8.421-course-intro.txt

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Given all this excitement, you have many reasons
to want to learn more about it, and this course is definitely
a good starting point.
Let me maybe tell you a little bit
what is the philosophy behind the course, and what you get.
That means, of course, at the same time,
what you will not get.
This course is meant as an systematic basic introduction
into AMO physics.

It should really lay the basic foundation,
that when you talk about atoms, when you talk about light,
you are really an expert and you can talk about it at the most
profound level.
So it's important here, and this is the goal of this course,
to provide enough knowledge, enough foundation for that.
So it's not a course where I just
try to send you highlights of the field,
and provide you with a semi understanding of all
this wonderful phenomena.
I rather try to focus on selected basic things, but then
also exciting things.
But rather explain them thoroughly
and teach you by example than teaching you a big overview.

The course, if I want to characterize it,
I would say it is a conservative course.
It's also in this sense traditional.
One reason for that is MIT-- the tradition we have at MIT.
At MIT, we have this several generation of atomic physicists
who have shaped the field.
And I learned atomic physics as a postdoc
from Dave Pritchard, who was a graduate student of Dan
Kleppner.
Dan Kleppner was a graduate student from Norman Ramsey.
And Norman Ramsey was a postdoc with I. I. Rabi.
And Rabi resonances, RF resonances,
this is the beginning of atomic physics-- the resonances.
That's what we also focus on today and in the first week.
This is the most important concept in atomic physics.
To really understand the nature of resonances
and all its implication.
So I should say when I-- late in my life--
I was already past 30 when I took the first atomic physics
class in my life.
I took it from Dave Pritchard, and I was really sort of amazed
about a course which had the traditional topics,
but provided a lot of insight.
You can teach traditional physics
from the perspective of somebody who does research today.
So I want to give you all the connections,
but at the same time, I like a lot
about the traditional approach, and some of it
can be traced back to Norman Ramsey.
So eventually over the last years,
I was the main person who has shaped the atomic physics
course.
I expanded it from one semester to two semesters.
But when I created a lot of new topics,
I always looked through Dan's and Dave's note and made sure
the best of what they taught, the best ideas
they put into the course-- they still survive
until the present day.
So this course is the development and continuation
of a longstanding tradition.
I should say I have immensely enjoyed
to co-teach the course on a couple of occasions with Vladan
Vuletic and Ike Chuang.
And Ike has made major contributions
to the second part of the course,
and Vladan especially to what we will be discussing
in the next few weeks.

