Bose and Einstein.In 1924an Indianphysicist named Bose studied the quantum behaviour of acollectionofphotons. Bose sent his work to Einstein,who realized that it was important.: Einstein generalized the idea to atoms, considering them as quantum particleswithmass.· Einstein found that when the temperature is high, they behave like ordinarygases.However,when the temperatureis verylow,they will gather together at thelowestquantumstate.ThisiscalledBose-Einsteincondensation
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）andBosons（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.* Singleprotons,neutrons and electrons havea spin of .They are called fermionsThey cannot appear in the same quantum state.BEC cannot take place.Someatomscontainan even number of fermions.Theyhavea total spinof wholenumber.Theyarecalledbosons*Bosons show strong"social"behaviour,and can have BECExample:A23Naatomhas11protons,12neutronsand11electrons
Fermions (費米子) 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
TheMaterial ForBEC★BECwasfoundinalkalimetalse.g.87Rb（金如），23Na（纳），7Li（鲤）because:*Theyarebosons.Each atom is a small magnetic compass,so that a cooling technique calledmagnetic cooling canwork.*The atoms havea small repulsion,so that they do not liquefy or solidify downtoaverylow temperature
The Material For BEC BEC was found in alkali metals e.g. 87Rb (金如), 23Na (鈉), 7Li (鋰) 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 AtomsSeetheanimation:When the temperature is high,the atoms have high energies on average.Theenergy levelsare almost continuous.It is sufficient to describe the systembyclassicalphysicsWhenthetemperatureis low,theatomshavelowenergies on average.It isnecessarytodescribethesystembyquantumphysics.*Whenthetemperatureisverylow,alargefractionofatomssuddenlycrashinto the lowest energy state. This is called Bose-Einstein condensation
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