M0L1a.txt

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So when you take a course in AMO physics,
the obvious question is what is AMO physics.
What defines our field?
Actually, by far, the best definition
of what AMO physics is, it is what AMO physicists do.
It is defined by the community of AMO researchers.
And this is really characteristic for our field.
I will tell you in the next 10 minutes
or so about this enormous dynamics
of the field, how AMO physics has changed
in a fraction of my lifetime.
And what happened is I felt whatever I was doing,
and it turned out to be very different of what I
did 10 and 15 and 20 years ago.
We stayed in the center of AMO physics.
So myself and the whole community,
we are moving and took the field along.
So therefore, AMO physics is what
gets AMO researchers excited.
It's not a joke.
That's really happened.
Well, a little bit more mechanically
we can define AMO physics.
AMO physics is what is made out of the building
blocks we have in AMO physics.
So defined by the building blocks.
And in AMO physics, we build systems
out of atoms or molecules, and then photons or light.
And in general, electromagnetic fields.
Electric magnetic field.
Microwaves and all this.
So in other words, everything which is interesting.
And we can put together with those building blocks.
This is AMO physics now, and this may redefine AMO physics
in the future.
Now, historically this meant that those building
blocks-- atoms and molecules-- were
available in the gas phase.
So you had gases of atoms, gases of molecules.
And you studied them.
And almost all of AMO physics was actually
about individual particles, individual atoms,
individual molecules.
Because in the gas phase at high temperature,
pretty much you learned that in statistical mechanics
each particle is by itself.
And the partition function of the whole system
is just vectorizes into partition functions
of the individual particles.
Well, maybe with some exception, occasionally particle collide
in the field of collisions.
Collisions between atoms and atoms, atoms and ions, atoms
and molecules, molecules and photons.
This was widely studied in AMO physics.
Well, now the field has really moved away
from just individual particles and collisions
between two particles.
What is in the center of attention is few-body physics.
And that, of course, takes us to entanglement.
For instance, people study entanglement of [? eight ?]
ions in ion traps.
And of course, this is deeply related
to [? quantum ?] information science.
Or if you want to go from few-body to many-body,
this is now starting to overlap with condensed metaphysics.
And this is now widely studied in the field of [? ultracold ?]
atoms and quantum gases.
When we talk about many-body physics,
we get, of course, into overlap with condensed metaphysics.
I would actually say now a larger fracton of AMO physics
and condensed metaphysics overlap strongly.
[? That ?] we speak the same language.
We study the same Hamiltonian.
A lot of theorists apply the same methods
to topics coming from either field.
However, often in technology, how the systems are studied,
there's a different culture, different tradition, and still
two different communities.
The overlap comes about because there
are systems in nature, which you can see-- well, natural.
The unnatural two- or few-level systems, of course,
will help nature a little bit by engineering.
Those systems are, for instance, quantum dots, or in-recent, as
in [? diamant. ?]

Or another overlap with condensed metaphysics
is that [? one frontier of ?] AMO physics
is the optical control of mechanical oscillators.
Microcantilevers, membranes, tiny mirrors in cavities.
They have mechanical motion, and the mechanical motion
is strongly coupled to the photon field.
Of course, for fundamental AMO physicists,
a mechanical oscillator is nothing else
than a harmonic oscillator.
So [INAUDIBLE]-- and this is sort of my third attempt
in defining for you what AMO physics is,
AMO physics is almost defining itself by low-energy quantum
physics.
So all the quantum mechanics which doesn't take place
at giga and tera electron volt, which
takes place at low energy, this is AMO physics.
Of course, maybe not all of it.
Usually when it is in wet solution, it's biophysics,
and it's distinctly different from AMO physics.
Or if the solid state involved, then it's
more condensed metaphysics.
But I would say there is one part of solid-state physics
which is already becoming interdisciplinary
with AMO physics.
And this is when the control of the system
is not done by wires and [INAUDIBLE],
but it is done by lasers.
So if you have a solid-state system
and you use all the methods you apply to atoms,
you use electromagnetically induced transparency coherence,
all those concepts, then AMO physicists feel at home.
And they don't care if the two-level system
or the harmonic oscillator is an ion oscillating in an ion trap
or whether it's a small cantilever.
The methods and concepts are the same.

So therefore, one could say AMO physics
is sort of the playground where we
can work on extensions of simple systems
which we understand and cherish.
And of course, our exactly solvable problems
are the hydrogen atom.
My colleague Dan Kleppner would have said maybe the hydrogen
atom-- if you understand the hydrogen atom,
you understand all of atomic physics.
I'm not so sure.
I would actually say in addition to the hydrogen atom,
you have to know the two-level system.
And of course, you have to understand the harmonic
oscillator.
So these are three paradigmatic Hamiltonians.
And a lot of understanding of much more complicated system
really comes from taking the best features of those three
systems and combining them.

If you have questions or comments.
This is an interactive class.
Feel free to speak out or interrupt me.

