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 work

o Measuring MOT characteristics

o Measuring MOT loading rates

o Discussion of results

•Questionshttp://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) formation

o 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

guidance

o Will replace laser interferometers/gyroscopes

•Atom Interferometry more sensitive than with light =

BETTER

o 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 wire

o Causes wavefunctions in trap to

change shape, spliting from

“single well” of atoms to “double

well”

•Atom wavefunctions recombine

o 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

microtraps

o 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 MOTImage 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

chamber

o 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 MOT

o The more atoms the better

•Measure density of atoms in MOT

o Denser is better

•Measure loading rate of MOT

o 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

circui

t

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 TorrImage courtesy of Arne

SchwettmannF= 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 curve

UV LIAD fitted curve

UV LIAD

Background pressure

Continuous

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

Pulsed Source MOT Loading Methods

•10A current pulse

gives fastest loading

rate

o 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

2s pulse

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

Questions?