So what I think is unusual-- you won't find it
in many textbooks-- is that we start out
by discussing the phenomenon of resonance
of the harmonic oscillator.
And we will emphasize for a while the classical part,
but then also, of course, go to the quantum
mechanical aspects of resonance.
And I have to say, this balance between classical and quantum
mechanics is something I will emphasize again and again
in the course.
I can guarantee you in this course,
I will sometimes ask you interesting questions
which challenge your intuition.
And you will most likely recognize
that often when your intuition goes completely wrong,
it happens because you believe too much,
or you overinterpret one aspect of quantum physics.
If I then tell you, but wait a moment.
Now think classically.
Push the classical concert further.
Regard the electron in the atom as an harmonic oscillator.
Regard light scattering as the effect,
not of a quantum mechanical atom,
but of a driven harmonic oscillator.
Suddenly, a lot of things which come out of quantum mechanics
make much more sense.
So I've often seen when I had a conflict in my understanding
and that there is a semi-classical
and a quantum mechanical explanation.
I've learned to trust much more they
semi-classical explanation.
So that's why I feel it is important to understand
the classical aspects.
And usually I would also say understanding means
to really understand its limits.
And often I feel you can understand a phenomenon only
when you have a quantum aspect, a classical aspect,
and you know exactly where they overlap and where they differ.
So to see even quantum mechanical objects occasionally
from the classical perspective provides additional insight.
So therefore I will emphasize classical aspects.
And for instance it may come for many of you as a surprise--
and we will see that next week-- that some aspects,
like the generalized Rabi frequency.
Which you've all, or many of you,
have seen for a two-level system.
We'll find it in classical resonance.
Just the classical equation of motion of a gyroscope
has a generalized Rabi frequency.
And I do feel that it is absolutely
important for the understanding of concepts
that you know, where do the concepts emerge?
Where are they?
Are they already there in classical physics and survive
in quantum physics, or is it something new
which is genuinely quantum?
So yes I will teach a little bit more classical physics
that in a standard AMO course, but because I've
seen within my own research experience
that it helped me to shape the intuition
for the full understanding of the systems we're dealing with.
So resonance is an overarching theme here.
But then we have to introduce our main players.
The atoms come to stage.
And we want to understand the electronics
structure, the fine structure, the hyperfine structure.
We want to understand what happens in
magnetic, electric, and electromagnetic light fields.
We want to understand in a deep way,
how do atoms interact with radiation?
This also leads us-- there's a big difference.
You would say, oh, what's the difference when
atoms interact with microwaves and atoms interact with light?
Well, light-- or at high frequency, spontaneous emission
becomes important.
And then you have an open quantum system.
You have Hamiltonian, which couples automatically
to many, many states.
So that's why radiation is different from just
electric and magnetic fields, because of the presence of all
of the vacuum modes, and we'll talk a lot about it.
There's one special aspect about the course,
which I don't think I've seen in textbooks in the same way.
We are singling out in a rather long unit
the aspect of line shape.
And OK, we talk a lot about an atom as a resonance.
But when you measure the resonance,
there is a line shape.
And I found it extremely insightful,
when I first saw Dave Pritchard doing it in his atomic physics
course, to just talk about all aspects which
modify the resonance from a delta function-- from a stick
diagram-- into a real shape.
It can be Doppler broadening.
It can be finite lifetime broadening.
It can be inhomogeneous field.
But there are lots of interesting effects,
and by discussing them all together,
we gain major insight.
So we discuss how photon recoil, how the velocity of the atoms
affects the line shape.
And if you think you've understood everything,
I will talk to you about a very counterintuitive aspect of line
shapes, the Dicke narrowing.
If you put atoms in the environment, you would say
and they collide.
This should lead to collision and broadening.
But there is one aspect where collisions lead to narrowing.
And that's sort of a highlight of this chapter which really
shows you how all those broadening mechanisms are
somehow connected.
And more towards the end of the course,
we want to understand what happens
when atoms interact not just with one photon,
but with several photons.
And then we talk about multiphoton processes.
I should actually say that I'm also emphasizing
the multiphoton process a lot.
Often we just simply do a transition between two levels,
and there is an operator, and it can be single-photon
or two-photon operator, yes.
But to understand the multiphoton
aspect is important, and maybe to just
give you one aspect of it.
When you think you do one photon physics,
often you do two-photon physics.
A lot of people think atoms can absorb a photon.
I've never seen in my life an atom
which has absorbed a photon.
The photon is immediately re-emitted.
It's a scattering event.
An atom cannot absorb a photon for good,
because the lifetime of the excited state is short.
So when you think absorption is a single photon event--
there is a limitation where yes, you
are allowed to think about it.
But if you get confused-- and I will confuse you-- then
you need the fact that every absorption process is actually
a two photon process.
Photon in, photon out.
And sometimes by remembering that it's not
single-photon-- there are always two photons involved--
it helps you to avoid some pitfalls
of the single-photon picture.
So therefore multiphoton, yes.
It's not just high intensity, two-photon transitions
in atomic hydrogen or such.
It's also about a deeper understanding.
How does a single photon interact with atoms?
And finally, there is something which
has fascinated many physicists-- the question about coherence.
And coherence is as fascinating as it is diverse.
Because coherence has many aspects
and has many implications.
And I also like a lot in this traditional MIT course
that coherence is sort of singled out as a chapter,
and how I tell you about all the different phases of coherence
in this chapter, and not scattered
throughout the whole course.
We have coherence in single atoms.
The simplest one is a coherent superposition
of two-levels, which is so simple that it's almost boring.
But there is an enormous richness
when we put in a third level.
About 20 years ago, an understanding
of three-level physics has really
created a new frontier in the field.
And let me just tell you buzzwords.
Lasing without inversions.
Electromagnetically induced resonance.
Those concepts happen due to coherence between three levels.
And we talk about them towards the end of the course.
Well, we have coherence within an atom, between two
different or three different energy levels,
but we can have also coherence between atoms.
And at that point, the atoms interact not individually,
they act collectively.
And of course, coherence between atoms
can be the coherence of many atoms
in the Bose-Einstein condensate, where
they form one big metawave.

But it can also be the coherence.
The atoms are not coherent because they're
from the Bose-Einstein condensate,
but they interact in a coherent way with light.
So there's only one aspect where the atoms act coherently.
They may be in different quantum states,
but the interaction with the light is absolutely identical.
And when it then comes to optical properties
of the system, the light doesn't care
if the atoms are different.
The light only cares whether the atoms interact
with the light in an absolute identical way,
and then you have certain symmetries of the light, atom,
and direction.
And this coherence between many atoms
in the interaction with light leads to-- I'll
just give you the buzzwords-- is responsible for the process
of phase matching.
When you have a crystalline frequency [? doubled ?]
laser light, you want all the atoms to interact coherently.
And it is also important for the phenomenon of superradiance.
I found the subject of coherence particularly fascinating.
I should say it was the subject of coherence
where, some maybe 10 years ago, I
was in a long-lasting controversy
with some colleagues in my field.
They're people like Bill Phillips.
When I meet him, he's one of the smartest atomic physicists,
and one of the fastest ones, and ideas just fly back and forth.
And there was only one example where we disagreed
over a long period of time.
Where he had good intuitive arguments,
I had good intuitive arguments, and we couldn't agree.
And this was related to the question when
it came to atom amplification, some coherent process,
whether it is really necessary to a Bose-Einstein condensate,
or whether you can get away with less, which is more
the superradiant way, where the atoms are
different on different states, but they have an identical way
to interact with light.
And in the end I could prove that certain aspects which
all people thought in the field were
due to the coherent nature of atoms.
That due to the fact that these atoms can be regarded
as an atom laser, it was just some form
of superradiance in disguise.
But so anyway, you will notice some of my own interest
in the chapter of coherence when I teach it.
But it's something which is-- yes,
phase matching and superradiance is the physics of the '50s,
but a deeper understanding of it really
developed when we had Bose-Einstein condensates