Determination of fundamental constants using laser cooled molecular ions

1. Determination of fundamental constants using laser cooled molecular ions

2. C. W. Chou, D. B. Hume, J. C. J. Koelemeij, D. J. Wineland, and T. Rosenband, PRL 104, 070802 (2010) Jeroen Koelemeij, PostDoc

3. Working on molecular ions Mohammad Ali Haddad, PhD student Jeroen Koelemeij, PostDoc

4. Outline • The proton-to-electron mass ratio μ • Measuring vibrations in HD+ • Ion trap • Laser Cooling Atoms • Cooling molecules • Measurements • Results

5. μ • In general, value of physical constants can depend on units • Dimensionless constants do not depend on the stick you measure with • Two dimensionless parameters needed for gross structure of atoms and molecules • Fine structure constant α • The proton-to-electron mass ratio μ

6. μ • Vibrations and rotations in molecules and vibrations in crystal lattices depend on μ • Properties of matter like specific heat capacity, thermal conductivity depend on these motions • μ=mp/me=1836.1526724718(80) • Want an even more accurate value • Compare results of different techniques

7. HD+ • Study vibrations of simplest molecule, like H2+ • HD+ has a permanent electric dipole which allows for a vibrational transition within an electronic state • QED Calculations on vibrational transitions limited by knowledge of μ • Vary μ to fit calculations to experimental data • ftheory(μ)=fexp

8. Measuring • Movement disturbs measurement • Vacuum • Cooling

9. Ion trap • Ions are trapped using electric fields • Impossible to trap them with static fields • RF-fields create a harmonic pseudopotential

11. Laser Cooling • Atoms accelerate or decelerate when absorbing light • Need absorption only when would decelerate an atom • When atom is moving opposite to laser beam • Absorb light only at resonance frequency • Doppler effect • Laser detuned below resonance

12. Cooling cycle • Need many photon absorptions • Requires a two-level system • Molecules can decay to many rovibrational levels • They do not end up in the same initial state

13. Cooling of molecules • No direct laser cooling • Use sympathetic cooling • Energy transfer from HD+ to atomic ions due to Coulomb interactions

14. Cooling of molecules • Frequency of radial harmonic motion ωr in RF-pseudo-potential scales with 1/m • Potential energy scales with (m ωr2)∝1/m • Use ions with small mass for greater Coulomb-interaction • Be+ B. Roth, J. Koelemeij et al., PRA 74, 040501 (2006)

15. Laser cooling • Beryllium ions • Tuning to the right wavelength • We need 313 nm • Frequency doubling of 626 nm dye laser • 532 nm solid state laser to pump the dye

18. Measuring • Measure the absorption frequency • Detect absorption of probe laser while changing its frequency • Transition to other vibrational state • Low spontaneous emission rate • Fluorescence is very weak

19. Measuring • Dissociating HD+ • Laser dissociates molecules from excited  state • Change in response of the ion cloud if HD+ was in excited state

20. Measuring • Be+ is visible because of cooling laser • Fluorescence depends on temperature • Drive HD+ harmonic motion in trap • Temperature depends on amount of HD+ • Fluorescence depends on amount of HD+ B. Roth, J. Koelemeij et al., PRA 74, 040501 (2006)

21. No measurement data yet • Still setting up 780nm laser • Stabilization, frequency calibration • Ion trap can still be improved

22. But we do have trapped ions

24. Sources used for pictures: • http://www.informaworld.com/ampp/image?path=/713172969/713548228/F0001.png • http://gottatopic.com/how-long-is-a-meter/ • http://www.andybrain.com/sciencelab/2008/04/26/learn-about-volume-and-space-with-ice-water/