Laser cooling involves using lasers tuned slightly below an atomic transition to reduce the kinetic energy and temperature of atoms. Photons from lasers traveling in opposite directions are more likely to be absorbed by atoms moving towards the laser, transferring momentum. Repeated absorption and emission lowers the atom's average velocity over time. Precisely tuned lasers, magnetic fields to control atom movement, high vacuum, and multiple laser directions allow laser cooling to produce ultracold atomic samples for quantum physics experiments near absolute zero.

2. INTRODUCTION TO COOLING
IN OUR PREVIOUS CLASSES WE HAVE LEARNT ABOUT KINETIC
THEORY OF GAS MOLECULES ACCORDING TO WHICH
T = 2 __K___
3 N.Kb
WHERE T IS TEMPERATURE OF THE MOLECULE
K IS KINETIC ENERGY OF GAS MOLECULES
N IS NUMBER OF MOLECULES
Kb IS BOLTZMANN CONSTANT
HERE WE CAN CLEARLY SAY THAT THE
TEMPERATURE IS DIRECTLY PROPORTIONAL TO KINETIC ENERGY OF
THE GAS MOLECULE SO BASICALLY THE COOLING IS SOMETHING
WHICH HAPPENS DUE TO DECREASE IN THE KINETIC ENERGY OF THE
MOLECULES IN THE PRESENT SAMPLE OF GASES .
IF WE TAKE EXAMPLE IN OUR EVERY DAY LIFE WHEN WE BOIL
WATER THE KINETIC ENERGY OF THE WATER MOLECULES
INCREASES AS A RESULT THE TEMPERATURE OF THE WATER SAMPLE
INCRESES.
THIS IS THE BASIC PRINCIPLE WHICH WE ARE GOING TO APPLY
INTO OUR PROJECT.

3. COOLING USING LASERS
LASER COOLING REFERS TO THE NUMBER OF TECHNIQUES IN WHICH
ATOMIC AND MOLECULAR SAMPLES ARE COOLED THROUGH THE
INTERACTION WITH ONE OR MORE LASER LIGHT FIELDS.
• THERE ARE MANY TECHNIQUES WHICH ARE USING THIS PRINCIPLE FOR
COOLING THE GAS BY APPLYING LASERS FROM DIFFERENT
DIRECTIONS.
• SOME EXAMPLES ARE
 Doppler cooling
 Sisyphus cooling Resolved sideband cooling
 Velocity selective coherent population trapping (VSCPT)
 Anti-Stokes inelastic light scattering (typically in the form of fluorescence or
Raman scattering)
 Cavity mediated cooling
 Sympathetic cooling
• Laser cooling is primarily used for experiments in Quantum Physics to achieve
temperatures of near absolute zero (−273.15°C, −459.67°F). This is done to
observe the unique quantum effects that can only occur at this heat level.
Generally, laser cooling has been only used on the atomic level to cool down

4. DOPPLER COOLING
Brief Explanation
Doppler cooling involves light whose frequency is tuned slightly below
an electronic transition in an atom. Because the light is detuned to the
"red" (i.e. at lower frequency) of the transition, the atoms will absorb
more photons if they move towards the light source, due to the
Doppler effect. Thus if one applies light from two opposite directions,
the atoms will always absorb more photons from the laser beam
pointing opposite to their direction of motion. In each absorption
event, the atom loses a momentum equal to the momentum of the
photon. If the atom, which is now in the excited state, emits a photon
spontaneously, it will be kicked by the same amount of momentum but
in a random direction. The result of the absorption and emission
process is a reduced speed of the atom, provided its initial speed is
larger than the recoil velocity from scattering a single photon. If the
absorption and emission are repeated many times, the mean velocity,
and therefore the kinetic energy of the atom will be reduced. Since
the temperature of an ensemble of atoms is a measure of the random

5. Some Questions which Need to be
Answered
1. Usually light incident on any matter just
increases its temperature but in this case lasers are
cooling it down how?
Ans. The basic theory used behind this is let the
photon bounce back from the atom for ex. If we
have a big rubber ball rolling we can hit it with small
rubber balls and can slow down the speed of the
big ball by transferring the momentum. In this case
laser lights photon is the smaller rubber ball and the
atom is the big rubber ball which is bouncing the
photons from the surface.
Following scientists got Nobel Price in field of
physics for discovering the cooling principle.

