# Absolute Zero.

1. Jun 5, 2007

### Stevedye56

I am aware that absolute zero can not be achieved by cooling a substance since absolute zero is zero-point energy. I was just wondering what is used (coolant wise and apparatus wise) to cool something near this temperature since there some experiments such as the Boise-Einstein Condensates which require near absolute zero temperatures.

-Steve

2. Jun 5, 2007

### waht

With liquid helium you get down to 4 Kelvin. After that elaborate set ups cool it even further using lasers. Instead of a laser heating up the gas, it is actually taking heat away.

3. Jun 6, 2007

### Nerro

I believe heat can be extracted letting for instance liquid hydrogen evaporate on the surface of the container. The evaporation is endothermic and will absorb some of the energy from whatever you want to cool. I'm not quite clear on how to cool something with a laser as a laser will add energy rather than take it away...

4. Jun 6, 2007

### smallphi

The lasers are set up in such a way so that on average they kick the atoms in direction opposite of the atomic velocity i.e. cooling them.

That is achieved by setting the laser frequency a little below the frequency at which the atoms will absorb it. That way, only atoms moving AGAINST the laser light will see it dopler shifted to the right higher frequency, absorb it and get a kick in direction opposite of their motion.

In this way, on average, atoms get more absorbtion kicks opposite to their motion. Of course, later they reemit the light by spontaneous emission but the kicks of the spontaneous emission have no preferred direction. The result is on average, atoms are kicked opposite of their motion and slowed down. Since temperature is a measure of the atomic translational kinetic energy, the atomic gas is cooled down.

The most basic set up is two laser beams opposing each other and cooling in two opposite directions and the atomic sample in the middle. More elaborate set up is 3 couples of opposite beams along the X, Y, Z directions.

Last edited: Jun 6, 2007
5. Jun 6, 2007

### DaveC426913

Thank you. That is an extremely succinct explanation of something that was heretofore a mystery to me.

6. Jun 6, 2007

### Stevedye56

Thanks, that was a great explanation.