8.421-atomic-physics-topic-intro.txt

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Welcome to a new teaching, a new lecturing of 8421.


AMO science is booming and is rapidly advancing,
and a lot of it is really due to, well, of course,
new insight, new ideas, new breakthrough but also combined
with technology.
We have seen over the last couple of decades
a major development in light sources.
If I remember what lasers I have used in my PhD
and what lasers you are using, wow, there's
a big difference-- big difference in performance
but also big difference in reliability and convenience.
But just a few systems which didn't exist a few decades ago
are Ti-sapphire laser, which has really
become the workhorse of generating lots
and lots of power in the infrared domain,
but then it can also be frequency-doubled
to the visible.
When I was a post-doc in the early '90s,
people just started to use diode lasers in atomic physics.
Well, here you are 20 years later.
We see a much more solid-state lasers,
and I would say even in the last five to 10 years
there has been another, well, revolution
is too strong a word, but another major advance by having
extremely high power fiber lasers, which are covering more
and more of the spectral range.
So we have seen major advances in shaping short pulses.
I remember when I was a student how femtosecond lasers were
the latest.
Well, they required a big laboratory,
and femtosecond pulses could only
be produced in a few laboratories in the world.
With the discovery of the Ti-sapphire laser and Kerr-lens
modelocking this has now become standard
and is even commercially available.
But researchers have pushed on, and attosecond pulses are now
the frontier of the field.
Well, if you have very short pulses,
that also opens up the possibility
to go to really high intensity.
You don't need so much energy per pulse.
If the pulse is very short, you reach a very high intensity,
which is in the range of terawatt,
and it is now pretty standard.
If you focus the short-pulse laser,
in the focus of the short-pulse laser
you create electric field strings,
which are stronger than the electric field in an atom.
So therefore the dominant electric field
is the one of the laser, and then you
may add perturbatively or in whatever scheme
the field between the electrons or the electron and the proton.
So this is the generation of light,
but light also wants to be controlled,
and this is done by using cavities.
A single photon would just fly by,
but if you want to a photon to really intimately interact
with an atom, maybe getting absorbed, emitted,
absorbed, emitted, if you really
want to have the photon as a quantum state
and not just as something which flies by, you need cavities,
and not just as something which flies by, you need cavities, And we have really seen peak advances
And we have really seen peak advances
in superconducting cavities, super coatings
in the optical regime, and cavity
QED in the optical and the microwave domain
have led to major advances, and a series
of spectacular experiments have formed now with single photons.
So the single photon is no longer
an idealized concept for the description of light atom
interaction.
It has been a reality, and single photon control
has as advanced greatly.
Well, you can make major advanced in terms of light,
find new lasers, shorter pulses, higher intensity pulses
and things like this, but the other part
of atomic physics, one is light, the other one are the atoms.
We haven't invented new atoms yet.
We still got stuck with the same periodic table,
but we have modified the way how we can prepare and control
atomic samples.
A big revolution in the '90s or '80s
has been the cooling of atoms that now
microkelvin and nanokelvin and with evaporative cooling,
even picokelvin regime has become possible.
In terms of atomic samples, this was
a revolution which took place during my time as a researcher.
Atoms always meant you had a sample of individual atoms.
Sometimes you started in the action
when two atoms were colliding, but atomic physics was really
the physics of single particles or two particles
interacting, colliding, or forming a molecule.
But the moment we reach, for cooling, nanokelvin
temperature, atoms move so slowly that they feel out
each other, and that means, suddenly, we
have a system to do many-body physics.
So the advent of quantum degenerate gases
and many developments after that with optical lattices and lots
of bells and whistles really meant
that-- and this is dramatic-- that atomic physics has
made the transition from single- and two-particle physics
to many-body physics.
And for several research groups in the Center
for Ultralcold Atoms this is, of course, an important frontier.
Well, somewhat related to that but more generally,
the precision in preparation and manipulation
which atomic physics has reached with quantum systems
puts now atomic physics in the leading position
at the forefront of exploring new aspects of Hilbert space.
One can see that Hilbert space is vast,
but what is realized with simple quantum
system is only a tiny little corner of Hilbert space.
And atomic physics, if I want to define it
in the most abstract way, the goal
is to master Hilbert space, and that
means we want to harness parts of Hilbert space
which are characterized by quantum entanglement--
maybe in simple form, between two particles
but also between many particles.
And, of course, this has led to whole new frontier
in quantum computation and quantum information processing.
So this sort of should show you that how technology, new ideas,
control, manipulation is suddenly
opening up whole new scientific directions.
And just to add something more recent to the list,
we have now a major research direction
in AMO physics dealing with cold molecules,
and they're even prospects of rewriting chapters
and they're even prospects of rewriting chapters What happens when you do chemistry
What happens when you do chemistry
but not in the ordinary way but at nanokelvin temperature,
or what happens when you do chemistry
where you have coherent control in such a way
that maybe the molecules before and after the reaction
are in the coherent superposition state?
So, in that sense, the conclusion of the introduction
is atomic physics has been successful because it continues
to redefine itself.
