What gas is used in BEC

H. How can the conditions required for Bose Einstein condensation be achieved?

In the section on quantum statistics it was already mentioned that quantum effects, in gases made of atoms, only appear under extreme conditions. The condition is that the thermal deBroglie wavelength and the mean distance between the particles in the gas must be of the same order of magnitude.

There are two ways to achieve this. The first possibility would be to increase the density of the gas. This is not particularly practical because it is very difficult to increase the density of a gas by more than two or three orders of magnitude. In addition, a gas can become liquid if it is compressed too much, which makes Bose Einstein condensation very difficult, if not impossible.

Therefore, the method of choice is to increase the thermal deBroglie wavelength by greatly reducing the temperature. In the section on quantum statistics it was also shown that very low temperatures are necessary to bring a gas made of atoms into the quantum regime. The temperatures to reach a Bose Einstein condensation are even lower.

So it came about that the first Bose Einstein condensate in a gas of rubidium atoms was created in 1995 by Anderson and his colleagues. The conditions under which the Bose Einstein condensation took place were very extreme; the density of the gas was only 1012 Atoms per milliliter. This value is 10 million times smaller than the density of a gas under standard conditions. As a result, the temperature required for the Bose Einstein condensation was much lower than the one previously calculated.

The condensation could take place at a temperature of

to be watched. It is only just above absolute zero. But that means that any Interaction with the environment (e.g. with the walls of a vessel) would increase the temperature of the gas and destroy the condensate.

Therefore the atomic cloud is trapped in a magneto-optical trap. As the name suggests, the trap consists of magnetic fields that are generated by current-carrying coils and of optical laser beams that are used to cool the atoms.

This means that the condensate floats in the room and does not come into contact with the walls of the vessel. A similar technique is used in some experimental fusion reactors. However, the inclusion does not serve to prevent the gas from heating up, but rather to cool it down.

The above drawing shows the basic structure of a magneto-optical trap. The current-carrying coils (green and blue) generate a static magnetic field. Lasers rays (red) are used to cool and contain the gas.

Now that we have understood how to create a Bose Einstein condensate, the question arises:

What are the properties of a Bose Einstein condensate?