bose爱因斯坦.ppt

玻色-爱因斯坦凝聚
2001 Nobel Prize in Physics
The Prize
“for the achievement of Bose-Einstein condensation (玻色-爱因斯坦凝聚态) in dilute gases of alkali atoms (碱原子) , and for early fundamental studies of the properties of the condensates"
The Winners
Eric A. Cornell
JILA & NIST,
Boulder, Colorado.
1961-
Wolfgang Ketterle
MIT
1957-
Carl E. Wieman
JILA & University of Colorado, Boulder.
1951-
Bose and Einstein
In 1924 an Indian physicist named Bose studied the quantum behaviour of a collection of photons.
Bose sent his work to Einstein, who realized that it was important.
Einstein generalized the idea to atoms, considering them as quantum particles with mass.
Einstein found that when the temperature is high, they behave like ordinary gases.
However, when the temperature is very low, they will gather together at the lowest quantum state. This is called Bose-Einstein condensation.
Fermions (费米子) and Bosons (玻色子)
Not all particles can have BEC. This is related to the spin of the particles.
The spin quantum number of a particle can be an integer or a half-integer.
Single protons, neutrons 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 pass, so that a cooling technique called ic 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: sics/2000/bec/
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 class