Taking quantum leaps

Quantum physics promises a revolution in technology as quantum computers and communication systems come on line. The new breakthroughs in quantum mechanics are just as fascinating and mind-boggling as ever.

Quantum leap for gravitational wave astronomy

The Department of Quantum Science project has specialized in Quantum Squeezing, a method for drastically reducing the noise in laser signals for thirty years. They have developed squeezing system to improve the sensitivity of the gravitational wave detectors that is being installed at LIGO.

"It will expand how far we can see out into the dark universe 1000 times," says Professor David McClelland.

Quantum curiosities key to gravity measurements

Bose-einstein condensates might have begun as quantum curiosities, but now Professor John Close has shown how these collections of ultra-cold atoms can be used to make extremely sensitive measurements of gravity.

The potential has intrigued the Defence Department who have granted $2.7M over 3 years to transform the sensitive lab experiment into a field-ready device.
 

Quantum weirdness experiment probes the nature of reality.

"Those who are not shocked when they first come across quantum theory cannot possibly have understood it," said Niels Bohr, one of quantum theory’s great pioneers, summing up how great a stumbling block it is for students.

Those who are not shocked when they first come across quantum theory cannot possibly have understood it

One hundred years later, quantum experiments at ANU are still confounding common sense, with Associate Professor Andrew Truscott demonstrating yet another form of quantum weirdness, in a quantum interference experiment with helium atoms.

Dr Truscott’s team in the Laser Physics Centre conducted the famous 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?

Andrew Truscott and Roman Khakimov

Andrew Truscott and Roman Khakimov

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 Truscott.

Making quantum technology a reality

ANU hosts a node of the Centre for Quantum Computation and Communication Technology, which is preparing quantum technology to take a practical place in computing technology. With nodes across Australia it is one of the largest combined and focused efforts in quantum computation and communication in the world.

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 in the Laser Physics Centre.

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.

Quantum Hard Drive record

Associate Professor Matt Sellars' team at the Laser Physics centre set a record for a quantum hard drive, storing fragile quantum information for 6 hours.

The hundred-fold improvement, achieved by storing quantum information in atoms of the rare earth element europium embedded in a crystal, brings a worldwide data encryption network based on quantum information a step closer.

"We can now imagine storing entangled light in separate crystals and then transporting them to different parts of the network thousands of kilometres apart," said lead author of their Nature paper, Manzin Zhong, a PhD student in the Laser Physics Centre.

"We are thinking of our crystals as portable optical hard drives for quantum entanglement."

The team's record storage time of six hours, which could be used for banking transactions and personal emails.

"We believe it will soon be possible to distribute quantum information between any two points on the globe," said lead author Manjin Zhong.

"Quantum states are very fragile and normally collapse in milliseconds. Our long storage times have the potential to revolutionise the transmission of quantum information."

The team of physicists at ANU and the University of Otago stored quantum information in atoms of the rare earth element europium embedded in a crystal.

Their solid-state technique is a promising alternative to using laser beams in optical fibres, an approach which is currently used to create quantum networks around 100 kilometres long.

"Our storage times are now so long that it means people need to rethink what is the best way to distribute quantum data," Ms Zhong said.

"Even transporting our crystals at pedestrian speeds we have less loss than laser systems for a given distance."

"We can now imagine storing entangled light in separate crystals and then transporting them to different parts of the network thousands of kilometres apart. So, we are thinking of our crystals as portable optical hard drives for quantum entanglement."

Updated:  24 March 2017/ Responsible Officer:  Director, RSPE/ Page Contact:  Physics Webmaster