|Artist's impression of the speckle pattern created by a multimode light beam (top, red), and the image measured in this experiment created by a multimode beam of atoms (top, blue). Source: Dr Tim Wetherell, ANU.|
"In an optical fibre, many modes of light can be conducted simultaneously, and they can interfere to produce a speckled pattern of light," said team member Professor Ken Baldwin from the ARC Centre of Excellence for Quantum-Atom Optics at ANU.
"We have shown that when atoms in a vacuum chamber are guided inside a laser light beam, they too can create a speckle pattern - an image of which we have captured for the first time".
The team trapped a cloud of cold helium atoms at the focus of an intense laser beam pointed downwards at the imaging system, and then gradually turned down the laser intensity until the speckled image appeared. The work was done with PhD students Sean Hodgman and Andrew Manning.
"We then made the atoms even colder," says team leader Dr Andrew Truscott, "until they behaved more like waves than particles, forming a single quantum wave called a Bose-Einstein condensate (BEC). When the BEC was loaded into the guide, the speckle pattern disappeared, showing that just one mode was being transmitted - the single quantum wave."
The physicists demonstrated that by measuring the arrival time of the atoms on the imaging system, they were able to distinguish between the multimode (speckled image) guiding, and the single-mode (smooth image) guiding.
"Measurements for the multi-mode beam showed the atoms arriving in groups as a result of their interference - so-called atom bunching," said team member Dr Robert Dall. "However, the BEC represents just a single quantum mode with no interference, so when we guided the BEC - we saw no bunching."
The guiding behaviour agreed with a theoretical model developed by team member Mattias Johnsson. "We have shown that atoms can be guided in a laser beam of light, with the same properties as light guided in an optical fibre made of glass," said Dr Johnsson.
Source: Australian National University