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ScienceWise - Winter 2011

A speckle of matter

Article Illustration
A coherent beam of matter should generate a speckle pattern in just the same way as a coherent beam of photons from a laser
Article Illustration
Some members of the scientific team, Dr Andrew Truscott, Professor Ken Baldwin, Andrew Manning, Sean Hodgman and Dr Robert Dall

Final confirmation of the matter laser

Have you ever noticed that when you look at the spot of a laser beam on a piece of paper the halo around it has a fine speckle pattern? The reason for this is that the laser light is coherent, that is each photon in it is identical to the others in wavelength and phase. What this means is that when the light hits a rough surface like paper the microscopic peaks and valleys in the surface each scatter the light in different directions. They act as independent sources of coherent light, and the result is a strong interference pattern which we see as speckle. However, the phenomenon of speckle isn’t just confined to light.

One of the great physics discoveries of the twentieth century was de Broglie (or matter) waves in which a stream of particles such as electrons, atoms or even cannon balls, also exhibit wave-like behaviour. In this way electrons passing through an aperture are diffracted in just the same way as light or ripples on water would be. In a stream of atoms created at room temperature, the atom velocities and times of release all vary. This in turn causes their de Broglie wavelengths and phases to all be different. 

However, another of the great discoveries in modern physics was that of the Bose Einstein condensate, or BEC. This is a strange form of matter in which a collection of atoms are made so cold (within a millionth of a degree above absolute zero) that they all settle down into a single quantum state – something that would be completely impossible at normal temperatures.

In recent years scientists around the globe have been able to create BEC’s and then allow the atoms to leak out creating a beam of identical atom waves. Because they’re all in the same quantum state, their de Broglie wavelengths and phases are all the same too, so in effect what you have is a laser beam made of matter rather than light.

Of course theoretically you’d expect that such an atom laser would generate the same speckle pattern as a light laser does. However, observing this in practice is a very difficult thing to do because each stage - the creation of the BEC, the creation of the atom laser beam and the creation and detection of the speckle pattern is incredibly difficult to achieve. However, a team of scientists at ANU have recently become the first to observe just such speckle from a beam of ultra-cold atoms.

“This is exactly the result we would have expected from theory,” professor Ken Baldwin says, “But we’ve gone even further to show that the coherence properties of matter wave speckle match those of light speckle. This enables us to use such properties to determine whether atom lasers are suitable for use in future devices, such as atom interferometers for gravity detection.”

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