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Physicists solve quantum tunneling mystery

An international team of scientists studying ultrafast physics have solved a mystery of quantum mechanics, and found that quantum tunneling is an instantaneous process.

The new theory could lead to faster and smaller electronic components, for which quantum tunneling is a significant factor. It will also lead to a better understanding of diverse areas such as electron microscopy, nuclear fusion and DNA mutations.

“Timescales this short have never been explored before. It’s an entirely new world,” said one of the international team, Professor Anatoli Kheifets, from The Australian National University (ANU).

“We have modelled the most delicate processes of nature very accurately.”

At very small scales quantum physics shows that particles such as electrons have wave-like properties – their exact position is not well defined. This means they can occasionally sneak through apparently impenetrable barriers, a phenomenon called quantum tunneling.

Quantum tunneling plays a role in a number of phenomena, such as nuclear fusion in the sun, scanning tunneling microscopy, and flash memory for computers. However, the leakage of particles also limits the miniaturisation of electronic components.

Professor Kheifets and Dr. Igor Ivanov, from the ANU Research School of Physics and Engineering, are members of a team which studied ultrafast experiments at the attosecond scale (10-18 seconds), a field that has developed in the last 15 years.

Until their work, a number of attosecond phenomena could not be adequately explained, such as the time delay when a photon ionised an atom.

“At that timescale the time an electron takes to quantum tunnel out of an atom was thought to be significant. But the mathematics says the time during tunneling is imaginary – a complex number – which we realised meant it must be an instantaneous process,” said Professor Kheifets.

“A very interesting paradox arises, because electron velocity during tunneling may become greater than the speed of light. However, this does not contradict the special theory of relativity, as the tunneling velocity is also imaginary” said Dr Ivanov, who recently took up a position at the Center for Relativistic Laser Science in Korea.

The team’s calculations, which were made using the Raijin supercomputer, revealed that the delay in photoionisation originates not from quantum tunneling but from the electric field of the nucleus attracting the escaping electron.

The results give an accurate calibration for future attosecond-scale research, said Professor Kheifets.

“It’s a good reference point for future experiments, such as studying proteins unfolding, or speeding up electrons in microchips,” he said.

The research is published in Nature Physics.

Source: ANU

Experiment confirms quantum theory weirdness

The bizarre nature of reality as laid out by quantum theory has survived another test, with scientists performing a famous experiment and proving that reality does not exist until it is measured.

Physicists at The Australian National University (ANU) have conducted John Wheeler’s delayed-choice thought experiment, which involves a moving object that is given the choice to act like a particle or a wave. Wheeler’s experiment then asks – at which point does the object decide?

Common sense says the object is either wave-like or particle-like, independent of how we measure it. But quantum physics predicts that whether you observe wave like behavior (interference) or particle behavior (no interference) depends only on how it is actually measured at the end of its journey. This is exactly what the ANU team found.

“It proves that measurement is everything. At the quantum level, reality does not exist if you are not looking at it,” said Associate Professor Andrew Truscott from the ANU Research School of Physics and Engineering.

Despite the apparent weirdness, the results confirm the validity of quantum theory, which governs the world of the very small, and has enabled the development of many technologies such as LEDs, lasers and computer chips.

The ANU team not only succeeded in building the experiment, which seemed nearly impossible when it was proposed in 1978, but reversed Wheeler’s original concept of light beams being bounced by mirrors, and instead used atoms scattered by laser light.

“Quantum physics’ predictions about interference seem odd enough when applied to light, which seems more like a wave, but to have done the experiment with atoms, which are complicated things that have mass and interact with electric fields and so on, adds to the weirdness,” said Roman Khakimov, PhD student at the Research School of Physics and Engineering.

Professor Truscott’s team first trapped a collection of helium atoms in a suspended state known as a Bose-Einstein condensate, and then ejected them until there was only a single atom left.

The single atom was then dropped through a pair of counter-propagating laser beams, which formed a grating pattern that acted as crossroads in the same way a solid grating would scatter light.

A second light grating to recombine the paths was randomly added, which led to constructive or destructive interference as if the atom had travelled both paths. When the second light grating was not added, no interference was observed as if the atom chose only one path.

