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Characterization of the Loading
Dynamics of a Magneto Optic
Trap
Vilas Rao
NASA SHARP 2001
Dr. Georg Raithel
Assistant Professor of Applied Physics/Mentor
What is Laser Cooling?
It is the process of cooling, or slowing, atoms using lasers.
How can light cool something down?
Radiation Pressure - Light exerts a force!
Light is made up of photons. A photon striking an atom
slows the atom slightly.
After the atoms absorb the photon, it remits it in a random
direction. Called photon scattering.
Absorption
Reemission
Photons
atom
atom
What is a Magneto Optic Trap?
What is a Magneto Optic Trap?
Lasers coming from 6 directions cool the atoms into a ball of
atoms in the center of the trap
Collisions in the MOT
The MOT is only able to trap atoms traveling slower than 30
meters/second.
Faster atoms still move around in the MOT, sometimes hitting an
atom out of the cluster of cooled atoms.
Cooled atoms
Fast-moving atom
Atoms get knocked out
Loading Time and Steady State
of the MOT
Loading Time - The average time that an atom stays in the cluster of
atoms before getting knocked out.
After a period of time, the number of atoms in the MOT becomes
constant.
Rate of Atoms Being Cooled = Rate of atoms being knocked out
The constant number of atoms in the trap is known as the steady state
atom number
Calculating the Number of Atoms
and Loading Time
Reemission
Absorption
Photons
atom
atom
The cluster of atoms can be seen as a bright ball of light due to the
reemission of photons.
A photodiode converts photons that hit its surface into a current.
We place a photodiode such that the light from the atoms hit it, and
measure the electric current.
Using this value, we can calculate the steady-state atom number in the
trap as well as the loading time.
We found that 5.6 million atoms per second were trapped by the MOT.
Application of Laser Cooling
Studying Atoms:
Atoms that are traveling slower can be studied more easily.
Atomic Clock :
Cesium atoms move back and forth between two energy states.
Atomic clocks measure time based the oscillation between the states.
This can be measured more accurately for cooled atoms than fastmoving atoms.
Time can more accurately be measured.
Conclusion
Through the process of laser cooling, the MOT can trap atoms by
cooling atoms in all directions.
Potential Problems:
• The light reemitted by the atoms is not bright enough to measure
a current
• The lasers are not aligned precisely.
Cooling atoms has already revolutionized the study of atomic
phenomena, and will continue to show applications.