Laser cooling — an oxymoron?

We all know lasers can heat things up. It does this by moving particles around, which increases the particles’ kinetic energy. A higher kinetic energy means a higher temperature, so the temperature rises.



But lasers can also be used to cool particles to temperatures nearing absolute zero: 0º Kelvin.

Atoms have discrete energy levels. This means that if an electron jumps from the second energy level to the first energy level, it must give out some energy by emitting light. This is an atom in its ‘excited state’ returning to its ‘ground state’. The atom absorbs or emits light in discrete packets called photons, which have a definite energy.

Similarly, an atom in its ground state can also absorb a photon and become excited, but will eventually decay back to its ground state.


Since light also carries momentum, if you have light moving in one direction, and an atom moving in the opposite direction, it will slow down. However, atoms only absorb light of a specific wavelength, called the ‘transition wavelength’. The closer a light’s wavelength is to the transition wavelength, the more likely the photon will be absorbed by the atom.

So in a type of laser cooling called Doppler cooling, lasers are beamed to an atom in opposite directions.

The light from the lasers have larger wavelengths than the transition wavelength of the atom, so they appear ‘redder’, since larger wavelengths are said to be red while shorter ones are said to be blue.

When the atom begins to move towards a laser, it becomes ‘bluer’ and closer to the transition wavelength, while the other laser beam becomes ‘redder’ and farther to the transition wavelength. This means that the atom is more likely to absorb photons from the beam it is travelling towards, since it is closer to the transition wavelength, which reduces the atom’s momentum, thus overall slowing it down.

This is happening in three dimensions, so you need six lasers to accomplish this.

A much simpler, though not completely accurate, way of understanding it might be to think of the atom as a ball. If you push on that ball, it is going to absorb your energy and move in that direction. However, if your friend also pushes with the same force, but in the opposite direction, that ball will absorb both your energies, which will cancel each other out, and not move. Then imagine you have six friends, all pushing with the same force, all countering the forces of the friend opposite them. Then, the ball won’t move at all. At least, it won’t move much. So the ball loses most of its kinetic energy, which is what defines the temperature of an object, and hence becomes ‘cooler’. Laser cooling can cool particles to temperatures as cold as 150 microKelvins. That’s 0.000150 K, which is -273.14985 degrees Celsius, or -459.66973 degrees Fahrenheit. The major breakthroughs in the 1970s and 80s of using laser light for cooling led to several discoveries with temperatures just above absolute zero and improvements to preexisting technology. The cooling processes were utilised to make atomic clocks more accurate and to improve spectroscopic measurements, and led to the observation of a new state of matter at ultracold temperatures. This new state of matter was the Bose–Einstein condensate, observed in 1995. We've barely scratched the surface of laser cooling here, so do check out some more websites and research papers about this. Until you find those, though, here is a really “cool” video explaining laser cooling:

https://www.youtube.com/watch?v=fAEAZaXhD_Y

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