Transcript Slide 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
Outline
•Motivation
•Atom Interferometry
•Magneto Optical Trap (MOT)
•Cooling and trapping transition
http://www.aerospaceweb.org/aircraft/fighter/f22/f22_09.jpg
•Mirror MOT
•My work
o Measuring MOT characteristics
o Measuring MOT loading rates
o Discussion of results
•Questions
http://weblogs.newsday.com/sports/watchdog/blog/satellite-radio.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
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
Atom Interferometry: Why
•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
Mirror assembly for laser
interferometer
o more time to experience inertial changes
Fiber optic
gyroscope
www.aerospaceweb.org/question/
weapons/q0187.shtml
Ring laser gyroscope
www.answers.com/topic/michelson-interferometer
Atom Interferometry: How
•Atom well formed in MOT or
other similar means
•Radio frequency (RF) current
passed through a nearby wire
Atomic Wave Functions (split-> superposition)
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
MOT
•Cooling and trapping:
o Lasers create “Optical Molasses”:
atoms absorb photon from one
Laser Orientation in a MOT
(red= laser)
direction, then emit in all directions;
repeats
o Reduction of momentum and
kinetic energy of atoms results =
“cooling”
o Magnetic field gives a spatially
dependent absorption = “trapping”
Graphic courtesy of H. Grotewohl
MOT Animation
ΔP
ΔP
Photon
Atom
ΔP
Animation courtesy of Ernie Sanchez
Mirror MOT
Atom chip surface
•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
Schmiedmayer Paper, p. 4
Mirror MOT on atom chip
(red= laser, gray=chip/mirror)
mirror MOT
Graphic courtesy of H. Grotewohl
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/
Our Mirror MOT
•Rb-85 atoms in mirror
MOT
•Located 4.8mm below
mirror surface
Image courtesy of Arne Schwettmann
Future cooling
block location
Mirror
(or atom chip
mount)
•No chip in chamber yet;
just mirror
•T=~200μK
•FWHM 1.6mm vertically,
0.6mm horizontally
MOT
Mirror MOT Chamber Setup
Anti-Helmholtz Coils
Main Chamber
CCD Camera
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
•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
My Work
•Goal: to make higher quality MOT
MOT in Shaffer Lab
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
Image courtesy of Arne Schwettmann
Atom Number and Density in a MOT
•Calibrate photodiode with power
meter (measure in volts)
Variable Description
a = lens focal length
d = lens diameter
α = reduction factor of glass
•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
P = measured power
Pa = power per atom (constant)
PTOT = 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
•Two separate pulse timing schemes considered:
o 4s on, 16 seconds off with 5A current
o 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
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 pressure
Background fitted curve
UV LIAD fitted curve
UV LIAD
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
*Error in all data points measured is +/- 13%
2s pulse
2s pulse fitted curve
4s pulse fitted curve
4s pulse
Questions?