Laser Cooling And Bose-einstein Condensate
Scientists have demonstrated another method of cooling via the reduction of energy, using lasers instead of electromagnets. Laser cooling operates on the principle that temperature is really a measure of the energy of the atoms in a material. In laser cooling, the force applied by a laser beam is used to slow and nearly halt the motion of atoms. Slowing the atoms reduces their energy, which in turn reduces their temperature.
In a laser cooling setup, multiple lasers are aimed from all directions at the material to be cooled. Photons of light, which carry momentum, bombard the atoms from all directions, slowing the atoms a bit at a time. One scientist has likened the process to running through a hailstorm—the hail hits harder when you are running, no matter which direction you run, until finally you just give up and stop. Although laser-cooled atoms do not completely stop, they are slowed tremendously—normal atoms move at about 1,000 mi (1,600 km) per hour; laser-cooled atoms travel at about 3 ft (0.9 m) per hour.
Using laser cooling and magnetic cooling techniques, scientists have cooled rubidium atoms to 20 billionths of a degree above absolute zero, creating a new state of matter called Bose-Einstein condensate, in which the individual atoms condense into a superatom that acts as a single entity. Predicted many years before by Albert Einstein and Satyendra Nath Bose, Bose-Einstein condensate is has completely different properties from any other kind of matter, and does not naturally exist in the Universe.
The researchers first used laser cooling to bring the temperature of the rubidium atoms down to about 10 millionths of a degree above absolute zero, which was still too warm to produce Bose-Einstein condensate. The atoms held in the laser light trap were then subjected to a strong magnetic field that held them in place. Called a time-averaged orbiting potential trap, the magnetic trap had a special twist that allowed the scientists to remove the most energetic (hottest) atoms, leaving only the very cold atoms behind.
Since the initial demonstration in 1995, scientists have continued to work with Bose-Einstein condensate, using it to slow light to less than 40 MPH (64 km/h), and even to produce an atom laser which produces bursts of atoms with laser-like properties. Multiple studies are underway to help researchers understand the properties of this baffling material.
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