6. Born: 28 February 1948, St. Louis, MO,
USA
Affiliation at the time of the award:
Stanford University, Stanford, CA, USA
Prize motivation: "for development of
methods to cool and trap atoms with laser
light"
Field: Atomic physics
Born: 1 April 1933, Constantine, Algeria
Affiliation at the time of the award:
Collège de France, Paris, France, École
Normale Supérieure, Paris, France
Prize motivation: "for development of
methods to cool and trap atoms with laser
light"
Field: Atomic physics

7. Born: 5 November 1948, Wilkes-Barre,
PA, USA
Affiliation at the time of the award:
National Institute of Standards and
Technology, Gaithersburg, MD, USA
Prize motivation: "for development of
methods to cool and trap atoms with laser
light"
Field: Atomic physics
figure to describe the change in the momentum of the atom
Source-http://www.nobelprize.org

8. 2.But in case of atom it is not a true replica of the
ex. Given in the above question then how we can use it?
Ans. Actually the atom is absorbing the photon and then
in fraction of seconds it is releasing it back with more
momentum as it is also adding some of its momentum as
they are in continuous vibration. The time between
absorbing and releasing is such low that it is negligible
and we can consider it as bouncing of photon form the
atom.
Photon absorption and
emission

9. 3. The next thing is the atom require a photon of a
particular wavelength or better say of particular colour to
excite the atom so how it is done ?
Ans. This thing is done By making a light source of
varying frequency such that we can adjust it to a
particular frequency as per the requirement. But even a
vibration in the ground can effect the frequency of the
emitted light, we have to consider this also.
HERE THE RUBIDIUM ATOM IS
EXCITED BY USING LASER LIGHT
THE THING TO BE NOTED IS
THAT TRANSITON IS STRAIGHT
LIKE
IF A-B THEN B-A NOT B-C WHERE
C IS INTERMEDIATE BETWEEN A
AND B.

10. 4. The laser slow down the fast moving atoms but it
should also increase the speed of the slower moving atom
why it is not doing so?
Ans. This question was one of the most difficult question
which almost made this theory a waste but some clever
scientist solved this using a simple theory which is
Doppler effect and as the frequency changes with the
change in the position of the light source and atom the
atom neglects the most of the frequency and that’s how
they are able to affect only the atoms with higher vibrating
speed. This was the basis of the Doppler Shift called
THE FOLLOWING FIGURE SHOWS
THE DOPPLER SHIFT HAPPENING IN
hence the cooling is also known as DopplerDUE TO
THE RESONANCE
cooling.
MOVEMENT OF THE ATOM

11. 5. The previous answer was applicable for only one
direction what about the atoms which are moving in
random direction?
Ans. Answer to this question was also very easy as to use
laser from different directions so a machine was built
which was able to throw the laser to the sample from
360*360 deg. Angle. So it was like a sphere of small-small
laser sources bounded together. Cooling the atoms and
getting a bunch of cooled atoms is called as "optical
HERE THE ATOM IS BEEN STRIKED BY LASE
molasses“ SOURCES FROM ALL ROUND THE DIRECTIO
AS PER THE ABOVE QUESTION IS ANSWERE

12. 6.Now its ok with cooling but what about the atoms
moving in different directions they are not controlled as
they can hit the wall of the container and can gain the
heat.
Ans. This problem was also a tricky one but the answer
the scientists got was very easy as they use strong
magnets to control them as the atom are also tinny
magnets they respond to the magnetic field and get
attracted toward them they applied it from all the
directions to get the atoms in the centre of the container
HERE THE ATOMS ARE KEPT IN A
and not allowing them to hit the FIELD SO THAT THEY ARE KEPT
MAGNETIC walls.
IN THE CENTRE OF THE CHEMBER.

13. 7. Now what if the atoms in between themselves only
hit each other and get heated up.
Ans. For this the machine is having vacuum pumps
so there are no any excess air atoms also the
amount of atom taken in the container is very low as
it would not allow the atoms to hit each other.

14. LASER COOLING
ARRANGEMENT

15. WORK OF DIFFERENT PARTS
 1- Frequency stabilised laser diode.
 3-Two lens telescope arrangement which
expands the laser beam.
 4,6,7,9,10,11- divide the incident beam into three
beams of nearly equal width and equal intensity
travelling in the three mutually orthogonal
directions in the trapping cell.
 13,14,15 and 16 -The beam along the axis of the
magnetic coils should be circularly polarized in
the opposite direction which is achieved by
setting the axes of the four quarter wave plates
13, 14, 15 and 16.

16. • 5,8 and 12- Mirrors which retro-reflect the
circularly polarized light beams along the
three orthogonal directions after they pass
through the quarterwave plates(14,16 and
18).
• 20 – Laser diode which produces frequency
stabilised hyperfine laser beam.
• 23a and 23b – Pair of mirors.
• 24 and 25 – Circular coils which produce a
quadrupolar magnetic field