Scientists from MIT Invent a New Laser Cooling Method

This article was updated on the 24th April 2019.

MIT scientists have invented a faster way to make the Bose-Einstein condensate. When atoms are at room temperature they constantly move around in a frantic state. However, once atoms are cooled to a temperature just a fraction above absolute zero, they occupy a quantum form known as the Bose-Einstein condensate and start moving together in a wave-like motion.

Original Production of Bose-Einstein Condensates

The first successful Bose-Einstein condensates were produced by researchers in Colorado and by Wolfgang Ketterle at MIT in 1995. They were interested in the quantum properties to gain insight into phenomena such as superconductivity and magnetism. The original method of cooling atoms into condensates was slow and inefficient, with over 90% of the atoms in the original cloud being lost in the process; therefore, a more efficient method was needed.

Traditional Laser Cooling

The original process worked through a combination of laser and evaporation cooling. Laser beams are shone from several directions on a cloud of atoms and the photons in the beam bounce off much larger atoms, slowing them down with each collision. The photons also limit the motion of the atoms and compress the cloud, cooling them in the process.

There is a limit to how much a laser can cool, with denser clouds having less room for photons to scatter. When this happens, the method is switched to an evaporative cooling method.

New Laser Cooling Method

The new method established by Vuletić and his colleagues in 2017 avoids these limitations. They used laser cooling techniques to cool a cloud of rubidium atoms to just above the point at which atoms become so compressed that photons start to heat up the sample. They then switched to Raman cooling, in which they used a set of two laser beams to cool the atoms further.

When you remove an atom’s kinetic energy, you remove the random motions and they transition into a quantum behavior that resembles Bose-Einstein condensates. The condensates can then take a form when the atoms have lost their total energy and cooled sufficiently to reside in their lowest quantum states.

To reach the final point and completely cool the atoms into condensates, researchers introduced one further step. The lasers were tuned away from atomic resonance so that the light could more easily escape from the atoms without moving and heating them. The incoming photons were then less likely to be absorbed by atoms and produce heat.

Conclusion

The new technique is faster compared with conventional methods and also conserves a large fraction of the original atoms. It utilizes a new process of laser cooling to cool a cloud of rubidium atoms from room temperature to 1 microkelvin, or less than one-millionth of a degree above absolute zero.

Scientists can now cool up to 2,000 atoms and generate a condensate of 1,400 atoms, conserving 70 percent of the original cloud, and the new process is 100 times faster than the conventional method. Their results have been published in Science.

References

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