2 March 2011

Scientists confirm cold atoms act like lasers

Cosmos Online
A beam of helium atoms has been shown to have properties similar to a laser light beam, according to Australian scientists who have confirmed - for atoms - a theory first developed for light nearly 50 years ago.
atomic laser

A beam of identical atoms, shown in red, pour out of a Bose-Einstein Condensate, shown in blue, to form an atomic laser. Australian scientists have shown that when cooled enough to form this condensate, a beam of atoms can be made to behave in the same way as a laser beam. Credit: Australian National University

SYDNEY: A beam of helium atoms has been shown to have properties similar to a laser light beam, according to Australian scientists who have confirmed – for atoms – a theory first developed for light nearly 50 years ago.

By showing that an atom laser can behave exactly like a light laser, the researchers have achieved a major advance in the understanding of this behaviour that may help scientists to improve other technologies such as atom holography.

“Lasers have a property called coherence, which means that the particles of light (or photons) all march in step”, said team leader Andrew Truscott from the Australian National University in Canberra. “If you measure the time between the arrival of photons in a laser beam, you find that the photons are randomly spaced, with all arrival times between photons being equally probable.”

Both wave-like and particle-like properties

According to Andre Luiten from the University of Western Australia, “One of the most curious aspects of quantum mechanics is so-called wave-particle duality: the fact that a single object can show wave-like properties, such as interference or diffraction, while also showing particle-like characteristics such a well-located lump of mass or energy.”

“This aspect of light has long fascinated and confused physicists, raising questions such as how it is possible for a single photon to be in two places at the same time? How can a single particle carry all the information required to properly exhibit wave-like behaviours?” he said.

In 2005, American theoretical physicist Roy Glauber won the Nobel Prize for providing a theoretical framework to answer these questions in the context of light waves.

“Now, for the first time, this Australian team has shown experimentally that what is true for light is also true for atoms,” said Luiten.

Cold atoms march in step

By making helium atoms extremely cold, the atoms were forced into a state of coherence causing them to march in step.

Thos allowed the scientists to create a beam that behaves almost exactly like a laser beam of photons.

“In order to create the Bose–Einstein Condensate (a coherent source of atoms), we must cool the atoms to within one millionth of a degree of absolute zero,” said co-author, Ken Baldwin from the Australian National University, of the research published in Science.

“These very cold atoms also had a random distribution of arrival times with no bunching, indicating that they were perfectly coherent”, he said.

Heated atoms caused photon bunching

When the helium atoms were heated above this particular temperature, the atoms no longer behaved coherently, and instead behaved just like the light from a conventional light globe – an incoherent light source.

In this state photon bunching occurs, which means the photons arrive in pairs or triplets known as second and third order correlations.

“Our experiment shows – for the first time – that the same second and third order coherence properties also apply to atoms,” said Baldwin.

Improving holography, mineral exploration

“The use of a laser with coherent properties has created technologies like holography and the hope is that similar technologies using atom laser beams will follow,” said Baldwin.

“The advantage would be that because atoms respond to gravity, they will be able to very sensitivity measure the Earth’s gravitational field, and the subtleties of the technology will improve the prospects for mineral exploration,” he said.

“Although, one could say that the experiment has merely confirmed our prior theoretical expectation, I think it is vital to check that our foundational assumptions are valid, particular when conditions are extremely different,” said Luiten.

“One can never be entirely sure that experiments such as this will give the ‘right’ result, so it is crucial to undertake them. It is fantastic that an Australian group has provided the answer to some important questions.”


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