LASER COOLING & TRAPPING: History Mechanism Experimental Setup Limiting Factor Applications

1. SIF 3001 ATOMIC PHYSICS
LASER COOLING AND
TRAPPING
Group Members Matric No.
Norsyazwani Binti Asmi 17142028
Aliff Izzudin Bin Ayob 17146452
Amirah Binti Basir 17145870
Saibah Nabihah Binti
Mohamad Baharuddin
17146639
Ahmad Ashshidiq Zamani
Bin Ahmad Nazeri
17161255
LINK: https://drive.google.com/drive/folders/11l96hfiWDuYjqDIDCGHeNzmHevJZalFT?usp=sharing

2. CONTENTS
BASIC CONCEPT
LASER TRAPPING
1
4
2
3
LASER COOLING
DOPPLER LIMIT
APPLICATIONS5

3. PART 1:
Basic Concepts

4. DISCOVERY
1975
The first to propose the cooling of neutral
atom in counter-propagating laser beams
Theodor
Hansch
Arthur
Schawlow

5. THE FIRST TEST
1997
Awarded Nobel Prize in Physics for the development of
methods to cool and trap atoms with laser light.
Steven Chu William Phillips Claude Cohen

6. BASIC CONCEPTS
Photons
✔ Light is an oscillating electromagnetic wave
and consists of particles that are called
photons
✔ Laser light has a special property called
“coherence”
✔ Particularly in laser beams, the individual
photon all has the same frequency, is in phase
with each other and they are all travelling in
the same direction
Figure 1: Photons in laser beams

7. Doppler Shift
✔ In the context of light wave, an observer will perceive a Doppler
shift in the frequency of light if the source of light is moving
relative to the observer.
✔ Doppler shift in laser cooling is a way used to control whether or
not an atom will absorb a photon, depending on whether the atom
is moving opposite to or in the direction of the laser beam.
✔ The basic principle is that absorption and subsequent spontaneous
emission of photons lead to light forces.
✔ These forces become velocity-dependent through the Doppler
effect

8. PART 2:
Laser Cooling

9. DOPPLER COOLING MECHANISM
Tune the laser light to
a frequency below
the atomic resonance
frequency (atoms
absorb photons from
the laser beams).
Momentum
transferred to the
atoms through
photon absorption &
emission
-decrease the
velocities of the
atoms
-reduces the
temperature of
the atoms
i) absorption ii) emission

10. Consider two energy levels E1 & E2. Two laser
beams coming at x-direction with frequency 𝜔𝜔𝐿𝐿
-Tune the frequency of the laser beam 𝜔𝜔𝐿𝐿 to get photon
energy < E2-E1.
-Rest atom between laser absorb equal amount of photons.
1
4
2
3
Photons of E= E2-E1 transfer their momentum to the
atoms through absorption and spontaneous emission
Slowing the atom
Initial atom at rest
Atom at rest experience equal forces & frequency 𝜔𝜔𝐿𝐿
from each direction and does not feel a net force from the
laser beams

11. Initial moving atom
• moving atom experience different frequencies from
each laser due to the Doppler shift.
• Atom moves to the left experience increase in the
frequency of Laser 1, 𝜔𝜔1, because it moves against
the direction& experience decrease in the
frequency of Laser 2, 𝜔𝜔2, because it moves in the
direction of L2.
• Atom absorbs photons from Laser 1 > Laser 2.
• Absorbing more photons from Laser 1 = reduces its
leftward velocity
• absorbing fewer photons from Laser 2 = atom
cannot be accelerated to the left.
• The same process happen when atom moves to
the right.
• The laser light always reduces the velocities of
the atoms, but never increases them.
• The atom sample begins to cool as the
velocities of the atoms in the sample decrease.

12. LASER
COOLING
EXPERIMENT
SET-UP

13. The trapping laser is
split into two with
equal intensity by
polarizing beam
splitter& reflected by
mirrors to send out of
injection locking
system
The collimated beam
then travel through a
system of auxiliary
optics where the
beam undergoes
several polarizations
& reflections
When the beam
entered the trap cell, it
will be divided in two
beams, in z-direction
& xy-direction. xy-
direction beam will
further spilt into x-
direction and y-
direction
Before these beams
entered vacuum
chamber containing Rb87
atoms, they will pass
through quarter-wave
plate, which converts
linearly polarized light
into circularly polarized
light
The circularly polarized
laser beams will be
reflected by mirrors in a
way that causes them to
intersect perpendicularly
inside the vacuum
chamber from each
direction in space,
allowing them to cool the
Rb87 atoms inside
1 3
42
5

14. PART 3:
Doppler Limit

15. • The frequency of the six laser beams in the
optical molasses can be reduced - the
slowest atoms are confined.
• Since the emission of a photon from an
excited atom is from random direction, the
atom can gain momentum from this
emission.
• An equilibrium between laser cooling and
the heating process arising from the random
nature of both the absorption and emission
of photons = the Doppler limit.

