文档介绍：玻色-爱因斯坦凝聚
The user can demonstrate on a projector or computer print the presentation and make it into e used in a widereutrons and electrons have a spin of ½. They are called fermions. They cannot appear in the same quantum state. BEC cannot take place.
Some atoms contain an even number of fermions. They have a total spin of whole number. They are called bosons.
Bosons show strong “social” behaviour, and can have BEC.
Example: A 23Na atom has 11 protons, 12 neutrons and 11 electrons.
The Material For BEC
BEC was found in alkali metals . 87Rb (铷), 23Na (钠), 7Li (锂) because:
They are bosons.
Each atom is a small magnetic compass, so that a cooling technique called magnetic cooling can work.
The atoms have a small repulsion, so that they do not liquefy or solidify down to a very low temperature.
Cooling Down the Atoms
See the animation:
When the temperature is high, the atoms have high energies on average. The energy levels are almost continuous. It is sufficient to describe the system by classical physics.
When the temperature is low, the atoms have low energies on average. It is necessary to describe the system by quantum physics.
When the temperature is very low, a large fraction of atoms suddenly crash into the lowest energy state. This is called Bose-Einstein condensation.
The Strange State of BEC
When all the atoms stay in the condensate, all the atoms are absolutely identical. There is no possible measurement that can tell them apart.
Before condensation, the atoms look like fuzzy balls.
After consdensation, the atoms lie exactly on top of each other (a superatom).
Q2: How Is BEC Made?
Laser beam
Other equipment: laser equipment, computer, electronics
Cost less than US$100,000
Laser Cooling (激光冷卻)
The technique of laser cooling was developed by the winners of the 1997 Nobel Prize winners.
In the physical world, the lowest temperatures approach a limit of