M5L22r.txt

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If you think you know already everything about how atoms
interact with light from two-level atoms,
I have to tell you that's not the case,
because a three-level system has many new effects.

So one, of course, is that atoms can now
interact with two electromagnetic fields.
And those two electromagnetic fields can affect each other.

And this can happen through coherent or incoherent
mechanisms.

In other words, you can say it simply.
If you hit an atom with light and give a two-level system,
there's no way how the atom can hide.
It's always excited by the laser.
But if you have a three-level system,
you may have a situation where you
have destructive interference between what the two lasers can
do to the atom, and suddenly there
may be a state where the atom is in the dark, where the atom can
hide from the laser beam.
And this is something which is fundamentally new
and has no counterpart in a two-level system.
So I've already pointed out that the lambda system
is the most important one, because it has two ground
states, which can be in a long-lived superposition state.

And so what we want to discuss as possible consequences
is that in a three-level system you
can realize a lasing operation without having
inversion of the population of the ground and excited state.
So if you always thought, if I want to build a laser,
the first thing I have to do is make
sure I have more atoms in the excited than in the ground
state, yes, this is valid for two-level system.
But it is no longer valid for a three-level system.

I mean, the reason why you want to invert a two-level system
is you want to have stimulated emission from the excited
state, which is stronger than absorption
from the ground state.
But if you take advantage of quantum coherence,
you may have a situation where two possibilities
for stimulated emission add up coherently,
but the two possibilities for absorption
add up destructively.
And therefore, you can avoid absorption.
You have only stimulated emission,
but you have not achieved that through inversion.
You have achieved that through quantum coherence,
a fundamentally new effect.
So we have lasing without inversion.
We have the phenomenon I mentioned already
that atoms can hide in the dark if the two laser
beams in the excitation mechanism destructively
interfere.
This is called electromagnetically induced
transparency.
Systems which have sharp resonances
in three-level systems are used for reducing the group
velocity of light, which goes under the name slowing light
or even putting the light to a standstill, stopping light.
And three-level systems are also used
for a quantum mechanical memories
for quantum computation.
Any questions?