16. • It is the minimum temperature
achievable with Doppler cooling.
• This limit is only applicable for a simple
two level structure atom.
• Temperatures below the Doppler limit
can be achieved by using Sisyphus cooling
• But, it is only applicable for real alkali
atoms with lack of hyperfine structure-
have degenerate energy levels.
1. Doppler Temperature
Find TDoppler by equating the
competing processes:
cooling = heating
MHz
kB = Boltzmann’s constant
ŋ = ħ = Planck's constant

17. PART 4:
Laser Trapping

18. ▪ Atom remained inside the trap
->Pump laser beam incident on atom shine slightly
higher frequency than trap laser beam
▪ Atom absorbs photons from pump laser beam and driven
to energy level just below E2
▪ Atoms drift out of trap
-> rate of atoms falling to different energy level state
than E1>rate the atoms jumped back to E1 by pump laser
The Lasers

19. Magnetic field

20. Atom located to the
right of z=0
Atom located to the
left of z=0

21. Quadrupole magnetic field
Coils
Zero-point
in field
Magneti
c field
lines

22. PART 5:
Applications

23. Quantum Computers
Quantum SimulationsAtomic Clocks
Atomic Interferometer Basis for Bose-Einstein
Condensation
The Technological Applications
Optical Tweezers

24. Atomic Clocks
✔ Numerous scientific and technological advances;
regional and global navigation satellite systems, and
applications in the Internet, which depends critically
on frequency and time standards.
✔ Used at some long wave and medium wave
broadcasting stations to deliver a highly precise
carrier frequency.
✔ Many scientific disciplines has been imposing atomic
clocks, such as for long-baseline interferometry in
radioastronomy.
✔ In 1997, Steven Chu et al., has made an atomic
“fountain” where cooled atoms are launched
upwards into a chamber in which they are slowed by
gravity.
✔ This technique is able to build atomic clocks with a
hundredfold greater precision than currently
possible.

25. ✔ Atomic clock uses certain resonance frequencies of atoms such as Cesium (Cs) or
Rubidium (Rb) to keep the time with extreme accuracy.
✔ The electronic components of atomic clocks are regulated by the frequency of the
microwave electromagnetic radiation.
✔ The quantum transition of the cesium atoms will be induced only when the radiation is
maintained at highly specific frequency.
✔ These quantum transitions are observed and maintained in a feedback loop that trims the
frequency of the electromagnetic radiation, then eventually these waves are counted.
The best Cs fountain atomic clocks are now predicted to be off by less than one
second in more than 50 million years!

26. All atoms of Cs-133
are identical and
produce radiation of
exactly the same
frequency, thus
makes the atom
perfect timepieces.
The timing process
begins by introducing
Cs(g) into a vacuum
chamber and directing
six infrared lasers to
compact and cool to a
near absolute zero
temperature
Then, two vertical lasers
are used to nudge the
atoms up to a metre
which creates a
“fountain” cavity.
Due to the round trip
via microwave cavity
takes about a second,
it is resulted in
greater timekeeping
accuracy.

27. REFERENCES
1. Foot, C. J. (2005). Atomic physics (Vol. 7). Oxford University Press.
2. Muckley, E. S. (2009). Constructing a Magneto-Optical Trap for Cold Atom Trapping.
3. Metcalf, H. J., & Van der Straten, P. (2007). Laser cooling and trapping of neutral atoms.
The Optics Encyclopedia: Basic Foundations and Practical Applications.
4. Muckley, E. S. (2009). Constructing a Magneto-Optical Trap for Cold Atom Trapping.
5. Metcalf, H. J., & Van der Straten, P. (2007). Laser cooling and trapping of neutral atoms.
The Optics Encyclopedia: Basic Foundations and Practical Applications.
6. Betts, J. D. (2017). Atomic Clock: Encyclopedia Britannica.
7. McCarthy, D. D., & Seidelmann, P. K. (2018). Time: From Earth Rotation to Atomic Physics:
Cambridge University Press.

28. THANK
YOU!