Researchers shine light on cool substance
Stuart Gary
ABC
Thursday, 28 November 2013
For the first time, scientists have found a way of studying Bose-Einstein Condensates without destroying them.
The method, reported in the New Journal of Physics, will allow researchers to examine the properties of Bose-Einstein Condensates, which are clouds of atoms cooled to temperatures of just 100 nano-Kelvin above absolute zero.
At such cold temperatures, atoms lose their individual identity and behave as a single macroscopic entity. Little movement can occur, making Bose-Einstein condensates ideal for probing atomic structure.
Studying these exotic substances will allow scientists to carry out fundamental research in new fields including atom lasers to precisely measure gravity, and models to study the emission of Hawking radiation from black holes.
Until now, it's been impossible to measure and control Bose-Einstein Condensates simultaneously. Using even a single photon of light to study them can heat them up enough to destroy them.
"Our new method uses a feedback mechanism which prevents the act of observing the Bose-Einstein condensate from destroying it," says one of the study's authors, Dr Joe Hope of the Australian National University in Canberra.
Solar system model
Most people picture an atom as a little version of the solar system, with a nucleus in the centre and electrons orbiting around it.
"That's a good approximation for everyday life, but when we look closer, we discover that they're actually waves. That's the theory around quantum mechanics," says Hope.
"When they get that cold, the fact that they're not particles but waves, becomes important, because they all slide into the same quantum state, becoming one big wave."
Scientists wanting to study these properties are hampered by the frailty of condensates.
"It's like taking a snap shot of everyone in the room, with the flash was so bright, that everyone runs away, preventing you from taking another picture," says Hope.
"So you get a single snapshot of a moment in time, but to get another, you have to find everyone and start again."
Hope and colleagues needed to determine if there was a way to keep cooling the condensate to compensate for the heating that was occurring while it was being studied.
"We've found we could get a net cooling effect, but had to design it very carefully," says Hope.
"It's a feedback mechanism, providing information about changes to the condensate which is constrained by a laser or magnetic trap, which adjusts its shape and position tens of thousands of times a second to compensate."
Cooling atoms
A Bose-Einstein Condensate is created by firing photons of laser light at the target atoms at lower frequencies that those of the atom, causing the light to pass by without affecting it.
If the atom moves towards the laser, the frequency changes through Doppler shifting until the two resonate pushing the atom back.
By surrounding the atom with lasers on all sides, it can be slowed down causing it to lose most of its energy, through laser cooling.
The remaining heat is removed by evaporation, as hot atoms move away, leaving only colder ones behind, a process which can lose over 99.9 per cent of all atoms in the condensate.
"We're now working on a method of removing the evaporation process completely, using only the trap feedback and laser cooling to reach condensate temperatures," says Hope.
