NZ quantum physics research featured in Nature

Physicist Ashton Bradley’s research into the behaviour of ultra-cold atoms is gaining international recognition for New Zealand cutting-edge science.

The work is published in the 16 October edition of the prestigious scientific journal Nature. Dr Bradley, most recently based at the University of Queensland, is now a Research Fellow at the University of Otago’s Jack Dodd Centre for Quantum Technology.

The Nature publication draws on more than a decade of theoretical work by Jack Dodd Centre director Crispin Gardiner and colleagues. Their research group studies the properties of Bose-Einstein condensates, a novel state of matter predicted by Einstein more than eighty years ago.

Bose-Einstein condensates are created when gas is cooled very suddenly to temperatures colder than outer space. Under these conditions, the atoms in the gas line up perfectly and develop bizarre, quantum properties. These properties are being explored by physicists around the world, and New Zealand scientists are among the leaders in this highly-specialised field.

Professor Howard Carmichael, the Dan Walls Chair in Theoretical Physics at the University of Auckland comments:

“The formation of a Bose-Einstein condensate can be compared to the way condensation forms on a window–when warm, moist air from indoors hits a cold window surface, droplets of water (fluid) are formed. A Bose-Einstein condensate happens through a rather similar process, but it occurs in a gas cooled to the very lowest temperatures ever reached anywhere in the universe!

“The resulting fluid droplet is then extra special, because its motion—the way it flows—obeys the equations of quantum mechanics…the same equations that govern how electrons circulate inside an atom.

“The research described in this paper investigates the dramatic events that accompany the formation of this extra special droplet. Dramatic I say, because the formation of the droplet is often accompanied by the simultaneous formation of a vortex, a mini whirlpool (or tornado) within the droplet.

“This research combines an experimental investigation that creates Bose-Einstein droplets and detects the vortices (whirlpools) in them, with a theoretical investigation that simulates the droplet and vortice creation on a computer. The close agreement between the simulations and experimental observations is highly significant, particlarly so, because, as I say, the creation is a dramatic event…one not so readily captured by a computer model.

“It is notable that the equations upon which the reported computer simulations are based were developed in New Zealand, by Crispin Gardiner, Director of the University of Otago’s Jack Dodd Centre, and Matthew Davis, now at the ARC Centre of Excellence for Quantum-Atom Optics at the University of Queensland. Co-author on this work is Ashton Bradley, a young New Zealand scientist who recently joined the Physics Department at the University of Otago. Ashton’s contribution as theoretician is an essential part of the combined experiment-theory impact of this piece of science, at the forefront of a very exciting field.

“It is just one example of how a young scientist returning to New Zealand can bring the spotlight of the world’s leading scientific endeavors to the country. I am struck by the fact that two other young physicists who recently returned to New Zealand, also to the University of Otago, have subsequently moved on to greener pastures. One must only hope that Ashton receives the encouragement and support required to induce him to stay and continue to attract the scientific spotlight to New Zealand.”

Professor John Harvey, Department of Physics, University of Auckland comments:

“The paper in Nature reports major progress on understanding the creation of quantized vortices which occur in superfluids. Lest this seem mindbogglingly abstruse, the work has relevance to understanding the creation of the universe and to the development of high precision instrumentation.

“It is a product of a series of international collaborations which had their origin in Ashton’s PhD thesis completed in Wellington in 2002, and I am particularly pleased to see people such as Ashton returning to New Zealand.

“This paper is evidence that our New Zealand scientists can not only participate in international scientific research at the forefront of their field, but that they can develop their careers using world class facilities here. For far too long New Zealand has simply exported top scientists for the benefit of other countries.”

Professor Robert Ballagh, Head of the Department of Physics at University of Otago comments:

“This work is a very significant achievement by a very talented young theoretical physicist, who has recently returned to New Zealand. The work represents an important contribution to our understanding of the behaviour of matter in the new pure quantum states that we have only recently been able to create. The experiment reported, and its analysis, is the first time we have been able to observe and understand the spontaneous formation of quantum vortices. The experiment has been done using ultra-cold atoms, but it also has some broad implications for other areas, because this mechanism may be important in the early stages of the formation of the universe.

“Ashton did his Ph D work in New Zealand, in the area of fundamental quantum theory, for which New Zealand has earned a very strong international reputation over the past three decades or so. He has built on foundation work of New Zealand physicists, and has developed a powerful methodology which gives detailed understanding of these complex modern experiments. He has fantastic technical skills, and very deep insight into the fundamentals of quantum theory. It is wonderful that we have to be able to attract him back to New Zealand with the help of a FRST postdoctoral Fellowship. It’s a ‘good news’ story of reversing the brain drain with one of our brightest young New Zealanders, and we are delighted that our group has been able to offer the stimulating environment that has helped to bring him back.”

Joachim Brand, Associate Professor, Massey University Institute of Fundamental Sciences, comments:

“More than 30 years ago Tom Kibble proposed that structures of enormous size that are still observed in our universe could have been formed during a period of rapid cool down not long after the big bang. While separate growing regions of space would change their phase similar to water vapour condensing into rain drops, Kibble’s theory predicts that string-like objects could remain where the boundaries of these space regions met. A collaboration of researchers from the USA, Australia, and New Zealand has now put Kibble’s theory to the test in a laboratory experiment.

“Instead of experimenting directly with cosmic strings, which would hardly fit into a terrestrial laboratory, the researchers have studied the shock frosting of an ultra-cold gas of Rubidium atoms. It has been suggested by W. H. Zurek that Kibble’s theory should also apply to superfluids, i.e. liquids without viscosity, such as found in liquid Helium and Bose-Einstein condensates of ultra-cold atomic gases.

“During the rapid cooling down to nano-Kelvin temperatures, the atomic gas in the current experiment undergoes a transition into a Bose-Einstein condensate, a superfluid state that behaves like a liquid without viscosity. As predicted by Kibble’s and Zurek’s theory, the researchers see in some of their repeated experiments that vortices appear. Vortices are little tornadoes that from a string-like structure in the Bose-Einstein condensate form just like the cosmic strings predicted by Kibble.

“This is not the first experiment to study the Kibble-Zurek mechanism of defect formation in a condensed matter context. Earlier experiments have been done with superfluid Helium. Nevertheless, the current experiment with a Bose-Einstein condensate sets a landmark by providing unprecedented control and diagnostics. It is also significant that simulations have been able to find quantitative agreement with the experimental findings. This opens the door to further our understanding of the Kibble-Zurek mechanism on a quantitative basis. This is also a great success for the theoretical understanding of Bose-Einstein condensates at finite temperatures, to which New Zealand scientists have made significant contributions.

“This work directly involves a young New Zealand scientist who returned to New Zealand after spending time abroad. I am very happy to see this happening and resulting in work that is recognised on a world scale. This success of this work shows that New Zealand is playing a significant role in global science and it is very important that this is realised in New Zealand.”

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