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M1L1d.txt
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#
# File: content-mit-8422-1x-captions/M1L1d.txt
#
# Captions for 8.422x module
#
# This file has 50 caption lines.
#
# Do not add or delete any lines. If there is text missing at the end, please add it to the last line.
#
#----------------------------------------
What I've discussed so far is that the standing wave of light
creates now a periodic potential for the atom,
and we will understand it from many, many different
perspectives, from the photon-type quantum optics
perspective to the most classical description.
So what immediately comes to my mind
is that we now want to look at this periodic potential in two
different situations.
Well, whenever you have light, spontaneous emission
is a possibility.
And we may ask what happens when spontaneous emission is not
negligible?
And we discuss it approximately in week nine of the course.
Then what happens is an atom has an excited and a ground state.
And both in the excited and the ground state,
do the atoms feel the periodic potential?
Well, eventually, we have to generalize
the notion of ground and excited state into three states.
And I will tell you all about it in a few weeks.
But the situation can now be when
an atom is in the ground state, it
has to move up the standing wave potential.
Then it's getting excited with a laser.
It has to move up again the standing wave potential,
and then there may be spontaneous emission.
So what I'm just describing here is a situation
where the atom is mechanically moving up the potential.
It's excited at the top of the potential,
and it emits when it's on top of the potential.
So the atom is doing mechanical work,
and this is called Sisyphus cooling.
This is one method, one mechanism of laser cooling
which leads to the lowest temperatures in the laboratory
before evaporative cooling is used.
So that's some cool aspect we will come along
by discussing motion in a standing wave,
but taking into account that photons are emitted
and really make understanding when and where are
the photons emitted.
Well, the second situation is, of course,
if spontaneous emission is completely negligible,
then we have a situation where all what matters
is the potential.
And we can completely forget that it's photons,
it's quantum optics which has created this potential.
We can simply use a classical potential