However, the random number determining whether the grating was added was only generated after the atom had passed through the crossroads.

If one chooses to believe that the atom really did take a particular path or paths then one has to accept that a future measurement is affecting the atom’s past, said Truscott.

“The atoms did not travel from A to B. It was only when they were measured at the end of the journey that their wave-like or particle-like behavior was brought into existence,” he said.

The research is published in Nature Physics.

Source: ANU

Dr Vladlen Shvedov (L) and Dr Cyril Hnatovsky adjusting the hollow laser beam in their lab at RSPE. Image Stuart Hay, ANU

ANU Physicists build reversible tractor beam

We have seen use of laser tractor beams from space ships catching or repelling space ships, objects and people. Science and technology have not developed that much to achieve such feats but Physicists at the Australian National University have done something amazing to push the boundaries science and technology a bit more and closer to that goal.


ANU Laser physicists have built a tractor beam that can repel and attract objects, using a hollow laser beam that is bright around the edges and dark in its centre.

Dr Vladlen Shvedov (L) and Dr Cyril Hnatovsky adjusting the hollow laser beam in their lab at RSPE. Image Stuart Hay, ANU
Dr Vladlen Shvedov (L) and Dr Cyril Hnatovsky adjusting the hollow laser beam in their lab at RSPE. Image Stuart Hay, ANU

It is the first long-distance optical tractor beam and moved particles one fifth of a millimetre in diameter a distance of up to 20 centimetres, around 100 times further than previous experiments.

“Demonstration of a large scale laser beam like this is a kind of holy grail for laser physicists,” said Professor Wieslaw Krolikowski, from the Research School of Physics and Engineering.

The new technique is versatile because it requires only a single laser beam. It could be used, for example, in controlling atmospheric pollution or for the retrieval of tiny, delicate or dangerous particles for sampling.

The researchers can also imagine the effect being scaled up.

“Because lasers retain their beam quality for such long distances, this could work over metres. Our lab just was not big enough to show it,” said co-author Dr Vladlen Shvedov, a driving force behind the ANU project, along with Dr Cyril Hnatovsky.

Unlike previous techniques, which used photon momentum to impart motion, the ANU tractor beam relies on the energy of the laser heating up the particles and the air around them. The ANU team demonstrated the effect on gold-coated hollow glass particles.

The particles are trapped in the dark centre of the beam. Energy from the laser hits the particle and travels across its surface, where it is absorbed creating hotspots on the surface. Air particles colliding with the hotspots heat up and shoot away from the surface, which causes the particle to recoil, in the opposite direction.

To manipulate the particle, the team move the position of the hotspot by carefully controlling the polarisation of the laser beam.

“We have devised a technique that can create unusual states of polarisation in the doughnut shaped laser beam, such as star-shaped (axial) or ring polarised (azimuthal),” Dr Hnatovsky said.

“We can move smoothly from one polarisation to another and thereby stop the particle or reverse its direction at will.”

The work is published in Nature Photonics.

Source : ANU News

Time to move to a post-carbon world: ANU VC Professor Ian Young responds to criticism over divestments

Australian National University’s decision to divest from some companies due to concerns mainly related to environment or carbon pollution. The decision has sparked fury from some business and political interest groups.

Responding to the allegations, Professor Ian Young wrote on ANU’s website and The Sydney Morning Herald.

According to Mr. Young :

Just over a week ago, The Australian National University decided to sell shares worth approximately $16 million in seven companies, representing just one per cent of our investment portfolio, and a fraction of the market worth of the companies involved, which has sparked an extraordinary reaction.

From one side it has been attacked by elements of industry, media and some political figures as reckless, cowardly, superficial, anti-business, poorly conceived and as destroying jobs.

On the other side, my email account has melted down with emails of support, congratulating the University on its action, and the University’s Facebook page is awash with positive comments.

The reason for this extraordinary response is because the ANU decision is seen as another domino in the divestment-movement effect, involving individuals and institutions deciding to sell their holdings in fossil fuel-producing companies.

