Chemical Physics at the University of Sydney


Energy and charge transport

Image: Joanne Kim / freeimages

Exciton transport in photosynthesis

Photosynthetic organisms harvest light using large antenna complexes containing many chlorophyll molecules. The energy harvested by the antennas is then transported to the reaction centres, where the first chemical steps occur. Recent experiments have suggested that, surprisingly, the excitation energy can be transported in a coherent, wavelike manner. It was previously doubted that quantum effects could survive for so long in biological systems at room temperature.

We recently clarified the relationship between these experimental observations—carried out with pulsed lasers—and the behaviour of the complexes in incoherent natural light. In sunlight, energy transport occurs at steady state, meaning that the wavelike dynamics observed experimentally is, in a sense, an experimental artefact. However, we also identified mechanisms by which quantum effects can enhance light harvesting even in incoherent light, and we are currently working on applying those findings to the design of artificial light-harvesting systems.

Selected papers
Image: Konarka

Photophysics of organic photovoltaics

Organic solar cells differ from conventional inorganic ones not only in their flexibility, but also in their fundamental physics. Organic materials are disordered, meaning that charges tend to be localised and move by hopping, unlike the band transport in inorganic devices. In addition, the localisation of the electron and the hole increases their Coulomb attraction, making it harder to separate them and produce an electric current.

It is still unclear exactly how the charges overcome their considerable electrostatic attraction, sometimes with an efficiency approaching 100%. We are working to understand the fundamental photophysics involved, often in close collaboration with experimentalists.

Selected papers

Quantum computing and quantum simulation

Image: Matthew Broome / EQuS

Photonic quantum computing and simulation

Photons are promising candidates for quantum communication and computation because they are easily isolated from noise, retaining their quantum correlations as long as they are not absorbed. The ability to control them precisely has allowed us—in collaboration with experimental groups such as the UQ Quantum Technology Laboratory—to build simulators of quantum phenomena, such as topologically protected bound states, that have not yet been directly observed in nature.

Although the stability of photons is essential for quantum communication, it also makes it challenging to simulate quantum systems subject to environmental noise. We have developed several techniques for adding controllable amounts of decoherence to photonic circuits, allowing us to demonstrate effects such as environment-assisted quantum transport, which we previously described in the context of photosynthetic energy transport.

Selected papers
Image: net_effect / flickr

Quantum computing for chemistry

Quantum calculations of chemical and physical properties, such as molecular energies or reaction rates, increase in difficulty with the size of the system, sometimes exponentially fast. As a result, the most accurate techniques are restricted to small systems. The fundamental obstacle is the presence of entanglement (correlation), which is difficult to represent efficiently on a classical computer.

We have shown that quantum computers could circumvent this problem in a natural way, by using a controllable quantum process to run a simulation of the unknown process. We have developed a suite of quantum methods for chemical problems, including the simulation of chemical reactions and the determination of properties like the dipole moment. Our proposals have been implemented experimentally on small scales, and we continue to collaborate with experimentalists on pushing the limits of quantum simulation.

Selected papers