Using the Doppler Effect Problem:The laser can slow the approaching atoms,but it can also blast off the receding ones. ● Solution:Use Doppler shift. o When the atom is receding from the laser source. the wavelength is lengthened and there is a redshift. ● When the atom is approaching the laser source, the wavelength is shortened and there is a blueshift. PDF created with pdfFactory trial version www.pdffactory.com
Using the Doppler Effect l Problem: The laser can slow the approaching atoms, but it can also blast off the receding ones. l Solution: Use Doppler shift. l When the atom is receding from the laser source, the wavelength is lengthened and there is a redshift. l When the atom is approaching the laser source, the wavelength is shortened and there is a blueshift. PDF created with pdfFactory trial version www.pdffactory.com
Laser Trapping Suppose the laser has the right colour for the photons to be absorbed by an approaching atom,then the atom will be slowed down. However,the laser will not have the right colour for the photons to be absorbed by the receding atom because of Doppler effect.Hence the atom will not change in this case. When lasers are sent in from all the different directions, the atoms can get cold very quickly. This is called laser trapping,and the trapped atoms form an optical molass G光学粘胶). PDF created with pdfFactory trial version www.pdffactory.com
Laser Trapping ÙSuppose the laser has the right colour for the photons to be absorbed by an approaching atom, then the atom will be slowed down. ÙHowever, the laser will not have the right colour for the photons to be absorbed by the receding atom because of Doppler effect. Hence the atom will not change in this case. ÙWhen lasers are sent in from all the different directions, the atoms can get cold very quickly. ÙThis is called laser trapping, and the trapped atoms form an optical molass (光学粘胶). PDF created with pdfFactory trial version www.pdffactory.com
Magnetic Trapping *Problem:Laser cooling can cool the atoms down to 10uK,because atoms can spontaneously emit the absorbed photon.This is still too hot for BEC. Solution:Evaporative cooling ★ The atoms behave as tiny compasses.They can be pulled by magnetic fields. *A magnetic field can be designed to push the atoms inwards from both sides,forming a magnetic frap. PDF created with pdfFactory trial version www.pdffactory.com
Magnetic Trapping ÙProblem: Laser cooling can cool the atoms down to 10mK, because atoms can spontaneously emit the absorbed photon. This is still too hot for BEC. ÙSolution: Evaporative cooling ÙThe atoms behave as tiny compasses. They can be pulled by magnetic fields. ÙA magnetic field can be designed to push the atoms inwards from both sides, forming a magnetic trap. PDF created with pdfFactory trial version www.pdffactory.com
Evaporative Cooling 0000 2000 *Principle:Evaporation takes heat.A cup of tea gets cool after steam escapes,because faster atoms escape from the cun leavina hehind the slower ones PDF created with pdfFactory trial version www.pdffactory.com
Evaporative Cooling ÙPrinciple: Evaporation takes heat. A cup of tea gets cool after steam escapes, because faster atoms escape from the cup, leaving behind the slower ones. ÙTechnique: Lower the height of the trap quickly, so that there are still enough atoms left in the trap to get involved in BEC. ÙTry to trap the largest number of atoms in BEC in the animation PDF created with pdfFactory trial version www.pdffactory.com
Bose-Einstein Condensation of Rb 87 hyN-∠r'-naN-rw5 140 120J 10 0 试a Temperature nanoKelvin Te T PDF created with pdfFactory trial version www.pdffactory.com
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