He further said:

There has been growing sentiment from our community to not just get a good financial return from our investments but also to invest in companies which would have activities consistent with the goals of the University, and do not manifestly cause social harm. For instance, the University for many years has not, and would not now, invest in tobacco

The initial calls were to divest from all fossil fuels. This is difficult in Australia, as many of our companies are diversified. They may produce coal, oil or gas but they also do many other things. And given the world’s necessary dependence on such fuels for a long time to come, the ethical issues involved are complex. To address these issues ANU established a socially responsible investment policy.

Not only Mr. Young conveyed his view point on the criticism but also provided a broad picture about the debate:

The real debate for Australia should be about jobs in a carbon-constrained world. What will our industries be in 20 or 30 years’ time? I am confident they will not be in producing fossil fuels. Australia should not be an adopter of alternative energy, we should be a producer.

The real debate in climate should be about producing cost-effective alternative energy. Sticking our collective heads in the sand and ignoring a changing world will ensure we do destroy jobs. Universities like the ANU should be the powerhouses to produce the new technologies for such a world.

The key here is for the various parties not to go to their collective corners and throw stones, but rather for us to work together and use the window of transition to ensure Australia is a technological leader in the post-carbon world.

In an email to Alumni, The ANU VC also urged former students to take part in the debate and give their views:

Dear student

As you may be aware, last week the University Council decided to sell a relatively small number of shares in seven companies. The decision has sparked an extraordinary reaction. I’ve written about the matter in an Op Ed published today.

ANU invests for the betterment of its community – students, staff and researchers. The returns on these investments fund scholarships, staff salaries, research projects and new infrastructure. The University has a responsibility to invest wisely but also in a manner consistent with the desires of our stakeholder students, alumni and staff.

To this end, the decision to divest was made after a review commissioned as part of our Socially Responsible Investment Policy. The review was undertaken by the independent Centre for Australian Ethical Research (CAER) and provided Environmental, Social and Governance Ratings on ANU-held domestic stocks. Using an internationally recognised methodology, our investments were assessed against environmental, social and governance criteria.

The ANU community – staff, students and alumni – has been very engaged in the debate about divestment. As the national university, we have a role to play in national and global debates of this kind.

As always, I welcome your views.

Professor Ian Young AO
Vice-Chancellor

The main post is available on ANU website link: http://vcdesk.anu.edu.au/2014/10/13/time-to-move-to-a-post-carbon-world/#comment-8291

Source: ANU

Dr Horst Punzmann (left) and Professor Michael Shats test their wave-generated tractor beam. Photo by Stuart Hay. Credit : ANU

Physicists create water tractor beam

Dr Horst Punzmann (left) and Professor Michael Shats test their wave-generated tractor beam. Photo by Stuart Hay. Credit : ANU
Dr Horst Punzmann (left) and Professor Michael Shats test their wave-generated tractor beam. Photo by Stuart Hay. Credit : ANU

Physicists at The Australian National University have created a tractor beam on water, providing a radical new technique that could confine oil spills, manipulate floating objects or explain rips at the beach.

The group, led by Professor Michael Shats, discovered they can control water flow patterns with simple wave generators, enabling them to move floating objects at will.

“We have figured out a way of creating waves that can force a floating object to move against the direction of the wave,” said Dr Horst Punzmann, from the Research School of Physics and Engineering, who led the project.

“No one could have guessed this result,” he said.

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The new technique gives scientists a way of controlling things adrift on water in a way they have never had before, resembling sci-fi tractor beams that draw in objects.

Using a ping-pong ball in a wave tank, the group worked out the size and frequency of the waves required to move the ball in whichever direction they want.

Advanced particle tracking tools, developed by team members Dr Nicolas Francois and Dr Hua Xia, revealed that the waves generate currents on the surface of the water.

“We found that above a certain height, these complex three-dimensional waves generate flow patterns on the surface of the water,” Professor Shats said. “The tractor beam is just one of the patterns, they can be inward flows, outward flows or vortices.”

The team also experimented with different shaped plungers to generate different swirling flow patterns.

As yet no mathematical theory can explain these experiments, Dr Punzmann said.

“It’s one of the great unresolved problems, yet anyone in the bathtub can reproduce it. We were very surprised no one had described it before.”

Source : ANU