Comparison of Methods to Load a 

Mirror Magneto-Optical Trap

Date: 14 May 2009Author: C. Erin SavellAdvisors: Dr. Shaffer and Arne SchwettmannAcknowledgement: Jonathan Tallant, Adrienne Wade, Herbert Grotewohl, Ernest Sanchez

Capstone Talk

PHYS 4300

Outline

•Motivation

•Atom Interferometry

•Magneto Optical Trap (MOT)

•Cooling and trapping transition

•Mirror MOT

•My worko Measuring MOT characteristics

o Measuring MOT loading rates

o Discussion of results

•Questions

http://weblogs.newsday.com/sports/watchdog/blog/satellite-radio.jpg

http://www.aerospaceweb.org/aircraft/fighter/f22/f22_09.jpg

Motivation

•To streamline MOT formation process; better MOTs allow better atom chip experiments

•Atom chip allows faster, cheaper BEC (Bose-Einstein Condensate) formationo 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

Graphic courtesy of H. Grotewohl

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

•Can be used for navigation gyroscope for inertial guidanceo Will replace laser interferometers/gyroscopes

•Atom Interferometry more sensitive than with light = BETTERo Atoms move at finite speed << c

o Longer sampling time

o more time to experience inertial changes

Atom Interferometry: Why

Mirror assembly for laser interferometer

www.answers.com/topic/michelson-interferometer

Ring laser gyroscope

Fiber optic gyroscope

www.aerospaceweb.org/question/weapons/q0187.shtml

Atom Interferometry: How

•Atom well formed in MOT or other similar means

•Radio frequency (RF) current passed through a nearby wireo Causes wavefunctions in trap to

change shape, spliting from “single well” of atoms to “double well”

•Atom wavefunctions recombineo Absorption imaging can detect

resulting interference pattern

Graphic courtesy of H. Grotewohl

Atomic Wave Functions (split-> superposition)

MOT

Graphic courtesy of H. Grotewohl

Laser Orientation in a MOT

(red= laser)

PhotonΔP

Atom

ΔP

ΔP

Animation courtesy of Ernie Sanchez

MOT Animation

Mirror MOT

•Same principle as a basic MOT, but uses a mirror to reflect the laser

•Easier for trapping atoms near a surface

•Provides good source of cold atoms for loading of atom chip microtrapso Atom chips can be used as the mirror

in a mirror MOT

Schmiedmayer Paper, p. 4

Atom chip surface

Mirror MOT on atom chip (red= laser, gray=chip/mirror)

Graphic courtesy of H. Grotewohl

Cooling and Trapping Transitions of Rb-87

http://jilawww.colorado.edu/pubs/thesis/du/

•Cooling laser: red-detuned to compensate for Doppler shift

•Repumping laser: recycles atoms from ground state back into cooling transition

Our Mirror MOT Image courtesy of Arne Schwettmann

MOT

Future cooling block location Mirror

(or atom chip mount)

•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

Mirror MOT Chamber Setup

CCD Camera

Main Chamber

Anti-Helmholtz Coils

Factors Affecting MOT Stability

•Background Pressure: ambient pressure inside chambero Pressure too low -> smaller number of atoms in MOT

o Pressure too high -> increased atom collisions shorten MOT lifetime by knocking atoms out of trap

•Laser Lock:o Necessity to minimize signal noise

o Stable lock = stable MOT

o No lock = no MOT

Rubidium Source

Saes Getters S. p. A Catalog, p. 10

Image courtesy of Arne Schwettmann

•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

My Work

•Goal: to make higher quality MOT for loading chip trap

•Count number of atoms in MOTo The more atoms the better

•Measure density of atoms in MOTo Denser is better

•Measure loading rate of MOTo Will compare rate and background

pressure of 3 different MOT loading methods

MOT in Shaffer Lab

Image courtesy of Arne Schwettmann

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

Variable Description

a = lens focal length

d = lens diameter

α = reduction factor of glass

P = measured power

Pa = PTOT =

power per atom (constant)power emitted by MOT

N = number of atoms in MOT

Photodiode Calibration Setup

iris linear polarizer

beam splitter

beam direction

power meter

photo diode

MOT Loading Rate Measurement•Fast loading rate and low background pressure are goals

•Compare rates and background pressure of 3 loading methods:o Continuous: source on nonstop

o Pulsed: source pulsed on/off

o 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

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

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

Pulsed Source

Experimental Parameters•The laser lockpoint was maintained at δ =-

10.7±1.6MHz from the trapping transition 85Rb 5 S1/2� F

= 3 5 P3/2� F = 4

•Background pressure of chamber was maintained near 2.0x10-10 Torr

Image courtesy of Arne Schwettmann

F= 2 & 4 F= 3 & 4

F= 4

RESULTS

UV-LIAD, Continuous, and Background MOT Loading Methods•Background rate is slowest

•UV-LIAD improves atom number by factor of 2

•Continuous source best of the three

Background fitted curveUV LIAD fitted curveUV LIAD

Background pressure

ContinuousContinuous *Error in all data points measured is +/- 13%

Pulsed Source MOT Loading Methods

•10A current pulse gives fastest loading rateo 10 times faster than

continuous, fastest overall

•5A current half has fast, twice as long, smaller atom number present in trap

2s pulse fitted curve4s pulse fitted curve4s pulse

2s pulse

*Error in all data points measured is +/- 13%

Questions?