qubits (e.g. solid-state qubits). This has applications both for quantum repeaters and for single-photon sources, which are necessary for scalable quantum computing using linear optics.
Ultracold matter and quantum gases are perfect systems for exploring the emergence of macroscopic quantum phenomena from microscopic quantum mechanics. We seek to understand aspects of quantum many-body physics via the interpretation of experimental data, and are particularly interested in non-equilibrium phenomena, both in weakly and strongly interacting regimes. Bose-Einstein condensates (BECs) are a relatively new state of matter first observed in 1995 (and the discovery of which led to the Nobel Prize in Physics in 2001). They form at ultra-cold temperatures and have similar properties to light from a laser. However, they are rendered much more complex due to the fact that atoms are interacting - they undergo collisions. One intriguing possibility is the creation of the atom laser, which will be able to be used in ultra-precise measurement devices.
Fig.8: An Active Learning lecture in first year physics. Inset: The UQ Pitch Drop Experiment.
The Physics undergraduate degrees provide students with a broad education with streams covering thermodynamics and condensed matter physics, fields (including electromagnetism), quantum physics, dynamics, astrophysics, experimental physics and computational physics. Theory classes are supported by experiments across most core courses from first to third year.
Our teaching philosophy is to use the active learning approach including pre-reading and on-line pre-reading quizzes, interactive classes (Fig. 8) with feedback devices, and practical/concept laboratories.We continue to develop this approach supported by internal and external funding and have published the outcomes of our studies. The teaching program has been recognized by the Australian Institute of Physics as "innovative and progressive". Four current staff hold national teaching citations for their work.
Physics runs an extensive outreach program engaging with schools and school students, professional organisations and the wider community. The Science Demo Troupe presents demonstrations at schools across Queensland and in lectures for undergraduate classes with the aim of cultivating the interest and understanding of physics of students at all levels. Science at a higher level is promoted in a variety of ways participation in the Queensland Junior Physics Olympiad, the BrisScience public lecture series and the UQ Physics Museum.
A highly recognizable aspect of UQ Physics is the Pitch Drop Experiment, listed in the Guinness Book of Records as being the world's longest-running laboratory experiment. This experiment, started in 1927 by Prof. Thomas Parnell, consists of a funnel filled with a highly viscous tar substance. Since its inception, nine drops have "dripped" from the funnel and the watch is now on, live on the internet, for the tenth drop.
Acknowledgement: The author would like to acknowledge the contributions of staff members within the School of Mathematics and Physics, and particularly contributions from Prof. Michael Drinkwater (Astrophysics), Prof. Ben Powell (Condensed Matter Physics), and Prof. Halina Rubinstein-Dunlop and Andrew White (Quantum Physics).
Timothy McIntyre is an Associate Professor in Physics at the University of Queensland. He graduated with a PhD at the Australian National University in Canberra in 1989. He held postdoctoral positions at the German Aerospace Research Establishment (DLR) and at the Australian National University before moving to Brisbane in 1995. He performs research on laser diagnostic imaging particularly applied to studying hypersonic flow.