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On the path to Bose-Einstein condensate (BEC)

On the path to Bose-Einstein condensate (BEC). Basic concepts for achieving temperatures below 1 μK Author : Peter Ferjančič Mentors : Denis Arčon and Peter Jeglič. Introduction. Bose-Einstein condensate – Atomic gasses cooled to VERY low temperatures (< μK )

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On the path to Bose-Einstein condensate (BEC)

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  1. On the path to Bose-Einstein condensate (BEC) Basic concepts for achieving temperatures below 1μK Author: Peter Ferjančič Mentors: Denis Arčon and Peter Jeglič

  2. Introduction • Bose-Einstein condensate – Atomic gasses cooled to VERY low temperatures (<μK) • Predicted in 1925 by Bose and Einstein • produced by Eric Cornell and Carl Wieman in 1995 – Nobel prize in 2001 • Tc ≈ 3.3 (ħ2n2/3 )/ (m kb) • Foralkaliatoms at n=1014/cm3 Tc ≈ 0.1 μK

  3. What is Bose-Einstein condensate • 107 condensed gas atoms • large fraction of the bosons occupy the lowest quantum state – atoms become indistinguishable • Basically we have one single “super atom” • Potential uses: • Simulation of solid state physics systems • Precision measurement • Quantum computing

  4. Used techniques • Slowing an atomic beam • Optical molasses technique • The magneto-optical trap • Dipole / Magnetic trapping • Evaporative cooling

  5. Slowing an atomic beam • Photon momentum: p=ħk • Absorbedphoton – fixeddirection • Emittedphoton – randomdirection • For λ=589 nm and Na atom, recoilvelocityΔv=3 cm/s

  6. Slowing an atomic beam • Need to compensate for Doppler effect • Frequency shift ~1.7 GHz (Natural width ~10 MHz) • Zeeman cooling • Chirp cooling • Laser cooling –Nobel 1997

  7. Opticalmolassestechique • 3 pairs of counter-propagating laser beams • When moving towards beam, absorption increases → slowingforce • Forceproportional to velocity • Doppler cooling limit: ~3 cm/s

  8. Magneto-optical trap (MOT) • Atoms diffuse from molasses in seconds for 1 cm wide beam – weshould stop them! • Magnetic quadrupole – B=0 in the center, increases as we move away • Ifphotonsmovefrom center zeemaneff. causes resonance • atoms are pushed back by laser beams → F(x)

  9. MOT – how to cancelreppeling? • Circularly polarized lasers: ΔM = +1 for right handed or ΔM = -1 for left handed • Add polarized laser beams -> F(x) • Change only in rate of photon absorption • These are OPTICAL forces!!!

  10. First stage cooling experiment • First MOT then molasses • Prediction: ~240 μK • Result: an order of magnitude LOWER temperature • But why?

  11. Sisyphus cooling • A sort of optical pumping mechanism

  12. Dipole lightforce • Refractedlightexcertsforce on object (photonmomentum: p=ħk) • Particles are attracted to areas of high light intensity • = Opticaltweezers • Wavelength is far from resonance!

  13. Evaporative cooling • Atoms with high enough energy escape the potential – taking above average energy with them • Lowering borders speeds up the process

  14. The experiment • Laser slowing of an atomic beam 900 K-> ~5 K • Magneto-optical trap ~300 mK • Optical molasses ~240 μK • Sisyphus cooling ~ 10-100 μK • Evaporative cooling in dipole trap <100 nK • Bose-Einstein condensate!!! • (note: temperatures are informative and highly dependant on the experiment)

  15. De jure • 1 slowing beam • 3 pairs of counter propagating beams • 1 pair of coils • 2 dipole force lasers

  16. De facto

  17. Conclusion & future • What are other potential uses for BEC? • Bikes vs. Light races (c=25 km/h) • Light-> matter -> light transitions- 2007 • Single spin addressing • Excellent tool for quantum mechanics • 2010 – first photon BEC • Cold atoms today under 500 pK

  18. Sources • AtomicPhysics; Foot • http://www.colorado.edu/physics/2000/bec/ • http://electron9.phys.utk.edu/optics507/modules/m10/saturation.htm • http://webphysics.davidson.edu/Alumni/JoCowan/honors/section1/THEORY.htm • http://en.wikipedia.org/wiki/Bose-Einstein_condensate • http://theory.physics.helsinki.fi/~quantumgas/Lecture4.pdf • http://www.nobelprize.org/nobel_prizes/physics/laureates/1997/illpres/doppler.html • http://physicsworld.com/cws/article/news/41246 • http://arstechnica.com/science/news/2011/01/pqe-2011-small-atoms-big-ideas-in-gravity-detection.ars • http://www.deas.harvard.edu/haulab/slow_light_project/remote_revival/remote_revival.htm • http://prl.aps.org/files/RevModPhys.70.721.pdf • http://www.phys.ens.fr/~dalibard/publi2/EuroPhysNews_98.pdf • http://www.asu.edu/courses/phs208/patternsbb/PiN/rdg/polarize/polarize.shtml

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