Transcript On the path to Bose-Einstein condensate

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)
• 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)
• For alkali atoms at n=1014/cm3
Tc ≈ 0.1 μK
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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
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Used techniques
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Slowing an atomic beam
Optical molasses technique
The magneto-optical trap
Dipole / Magnetic trapping
Evaporative cooling
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Slowing an atomic beam
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Photon momentum: p=ħk
Absorbed photon – fixed direction
Emitted photon – random direction
For λ=589 nm and Na atom, recoil velocity
Δv=3 cm/s
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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
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Optical molasses techique
• 3 pairs of counter-propagating laser beams
• When moving towards beam, absorption
increases → slowing force
• Force proportional to velocity
• Doppler cooling limit: ~3 cm/s
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Magneto-optical trap (MOT)
• Atoms diffuse from molasses in seconds for
1 cm wide beam – we should stop them!
• Magnetic quadrupole – B=0 in the center,
increases as we move away
• If photons move from center
zeeman eff. causes resonance
• atoms are pushed back by laser
beams → F(x)
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MOT – how to cancel reppeling?
• 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!!!
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First stage cooling experiment
• First MOT then molasses
• Prediction: ~240 μK
• Result: an order of magnitude LOWER
temperature
• But why?
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Sisyphus cooling
• A sort of optical pumping mechanism
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Dipole light force
• Refracted light excerts force on object (photon
momentum: p=ħk)
• Particles are attracted to areas of high light
intensity
• = Optical tweezers
• Wavelength is far
from resonance!
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Evaporative cooling
• Atoms with high enough energy escape the
potential – taking above average energy with
them
• Lowering borders speeds up the process
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The experiment
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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)
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De jure
• 1 slowing
beam
• 3 pairs of
counter
propagating
beams
• 1 pair of coils
• 2 dipole force
lasers
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De facto
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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
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Sources
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Atomic Physics; 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.h
tml
http://physicsworld.com/cws/article/news/41246
http://arstechnica.com/science/news/2011/01/pqe-2011-small-atoms-big-ideasin-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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