Wednesday, 3 August 2011

Colder Than Ice

Beautiful levitating microspheres
from James Millen's website
ucloptomechanics.wordpress.com
LASERs are pretty great. When they were first invented they were called the solution looking for a problem, translation: awesome but useless. Fast forward 40 years and LASERs are probably more widespread than rats. I bet you can't move 1 foot without coming across a LASER. They are used for entertainment, communication, surgery, defence and manufacturing - so much of our daily life revolves around LASERs we'd all morph back into monkey-state if they suddenly ceased to work. When your fancy cd players stop working I’m taking over and putting BON JOVI on all night long.)

What is a Laser?
A laser is ‘made’ out of light. I use quotation marks because its not like you can knit yourself a laser from the yarn of light, because a LASER is just a very specific version of light. The light that the sun emits is made of the same stuff as in a LASER – light in the form of packets called photons*[technically not really true – the sun emits broadband radiation i.e. lets say these photons come in flavours like ice cream. You can think of the photon as one discrete scoop of ice cream. The sun emits lots of the flavours at once – it’s like neopolitan ice cream. A LASER is usually just one narrow range of flavour – i.e. plain vanilla. But my point is, it’s all still ice cream in the end.

The main difference between the Sun's emitted radiation and that from a LASER is just that LASERS have very synchronised photons which all have identical properties such as phase. The synchronisation makes the LASER a LASER - it gives it the characteristic; long and shaft-like beam shape and it also makes it very intense (the LASER set up also builds up a lot of energy too so you get a focussed beam of light). So if you shine a LASER at a piece of metal, the amount of power delivered per area is huge compared to using focussed sunlight or a lightbulb (our neopolitan ice creams) and cuts through the metal with great speed and ferocity (i.e. efficiency).  

However, Lasers are also used to cool things down.

What makes a LASER cold?

First, what is cold? Well, cold is actually described as temperatures under 1K, and ultracold below 1mK. What is a K? It's a Kelvin! It's an alternative temperature scale to the common Celsius one. It's the science version of the American Fahrenheit. To convert Celsius to Kelvin just add 273.14. So zero degrees Celsius becomes 273.14K. There's a good reason for this - it makes you realise just how cold 0K is! -273.14C. However 0K also has an important definition attached to it - it's the temperature at which all thermal (mechanical) motion ceases, not like sleeping-lions stationary, but stationary to the atomic scale. Till the particles stop moving. According to Wikipedia the lowest temperature recorded on the surface of the Earth is 184K.

There are lots of methods nowadays to achieve cold and ultracold gases. But the main one is Doppler Cooling. This involves using LASERs to slow down the velocity of particles and then trap them using magnetic fields. You can do this because of radiative scattering.
Let’s revive the ice cream analogy where a photon is one scoop. And atom absorbing a photon is like hurling the scoop into a catapult, causing the elastic band to start to stretch. This is ‘excitation’ of the atom after photon absorption. You make the atom want to move in the same direction as it takes in the photon momentum. Now, if this catapult stretches too much (too much excitement) it will want to spring back and throw back the photon in a random direction. This is a recoil reaction. However, because recoiling occurs in all directions, the average direction turns out to be in no direction at all. This is like the dodgeball effect if you imagine balls are being thrown at you in all directions; there’s no where to escape so you stay still. Velocity is linked to kinetic energy because velocity involves something travelling and to do this you need fuel or energy. Energy is basically a free-form - it can be changed from kinetic to potential to thermal. So a measure of kinetic energy also gives you the thermal energy. So low temperature means lower velocity. This is how you achieve 0K.

However, there's a limitation to this.  The atom you want to cool will initially have a certain amount of velocity, hence trying to minimise it. The atom will experience a problem known as the Doppler shift. The Doppler shift is better known as the ambulance conjecture - i.e. the sound of the siren shifting in pitch as it flies past you. This happens to the frequency of light too. If we split the spectrum of visible light into a rainbow, you'll find there is a difference in wavelength for red and blue light. Red has a higher wavelength than blue. If you move towards a source of radiation, the light which you perceive will become blue-shifted - i.e. the light will seem to have a lower wavelength, and moving away from a source causes a redshift. As the scattering process is highly dependent on wavelength because this is what dictates the spacing between the transition levels, a LASER which is perfectly tuned to a transition level quickly becomes out of tune because of the atoms movement. Therefore to stay in tune you send a LASER already red-shifted to counteract the Doppler effect.

You effectively trick the moving atoms into submission.

Once the atom is cooled you can conduct some experiments on it, for example using another set of LASERs to levitate it and move single atoms around. This is optical tweezering, and is responsible for micro-graffiti, where scientists will tag small components with really boring words. The atom can also be used as a very accurate clock using the transition levels, and even manipulated to display some quantum behaviours. In summary, the coolest thing about cooling is not the temperature reached, but the power of control it enables experimental physicists to have over the building blocks of the universe.