Laser cooling allows atoms to be slowed and trapped using laser light. The first proposal for cooling neutral atoms with counter-propagating laser beams was made in 1975 by T.W. Hänsch and A.L. Schawlow. When atoms move towards a laser, they see a higher frequency due to the Doppler effect which allows them to absorb photons, slowing the atoms. Atoms are cooled to a few millikelvins through this process. However, limitations exist such as a minimum temperature due to spontaneous emission and a maximum concentration to prevent photon absorption as heat. Laser cooling has applications in atomic clocks, atom optics, and observing Bose-Einstein condensation.

1. Laser cooling
Abhishek Tomar

2. The first proposal to cool neutral
atoms in counter-propagating
laser beams was made by T.W.
Hänsch and A.L. Schawlow in
1975.
Theodor Wolfgang Hänsch

3.  The Nobel Prize in Physics was awarded in 1997 to Steven Chu, Claude Cohen-
Tannoudji and William D. Phillips, for the development of methods to cool and trap
atoms with laser light.
Steven Chu
Claude Cohen-Tannoudji
William Daniel Phillips

4. ATOM TRAPPING

5. Magneto-optical trap

7. Doppler effect:
The relativistic Doppler effect is the change in frequency (and wavelength) of light, caused by
the relative motion of the source and the observer, when taking into account effects described
by the special theory of relativity.

8. LASER COOLING

9. • A stationary atom sees the laser neither red- nor blue-shifted
and does not absorb the photon.
• An atom moving away from the laser sees it red-shifted and
does not absorb the photon.
• An atom moving towards the laser sees it blue-shifted and
absorbs the photon, slowing the atom.
• The photon excites the atom, moving an electron to a higher
quantum state.

10. • If an atom is moving towards any of the LASER, that
particular beam would appear at a slightly higher
frequency (Doppler effect).
• The laser light is tuned to a frequency with an energy just
below the energy of an electronic transition of an atom to
excite the atoms to higher state.
• The atoms will always scatter more photons from
the laser beam pointing opposite to their direction of
motion.

11. • This small increase in frequency makes the frequency of LASER
exactly equal to the needed one for atomic excitation. The photons
are thus absorbed and atom excites.
• The atom recoils back due to the momentum provided by the
photons and re-emits a new photon in random direction coming
back to its ground state.
• Thus the atom slows down and thus the kinetic energy of the
system and thus, the temperature goes down.
KE = 3/2 KT = ½ mv*v
So, v α √T
• This makes the temperature down to a few milli-Kelvins.

13. LIMITATIONS
MINIMUM TEMPERATURE- The atom performs a random walk in
momentum space with steps equal to the photon momentum due to
spontaneous emission and photon absorption. This constitutes a heating
effect, which counteracts the cooling process and imposes a limit on the
amount by which the atom can be cooled
MAXIMUM CONCENTRATION- The concentration must be minimal to
prevent the absorption of the photons into the gas in the form of heat.
This absorption happens when two atoms collide which each other while
one of them has an excited electron. There is then a possibility of the
excited electron dropping back to the ground state with its extra energy
liberated in additionnal kinetic energy to the colliding atoms — which
heats the atoms.

14. ATOMIC STRUCTURE- Only certain atoms and ions have
optical transitions susceptible to laser cooling, since it is
extremely difficult to generate the amounts of laser power
needed at wavelengths much shorter than 300 nm.
The more hyperfine structure an atom has, the more ways
are for it to emit a photon from the upper state and not return
to its original state, removing it from the cooling process.

15. applications
 construction of atomic clocks
 atomic interferometers and atom lasers
 in the development of instruments for atom optics and atomic lithography.
 observation of a Bose-Einstein condensation in a dilute atomic gas

16. References
 http://www.chemeurope.com/en/encyclopedia/Laser_cooling.html
 http://www.nobelprize.org/nobel_prizes/physics/laureates/1997/advanced.html
 Modern physics-serway,moses,moyer
 You Tube
 Wikipedia