OK.
So now we know, or we don't know,
what AMO physics is, let me now address,
how has AMO physics developed?
And I mentioned to you that AMO physics
has done breathtaking evolution in my lifetime,
or even in the shorter part of my life,
which is my research career.
Well, traditionally, almost all fields in science
started with observing nature.

The pursuit of science was born out of human curiosity
to understand the world around us.
And atomic physicists, well, they
started to observe atoms and molecules, usually in the gas
phase, and what they were doing.
And already there was some evolution,
because original observations at low resolution
were taken to a completely new level when
high-resolution methods were developed, when lasers came
along, when people had light sources which
had fantastic resolution.
And eventually, finer and finer details
of the structure of atoms and interactions between atoms
were resolved.

But AMO physics is a field which has taken the pursuit
of science much further.
So there is not just observation of nature.
And I want to write that with capital letters--
there is CONTROL of nature.

And you maybe take it for granted,
but you should really appreciate it,
that controlling nature, having control over what you study,
modify it, advance it, take it to the next level,
is really something wonderful.
It is completely absent in certain fields
like astrophysics.
In astrophysics, all you can do is you can observe.
In atomic physics, we create the objects we can observe.
So the control of nature, the control of our atomic physics
system, developed in stages.
The first kind of control was exerted about internal states.
If you have an atom at thermal energies,
it would only come in hyperfine states which
are thermally populated, or molecules
come in rotational states.
And well, your limited control was simply
to raise and lower the temperature.
But with the advent of optical pumping--
this actually happened already with classical light sources
before the invention of the laser.
So with optical pumping, you can pump the internal population
of molecules into, let's say, a single rotational state.
So this is control over the internal Hilbert space,
and this was actually rewarded with the Nobel Prize
to Alfred Kastler in 1966.
Of course, the next step after controlling
the internal degrees of freedom is
have control over the external degrees of freedom.
And this means control motion.
This was, of course, pursued by understanding
the mechanical aspect of light.
How do photons mechanically interact with atoms?
This eventually led to laser cooling
and Bose-Einstein condensation.
And it was also those developments
were recognized with major prizes.
Well, there is more to it than controlling
internal and external degrees of freedom.
You can then also say, well, how much can we control
the number of building blocks?
And eventually, AMO physics advanced
to exert control on to single quantum
systems-- single photons, single atoms, a single atom
in a cavity exchanging a photon with a cavity thousands
of times.
So this control of single quantum systems
was actually just recognized with a Nobel Prize
a few months ago.
Well, at this point, I sometimes make the joke.
We have gone from big ensembles in many, many quantum states
to single photons, single atoms in a single quantum state.
A single quantum state for many particles
is Bose-Einstein condensation.
So we have really come down to single atoms, single photons,
single quantum states.
Well, what comes next?
To have no atoms and no light in vacuum?
Well, the vacuum has some very interesting properties.
And if you talk to Frank Wilczek,
the nature of the vacuum-- dark energy
is one of the big mysteries in physics and in science
in general.
But the study of that is definitely
outside the scope of AMO physics.
So what happens is when we have come down and have now
control over the building blocks,
now we can sort of go up again in a controlled way,
create complexity by assembling a few photons, a few atoms,
into new, entangled states.
So we can now take our system into very different regions
of Hilbert space.
So what is defining now?
The control is we want to use this pristine control
over the building blocks to now put in something which hasn't
existed naturally before.
Or when it existed, it was completely obscured, completely
hidden, by thermal motion or by, you can say,
in homogeneous broadening our lack of control.
And the best [? passwords ?] are here now--
entanglement and many-body physics.
It's hard to capture that in a diagram, but let me try that.
I don't know actually what I'm drawing,
but I think you get the message that this
is sort of Hilbert space.
And I have two axes.
One is sort of entropy-- high temperature,
and the other one is complexity.
And for quite a while, people studied hot gases.
So these are gases.
There's a lot of entropy in it.
The complexity is actually not particularly high.
And everything is described by a statistical operator--
the density matrix.
The pursuit of cooling and actually
control-- gaining information about a system
is also a way of cooling.
If you know in which state the atom is,
the entropy of the system is 0, even if you haven't
changed the state of the atom.
So control and cooling, control measurement and cooling,
has now taken us to the point where we have systems
which have no entropy anymore.
They are very, very well-defined.
And our goal now is to take now these systems
to much more complexity, where wave functions become
entangled, and we have strong correlations
in many-body systems.
This is now described by wave functions.
But here is the wave function of a single particle.
And here we have now highly correlated, highly entangled
many-body wave functions.
So at least for me-- but all predictions
are notoriously incorrect when you look at them
in a few years from now.
But for me, this is sort of where the future of our field
is moving, to get sort of into interesting regions of Hilbert
space where no person has been before.
As an experimentalist and with a lot of experimental graduate
students in this room, I want to emphasize
that a lot of those rapid developments of the field
are driven by technology.
