How do you cool down water at your home? You keep it in the freezer in your kitchen. Right? So using this easily available appliance at your home, you can freeze water to the ice by bringing it to 0°C, i.e., 273K. But what if you want to cool something to nearly absolute 0K? Can you use your refrigerator? Well, the answer is “No.” Not only yours, rather no refrigerator in the world can cool something to nearly absolute zero. And this is when and where we need to have some unconventional cooling methods like laser cooling.
What is laser cooling?
Now, you might be wondering that a laser is actually a source of light energy, so definitely, it should provide some energy to the atoms it would encounter, thus raising their temperature. So, why are we associating the term “cooling” with it? Well, surprisingly, lasers can actually be employed to cool down atoms to significantly low temperatures. I know it seems weird, but it’s true. Laser cooling is actually a broad field that includes a wide spectrum of techniques using lasers to cool down atomic and molecular samples to nearly absolute zero temperatures.
Principle of laser cooling:
In laser cooling, the basic principle used to cool down atoms is to slow them down. We know, each atomic species has thermal energy associated with it. So, slowing down an atom would lower its kinetic energy and thermal energy, which will eventually lead to a decrease in its temperature, hence cooling it down. As mentioned above, there are many sub techniques under laser cooling to initiate the cooling. However, one of the most famous and in-demand techniques in this regime is that of Doppler cooling.
Whenever a light beam is shone on an object, it exerts a force on it. The radiation pressure exerted by light is also why comet tails always point away from the Sun. However, if the incoming laser beam is such that its frequency is somewhat below the atom’s resonant frequency, then the laser beam can be used to slow down the atom. So what leads to this slowdown? The answer is the direction-dependent Doppler effect!
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In technical terms, Doppler’s effect can be defined as an observed change in frequency of a wave when an observer and source have relative motion between them. So, if the radiation source is moving towards the observer, then the frequency of those radiations as perceived by the observer is blue-shifted, which means the observer will observe a higher frequency of radiation traveling towards it than the actual frequency emitted by the source. However, the reverse process of redshift occurs if the source of radiation is moving away from the observer.
A common example of the Doppler effect is the change of pitch heard when a vehicle sounding a horn approaches and recedes away from an observer.
So now, owing to the Doppler effect, the laser light, which actually has a frequency lower than the resonant frequency of atom, is brought to resonance for the moving atom “sees” the incoming photon with a higher frequency than what it actually has and hence, that atom can easily absorb it. The photons that will subsequently get radiated by the atom will have a somewhat higher frequency than what was absorbed. To compensate for the gap between the energy of absorbed and emitted photons, the atom suffers a toll on its kinetic energy.
As the lack of energy is taken from the atoms’ kinetic energy, they suffer a decrease in their momentum and velocity and hence, are eventually cooled down. Since the atoms return from the optically excited state to the ground state after some 10 ns, the absorption-emission process repeats very rapidly. So mostly, an arrangement of six laser beams is employed so that the atoms’ motion can be slowed down effectively in all directions.
Remarkable achievements in field of laser cooling:
Apart from Doppler cooling, some other laser cooling techniques include Sisyphus cooling, Resolved sideband cooling, Raman sideband cooling, cavity mediated cooling, etc., to name a few. In 1997, the Nobel Prize in Physics was awarded to Steven Chu, Claude Cohen-Tannoudji, and William D. Phillips for the development of methods to cool and trap atoms with laser light. And now, with the improvements in technology, the methods of laser cooling have advanced enough to the point that extremely low temperatures of 10-9 K have been reached.
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Moreover, laser cooling processes are being widely utilized to make atomic clocks more accurate and improve spectroscopic measurements. Laser cooling also led to the observation of a new state of matter, the Bose-Einstein condensate, which was observed in 1995 by Eric Cornell, Carl Wieman, and Wolfgang Ketterle at ultracold temperatures. Undoubtedly, laser cooling is one of the booming fields these days and a lot more exciting is expected to come up in the coming years.
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