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Chad Orzel Union College Physics 100 Laser Cooling OR How to Get Atoms Really, Really Cold Laser Cooling Chad Orzel Union College Physics 100 Physics 100: 100. Freshman Seminar (Fall). Team-taught course introducing physics at Union. Topics covered may include astronomy, atomic and molecular physics, biophysics, chaotic dynamics and fractals, computational physics, laser physics, solid-state physics, and statistical physics. Describe current research What’s new, what’s cool My opinion: Laser Cooling Nobel Prize in Physics, 1997 Chad Orzel Union College Physics 100 Laser Cooling? NOT about keeping lasers from overheating Shine laser light on sample of atoms => Atoms get cold A little counter-intuitive... ... but it works: T ~ 100 µK (1 µK = 1/1,000,000 ºC above absolute zero) Chad Orzel Union College Physics 100 Laser-Cooled Atoms at NIST Laser-cooled and magneto-optically trapped sodium (NIST) T ~ 200 µK Chad Orzel Union College Physics 100 Temperature What do we mean by temperature? Average Energy of particles in sample KE = 3/2 kB T = ½ mv2 Hot Sample = Fast atoms Room Temperature Cold Sample = Slow atoms Liquid Nitrogen: v ~ 300 m/s T ~ 70 K Speed of Sound Half the speed Why Bother? Chad Orzel Union College Physics 100 Why do we do laser cooling? Want to study properties of atoms Main tool for looking at atoms in detail: Spectroscopy: What color light do they emit? Different atoms emit different colors: Neon: red light (neon signs) Sodium: yellow light (street lights) Learn about atomic properties by looking at light Chad Orzel Union College Physics 100 Doppler Effect Change in frequency of waves from moving source Stationary Source Moving Toward Higher Frequency Moving Away Lower Frequency Shift Depends on velocity: ∆ ω = -2 π v / λ = -k v Chad Orzel Union College Physics 100 Color and Frequency Light behaves like a wave: Color of light Frequency of wave Blue light: high frequency Red light: low frequency Fast-moving atoms have their light Doppler shifted Slow atoms no Doppler shift Want cold atoms for best measurements Chad Orzel Union College Physics 100 Cold Gases Want extremely cold samples of atoms to do measurements Problem: Anything but Helium is solid (And He is a liquid) Solids, liquids have interactions which mess things up Solution: Work at very low density Too dilute to solidify Problem with Solution: Can’t cool dilute samples by usual methods Laser Cooling Chad Orzel Union College Physics 100 Temperature What do we mean by temperature? Average Energy of particles in sample KE = 3/2 kB T = ½ mv2 Hot Sample = Fast atoms Room Temperature Cold Sample = Slow atoms T = 100 µK v ~ 300 m/s v ~ 10 cm/s Speed of Sound Speed of bug Atoms and Photons Chad Orzel Union College Physics 100 |e> |g> Atoms have discrete energy levels ground state, excited state Change states by absorbing or emitting light absorption emission Photons: “particles” of light little bundles of energy Also Carry Momentum Light Force Chad Orzel Union College Physics 100 Photons carry momentum => absorbing photon exerts force p p Atom gets “kick” in photon direction Very small velocity 87Rb λ=780 nm v=5.8 mm/s Lots of photons (1015 per second) What about emission? Also gives “kick”, but in random direction Over many absorption/ emission cycles, averages out Chad Orzel Union College Physics 100 Photographic Evidence Absorb, emit photons a million times.... Shine laser on Bose Einstein Condensate ~ 1,000,000 atoms As close to absolute zero as you can get Scattered Atoms BEC Light Force II Chad Orzel Union College Physics 100 Light Scattering Exerts force => change atomic motion Not enough for cooling! Problem: p p v v v Can make fast atoms slow down slow atoms speed up As likely to heat as to cool! Need clever trick to do laser cooling v Chad Orzel Union College Physics 100 Doppler Effect Change in frequency of waves from moving source Stationary Source Moving Toward Higher Frequency Moving Away Lower Frequency Shift Depends on velocity: ∆ ω = -2 π v / λ = -k v Doppler Cooling Chad Orzel Union College Physics 100 How to use Doppler effect to do cooling? |e> ω3 ω1 Transition between states depends on frequency ω2 Too high, too low => no absorption ω ~ ωo => light force |g> Doppler Effect changes frequency seen by moving atom Tune laser to lower frequency (red) ω < ωo Stationary Atom No Force p Moving Toward Laser p v v Cooling Force! Chad Orzel Union College Physics 100 Optical Molasses Use two lasers, in opposite directions: Laser 1 Laser 2 Atoms move left: Absorb from Laser 1 Slow down Atoms move right: Absorb from Laser 2 Slow down One-dimensional Cooling Force: Opposes motion in either direction F=−αv Like viscous fluid: “Optical Molasses” 3-D Cooling Chad Orzel Union College Physics 100 What about other dimensions? Use three pairs of beams Counter-propagating pairs along X, Y, Z axes Sodium in optical molasses at NIST Cool in 3-D Low velocities in all directions Chad Orzel Union College Physics 100 Laser-Cooled Atoms at NIST Laser-cooled and magneto-optically trapped sodium (NIST) T ~ 200 µK Why Bother? Chad Orzel Union College Physics 100 So, what’s this all good for? Basic Physics Atomic Spectroscopy Quantum nature of atoms becomes apparent Bose Einstein Condensation Atom Optics Precision Measurement Cold-atom devices already most sensitive detectors of: Rotation, Acceleration, Gravity Gradient Atomic Clocks – Time Standards Best clocks in the world Exotic stuff: Atom Lithography, Parity Violation Quantum Computing, ??? Chad Orzel Union College Physics 100 NIST-F1 Built by: Dawn Meekhof Steve Jefferts Accurate to 0.1 ns / day History of NIST clocks Chad Orzel Union College Physics 100 What’s It Good For? Global Positioning System: 24 Atomic Clocks on satellites Chad Orzel Union College Physics 100 Global Positioning Works by triangulation: Satellites broadcast time Measure travel delay from three satellites (Use fourth to determine time) Gives position to ~10 m