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Physicists Have Cooled Molecules Below The Doppler Limit – 50 MicroKelvin

[Estimated read time: 3 minutes]

Physicists have developed a new method that pushes the limits of freezing molecules to slightly above absolute zero. They have used a combination of magnetic fields and lasers to trap and cool down a molecule of calcium monofluoride to 50 millionths of a degree above absolute zero – 50 microKelvin, or minus 273.15 degree Celsius.

Although, it is not the coldest molecule known to science, that reward goes to a team of MIT, this approach works with many different substances than pure laser cooling. For instance, most of the labs need to create exotic molecules from elements like potassium and sodium. ‘These molecules do not contain all properties required for ultracold molecule applications’ said Michael Tarbutt, lead author of the research.

A few of those applications involve analyzing entire system of atoms governed by quantum mechanics, and studying superconductivity. This would help researchers better understand the capabilities of materials, and eventually, how to create those materials that can perfectly work at higher temperatures (superconductive materials should be cold in order to work properly) .

The Cooling Process

Temperature represents the movement of molecules in a substance – on average, how fast they are moving. The lower the temperature, lesser will be the movement. In this case, molecule in calcium monofluoride is slowed down.

Laser beams are used to slow down the speed of molecules. This involves a bunch of lasers set to fire at the molecule from opposite directions. The first laser strikes from the left side, letting the molecule absorb a single photon, which then reduces the momentum of molecule. This is something similar to collision of billiard balls  in opposite direction.

Reference: Molecules cooled below the Doppler limit

The molecule of calcium monofluoride does not lose its complete momentum after the first laser strike. Instead, it shifts toward the right direction, where the second laser is placed. As it approaches, the laser light seems to have a smaller wavelength from the molecule’s perspective, which is called Doppler shifting. The molecule absorbs one more photon (coming from second laser), and slows down further. This process of slowing down the motion of molecules using laser, in order to cool a substance, is known as Doppler cooling.

The lasers used in the experiment excite molecules to some extent, due to which they release photons (in random directions) after every absorption. So the average momentum of the substance does not change after multiple laser strikes.

Schematic of the experiment

However, the release of photons decreases the momentum of molecule, because each emission produces a very small ‘kick’, keeping the molecule still a bit ‘warm’. This little ‘warm’ temperature, which is very difficult to decrease further, is called Doppler limit.

Going Beyond the Limit

To further reduce the temperature, the research team used magnetic field to put the molecule at specific location, and hit it again with a laser.

You can imagine the molecule as it is placed at the bottom of a hill (potential energy), and lasers push it upward. Something like, when you kick a ball upwards, potential energy increases as it approaches the top, and kinetic energy decreases. The same thing happens to the molecule of calcium monofluoride, and this phenomena is known as Sisyphus cooling.

Read: Physicists Discover ANGEL PARTICLE That Is Both Matter and Antimatter

Scientists have achieved 50 microKelvin by reducing the kinetic energy the molecule. Before this, other physicists were always studying potassium-rubidium and other molecules that don’t naturally occur. Because calcium and fluoride atoms make molecules in nature, they do not need any special method to unite. This opens up infinities possibilities for future endeavors’ says Lincoln Carr, physics professor at the Colorado School of Mines, who wasn’t involved in the research.