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Cool laser makes atoms march in time
Atoms chilled to more than -270C start to behave just like light particles in a laser beam, according to new Australian research.

The discovery could lead to the development of exciting new technologies including atom holograms, says Dr Andrew Truscott, who led the research team, at the ARC Centre of Excellence for Quantum-Atom Optics at ANU in Canberra.

Truscott and colleagues showed that when helium atoms are extremely cold - within one millionth of a degree of absolute zero - they are forced into a state of coherence where they will travel in much the same way as photons travel in a laser.

They say their work, which appears today in the journal Science, is the best proof yet of a theory first developed nearly 50 years ago for light.

"Lasers have a property called coherence," says Truscott.

"If you measure the time between the arrivals of the light particles in a laser beam, you find that they are randomly spaced, with all arrival times between particles equally probable. Which means that the particles of light - or photons - all march in step."

On the other hand, incoherent sources of light - such as ordinary light bulbs - exhibit something called photon bunching, where it is more likely that the light particles arrive within a short space of time of each other.

This bunching in an incoherent light source is manifested by photons arriving in pairs or triplets, known as second-order or third-order bunching.

Team member Professor Ken Baldwin says their very cold atom laser also had a random distribution of arrival times with no bunching - indicating that it was perfectly coherent.

"Our experiment shows - for the first time - that atoms can be made to behave in the same way as light in a laser," he says.

When the researchers warmed the atoms, they no longer behaved coherently, and once again exhibited bunching in pairs and triplets.

"Our work is the most stringent test so far of this phenomenon," Truscott says.

In the future, atoms with these properties could be used to make holograms using atom beams, say the researchers. Other potential applications are devices that use the coherent atom beams to measure changes in the Earth's gravitational field.

"This work may open up new applications in the same way that lasers led to a whole range of new applications," says Baldwin.

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