<i><b><u><font color=#00000000></font></u></b></i>
And to prove the case I can say when
I tried to predict-- I didn't even try because I knew
it wouldn't work-- but if I tried to predict
10 years ago what would be the hot topics of today,
I would have failed.
What happens is just breakthroughs and discoveries,
and usually they happen in areas where they are not predicted.
As another angle, atomic physics has
seen more than its usual share of Nobel prizes
in the last two decades.
Maybe the prize in 1989 for ion trapping
and Ramsey spectroscopy.
Ramsey spectroscopy is used for all the generation
of atomic clock.
Ion trapping is the basic building block.
This was sort of given for some of the technology,
but this was the only prize in the long list I'm writing down
now which was given for something which was maybe
invented a few decades ago.
A lot of Nobel prizes are given decades after the discovery.
But all of the more recent Nobel prizes, and this speaks
for the vitality of the field, were
awarded for developments which had just happened in the decade
before the prize, whether it was laser cooling, just
invented in the '80s, whether it was Bose-Einstein condensation,
observed first six years before the 2005 prize
on precision spectroscopy with lasers and frequency comb.
This was also a development that happened just a few years ago.
And the most recent recognition for Serge Haroche and Dave
Wineland is about the manipulation
of individual quantum system, and the highlights of this
were accomplished just, let's say,
over the last five or 10 years.
<i><b><u><font color=#00000000></font></u></b></i>
Just sort of to make a general case here,
I continue to be amazed how interesting and rich
the physics of simple systems are.
I actually expect that maybe even two Nobel prizes
in the near future for pretty much understanding
the Schrodinger equation.
You would say this has been done in the old days of quantum
mechanics in the '20s and '30s, and, of course, lots of people
have been recognized.
But there are two aspects of the Schrodinger equation
which hadn't been understood or which have been understood only
recently.
One is the aspect of entanglement and error
correction.
Nobody, until 10 or 20 years, nobody until Shor]
and collaborators introduced error correction would
have thought that a quantum system can de-cohere,
but you can reestablish coherence
by what is called quantum error correction.
These are properties of the simple Schrodinger
equation for just a few, well, qubits
for a few particles, which were not known,
or even the expert in field would have flatly said,
no, this is not possible.
And another aspect of actually single particle quantum physics
which has been fully appreciated only recently
is the question of Berry phase and topological phase.
All the rave in condensed matter physics,
which is also spilling over to atomic physics,
of quantum hall effect, topological insulators and all
of it means that there are nontrivial
phases, nontrivial symmetries in the single particle Schrodinger
equation.
So just that as a case in point that the single particle
Schrodinger equation, lot of people
thought in the '40s and '50s, that's it;
there's nothing else to do research.
And now we see whole new fields emerging, exploiting
new aspects of the Schrodinger equation.
Will there be something else of the same caliber to be discovered
<i><b><u><font color=#00000000></font></u></b></i>
20 years ago, people would have said, no,
and, I just gave you two examples of major new insight which
has really changed our understanding of quantum
physics.
A few years ago I served on a National Academy
of Science committee trying to do the impossible,
to predict the future of the field.
But sometimes the National Academy of Sciences
is asked to give advice and try to provide the best wisdom
possible what is exciting.
Of course we didn't predict the future, but at least,
to the extent possible, we summarized
what are the frontier areas where we see rapid development
and where it would be worth investing further.
And you will actually see that a number of those frontier
areas are where your research happens.
One is the traditional area of precision measurements.
As long as atomic physics exists,
one of the specialties of atomic physics
is we can do precise measurements, atomic clocks,
and precision measurements of fundamental constants, and all
that, and that continues until the present day.
It was just two weeks ago that there
was a new Nature paper on the really
major advance in atomic clocks.
Strontium neutral atom clock has reached the precision
of 6 times 10 to the minus 18.
It's amazing.
It's amazing. You really have to carefully understand and measure
small changes in the black-body radiation because just
the black-body radiation creates frequency shifts, which
would interfere with that precision--
an amazing accomplishment for the field.
So precision measurements continue
to be an important frontier.
Of course, there's always the aspect
of metrology, determine time, frequency, and other things
with higher and higher accuracy, but there are also
applications.
Just one example is magnetometry.
Atomic physics methods can be now used.
If you probe an atom in an environment ,
you can measure the magnetic field.
So people are now talking about using atoms or artificial
atoms in the form of NV centers to measure
the magnetic field in biological cells and all that.
So measurement has fundamental aspects
but has also applied aspects.
Well, other frontiers are, of course you can use the work
'ultra--' ultracold.
We've talked about high intensity lasers--
ultraintense, ultrashort.
Atomic physics is more and more getting
involved with nanomaterials, materials with new properties,
maybe materials with a negative index of reflection,
metal materials, or, in general, plasmonic materials.
Nanomaterials can help to shape light and explore
new aspects of how light interacts with matter.
And, of course, a major frontier is a frontier
of quantum information.
<i><b><u><font color=#00000000></font></u></b></i>