Comparison of Methods to Load a Mirror Magneto-Optical Trap

1. Comparison of Methods to Load a  Mirror Magneto-Optical Trap Capstone Talk PHYS 4300 Date: 14 May 2009 Author: C. Erin Savell Advisors: Dr. Shaffer and Arne Schwettmann Acknowledgement: Jonathan Tallant, Adrienne Wade, Herbert Grotewohl, Ernest Sanchez

2. Outline • Motivation • Atom Interferometry • Magneto Optical Trap (MOT) • Cooling and trapping transition • Mirror MOT • My work • Measuring MOT characteristics • Measuring MOT loading rates • Discussion of results • Questions http://www.aerospaceweb.org/aircraft/fighter/f22/f22_09.jpg http://weblogs.newsday.com/sports/watchdog/blog/satellite-radio.jpg

3. Motivation • To streamline MOT formation process; better MOTs allow better atom chip experiments • Atom chip allows faster, cheaper BEC (Bose-Einstein Condensate) formation • requires less equipment and gets steeper magnetic field gradients • Atom interferometry can beat current methods used for inertial navigation by orders of magnitude, but systems need to be compact

4. What is an Interferometer? • Interferometer: instrument that separates beam of light into two and recombines them resulting in an interference pattern • Resulting pattern can be used to measure wavelength, index of refraction, or astronomical distances (Measures Phase shifts -> phase to intensity conversion) • A high precision method to measure speed of light and acceleration Graphic courtesy of H. Grotewohl

5. Atom Interferometry: Why • Can be used for navigation gyroscope for inertial guidance • Will replace laser interferometers/gyroscopes • Atom Interferometry more sensitive than with light = BETTER • Atoms move at finite speed << c • Longer sampling time • more time to experience inertial changes Mirror assembly for laser interferometer Ring laser gyroscope Fiber optic gyroscope www.aerospaceweb.org/question/weapons/q0187.shtml www.answers.com/topic/michelson-interferometer

6. Atom Interferometry: How • Atom well formed in MOT or other similar means • Radio frequency (RF) current passed through a nearby wire • Causes wavefunctions in trap to change shape, spliting from “single well” of atoms to “double well” • Atom wavefunctions recombine • Absorption imaging can detect resulting interference pattern Atomic Wave Functions (split-> superposition) Graphic courtesy of H. Grotewohl

7. MOT • Cooling and trapping: • Lasers create “Optical Molasses”: atoms absorb photon from one direction, then emit in all directions; repeats • Reduction of momentum and kinetic energy of atoms results = “cooling” • Magnetic field gives a spatially dependent absorption = “trapping” Laser Orientation in a MOT (red= laser) Graphic courtesy of H. Grotewohl

8. ΔP ΔP ΔP MOT Animation Photon Atom Animation courtesy of Ernie Sanchez

9. Mirror MOT Atom chip surface Mirror MOT on atom chip (red= laser, gray=chip/mirror) • Same principle as a basic MOT, but uses a mirror to reflect the laser • Easier for trappingatoms neara surface • Provides good source of cold atoms for loading of atom chip microtraps • Atom chips can be used as the mirror in a mirror MOT Schmiedmayer Paper, p. 4 Graphic courtesy of H. Grotewohl

10. Cooling and Trapping Transitions of Rb-87 • Cooling laser: red-detuned to compensate for Doppler shift • Repumping laser: recycles atoms from ground state back into cooling transition http://jilawww.colorado.edu/pubs/thesis/du/

11. Our Mirror MOT • Rb-85 atoms in mirror MOT • Located 4.8mm below mirror surface • No chip in chamber yet; just mirror • T=~200μK • FWHM 1.6mm vertically,0.6mm horizontally Image courtesy of Arne Schwettmann Future cooling block location Mirror (or atom chip mount) MOT

12. Mirror MOT Chamber Setup Anti-Helmholtz Coils Main Chamber CCD Camera

13. Factors Affecting MOT Stability • Background Pressure: ambient pressure inside chamber • Pressure too low -> smaller number of atoms in MOT • Pressure too high -> increased atom collisions shorten MOT lifetime by knocking atoms out of trap • Laser Lock: • Necessity to minimize signal noise • Stable lock = stable MOT • No lock = no MOT

14. Rubidium Source • Source controlled by current • Normally ~5.3A • Attaches by a mount on a flange that has electrical feed-throughs • Releases Rb from solid state to a gaseous state Saes Getters S. p. A Catalog, p. 10 Image courtesy of Arne Schwettmann

15. My Work • Goal: to make higher quality MOT for loading chip trap • Count number of atoms in MOT • The more atoms the better • Measure density of atoms in MOT • Denser is better • Measure loading rate of MOT • Will compare rate and background pressure of 3 different MOT loading methods MOT in Shaffer Lab Image courtesy of Arne Schwettmann

16. Atom Number and Density in a MOT • Calibrate photodiode with power meter (measure in volts) • Measure intensity of light (power, P) emitted from MOT and detuning of laser beams with power meter • Solve for PTOT • Deduce the number of atoms by calculation • Number of atoms and MOT volume used to calculate density

17. Photodiode Calibration Setup iris linear polarizer beam splitter beam direction power meter photo diode

18. MOT Loading Rate Measurement • Fast loading rate and low background pressure are goals • Compare rates and background pressure of 3 loading methods: • Continuous: source on nonstop • Pulsed: source pulsed on/off • UV-LIAD (Ultra-violet Light Induced Adsorption Desorption): UV lamp used to desorb Rubidium atoms from windows/sides of chamber Diode lasers from MOT setup

19. Building a UV LED Array for UV-LIAD • Built UV-LED array • Assembled circuit to support LED array • Tested circuit and assembled it in front of chamber window UV LED array circuit

20. Rubidium Source Continuously “on” • Utilizes lower current (~3A) • Slower, more controlled loading rate UV LIAD Rates • Rubidium source switched off • UV LED array switched on for entire loading period • Rb atoms on chamber walls become excited, adsorb from walls into gas, load MOT

21. Pulsed Source • Two separate pulse timing schemes considered: • 4s on, 16 seconds off with 5A current • 2s on, 18 seconds off with 10A current • More intense current will induce faster loading rate • Shorter pulse time keeps background pressure low, reducing background collisions of atoms in MOT • Fast; allows for more experiments per block of time

22. Experimental Parameters • The laser lockpoint was maintained at δ=-10.7±1.6MHz from the trapping transition 85Rb 5 S1/2F = 3 5 P3/2 F = 4 • Background pressure of chamberwas maintained near 2.0x10-10Torr Image courtesy of Arne Schwettmann F= 3 & 4 F= 2 & 4 F= 4

23. Results

24. Background pressure UV-LIAD, Continuous, and Background MOT Loading Methods Background fitted curve UV LIAD fitted curve • Background rate is slowest • UV-LIAD improves atom number by factor of 2 • Continuous source best of the three UV LIAD Continuous Continuous *Error in all data points measured is +/- 13%

25. Pulsed Source MOT Loading Methods 2s pulse • 10A current pulse gives fastest loading rate • 10 times faster than continuous, fastest overall • 5A current half has fast, twice as long, smaller atom number present in trap 2s pulse fitted curve 4s pulse fitted curve 4s pulse *Error in all data points measured is +/- 13%

26. Questions?