M5L23d.txt

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One important application of the dark state
is the STIRAP technique.
STIRAP is adiabatic population transfer.

So STIRAP stands for stimulated-- what is the R?
Stimulated Raman Adi-- Stimulated Rapid Adiabatic
Passage?
[INAUDIBLE] rapid and Raman.
Yeah, I've heard both [INAUDIBLE]

So we have two choices.
It's both Raman and rapid.
A is adiabatic.
P is passage.
And STI is stimulated, because it's
a coherent stimulated process.
The idea is the following.
With this concept of a dark state,
I can immediately explain to you how,
by changing the intensities of the two laser beams,
you can make an adiabatic transfer from one stage, G,
to the other state, F. And this is important
because in many atomic physics experiments,
you start with one state and then
you want to prepare another state.
You always have the option of having some suitable pi pulse
and going from one state to the next.
But the pi pulse has to be exactly pi.
You have to be careful what the intensity
and duration of your pulse is.
But if you can adiabatically go from one state
to the other one, this is [INAUDIBLE]
against pretty much everything.
And I want to now use the concept of the dark state
to tell you how this population transfer works.
But before I even get into any explanation,
the picture is the following.
The dark state-- if you change laser parameters,
you can change which state is the dark state.
I gave you this simple example.
If one laser beam is on, the state G is the dark state.
If the other laser beam is on, the state F is the dark state.
So if you switch one laser beam off and the other one on,
you have changed the definition of the dark state.
And as I want to show you, the dark
state-- we can set it up in the situation
that the dark state is the absolute ground
state of the system.
And you know when you change parameters of your Hamiltonian
adiabaticity tells you that you always
stay in the ground state.
See?
Now even without any equations you
understand what adiabatic population transfer is.
But let's work it out.
So we want to understand-- and this is our adiabatic process--
what happens when the laser parameters omega 1 and omega 2
change slowly.