So it's driven by technology advances.
In the '50s and early '60s, people
thought AMO physics is pretty much dead.
It's sort of only a few people with gray hair
continue what they have done, and the field
will eventually die.
But then technological developments
made it possible to do major [? conceptional ?] advances.
I've mentioned the [? conceptional ?] advances.
Let me now say a few words about the technology which
has driven it.
There was one phase of developing lasers
which I experienced when I was a student.
But those lasers were fantastic.
They were very narrow already, very stable,
but they were very expensive and very complicated.
So if you had one laser, this defined your laboratory.
And then you studied a lot of things
with the single laser you could afford.
Well, you're probably not used to sort of one big [? dye ?]
laser pumped with a big argon-ion laser.
It was a $250,000 investment for the lab.
And of course, you couldn't afford a second laser.
It required 50 kilowatt of electrical power,
and all this power had to be cooled away by gushing water
through thick pipes.
So it was an expensive undertaking,
but you could really do wonderful science with it.
So what I've seen in the last 20 years
is the proliferation of solid-state lasers,
starting with diode lasers and continuing
until the present year.
So now in a lot of our laboratories here
at MIT, we have 10 lasers.
And we've stopped counting them, because adding a laser
to the system is almost like adding
an amplifier to a circuit or adding another circuit
to a data acquisition system.
But it's not just the simplicity of the lasers
which we have now.
The robustness to have 10 lasers in the lab, it's fine.
Previously, if you had three lasers of the lab,
you spent 90% of your time just keeping the lasers running.
Those lasers, not so much continuously
as [? our ?] continuous wave lasers,
but pulse lasers have also very, very different properties.
Laser pulses brought very, very short femtosecond
or even attosecond.
Shorter pulses means that the energy is now
focused to a much shorter temporal window.
Therefore, laser pulses of very, very high intensity--
if you focus a short-pulse laser on an atomic system,
you can easily reach an electric field
of the laser which is stronger than the electric field
of the atom.
So in other words, if you have photons and electrons, or well,
maybe the outer electrons, not get down to the single protons,
but you have maybe an ionic core, and you have electrons.
Now you should first look at the motion of the electrons
in the strong field of the laser and at the atomic structure
as a perturbation.
It really takes the hierarchy of effects upside down.
So the appearance of higher-intensity lasers
has given rise to a whole new field of atomic physics.
Lasers got more precise.
The invention of the frequency combs,
recognized for the Nobel Prize in 2005,
meant now that we can control laser frequencies at a level
of 10 to the minus 17.
And this has completely redefined precision metrology
and has advanced the control over atoms and molecules
I've mentioned before.
Finally, another technical development,
which plays a major role in research being pursued here
at MIT and elsewhere, are the development
of high finesse cavities.
High finesse cavities in the microwave range,
then they are superconducting, or
high finesse optical cavities by having supermirrors.
It is actually those super-cavities
which have enabled the study of single photon physics,
because after all, photons move away with the speed of light.
And if you want to observe a photon in your laboratory,
it has to bounce around zillions of times
in order to have enough time for the photon
to do something interesting.
So sometimes a field at the frontier of science
is defined by paradigms by if you
want to explain to somebody why your field of interest
is cool and exciting, you usually
do it by picking a few really exciting examples.
And I sort of want to show you how over the years
that has advanced.
Definitely in the '50s and '60s, you
would have mentioned that we understand
now atomic structure, atomic structure
of multi-electron atoms.
Optical pumping just started.
So these were flagship developments of AMO science.
The cool thing to do in the '60s and '80s was use the new tool,
the laser, applied to atoms and do laser spectroscopy--
[? subtoplaspictroscopy, ?] [? subnatural ?] spectroscopy,
resolving hyperfine structure.
Wow, I mean, this was really exciting in those days.
And well, the older people I have met,
my teachers, my thesis adviser, these
were people who started their research career
before the laser was invented, but then
as a young researcher embraced this new tool
and helped to redefine the field.
Definitely in the '80s and '90s, the cool pictures
were those of magneto-optical trap atoms standing still
and traveling around.
So the new aspect were mechanical forces
of light, which led to laser cooling and trapped atoms.
In the late '90s, of course, the excitement
was about Bose-Einstein condensation.
And it was really Bose-Einstein condensation
which drove AMO physics from single atoms
and maybe two atoms colliding to many-body physics.
It's always easier to analyze those things
by looking backward.
So if I'm now getting closer to the past,
I have to be a little bit vague.
But in the 2000s, I think hot topics
were ultracold fermions and the study of entanglement
and correlations.
And what is the paradigm now or in the near future?
Well, I think you have to help to define it.
If you make an interesting discovery,
this is what people will be pointing to and would say,
this is what now defines AMO physics.
Some candidates are, of course, if there is major breakthrough
in quantum computation.
In the field of, quote, "atom science,"
we may actually do some focus towards topological states,
which have different symmetries and different properties.
And another emerging frontier is micromechanical oscillators.
In the last couple of years, we just
had the breakthrough for the first time--
mechanical objects were cooled to the absolute ground state.
So this, for that community, it was what, for many of us,
Bose-Einstein condensation was 15 years ago.

Any questions?