So, thanks to the organizers for putting together this conference, this workshop.
The program looks extremely exciting and I'm happy to be here.
The work I'll present is mainly been done at the University of Bonn.
I've moved in the meantime to the University of Kaiserslautern.
Usually, so that's not the physics building in Bonn, usually people show the building
of the institution.
In Bonn, it's just too embarrassing to show, so I show the main administration that's
the castle in Bonn and when Bonn was the capital, the big demonstrations were here
on the meadow and front.
Just before leaving Bonn, I learned that every professor has the right to feed his cow on
this grass.
I'm not sure about any of these rules in Kaiserslautern, but I don't have a cow.
What I'm going to tell you today, and you've seen already some introductions, is the combination
of two different worlds in neutral atom quantum physics.
So we have two very different approaches looking either at large systems, for example, both
Einstein condensates or other degenerate gases, in optical lattice and they've proven very
powerful in, for example, simulating effects from solid state physics or other quantum
effects because coherent many-body interactions, they're there and they're for granted at the
moment as soon as you enter the degenerate regime.
However, I still say that the high-resolution single-particle manipulation is challenging
in these systems.
On the other hand, there is the world of single-particle physics and I will devote my first part as
you'll see to introduce a little bit to the tools we have developed to control individual
particles and here is a picture of eight cesium atoms sitting in their pockets of a standing
wave and this system has been engineered for every atom to sit exactly at the lattice site
where it should be and in these systems we can, as I'll show you, not only control, prepare
and detect them at certain lattice sites, we also have access to the quantum degree
of freedom.
However, the challenge in these systems is the interaction between two atoms.
At the moment, these atoms are still too hot so that coherent interactions are not visible
and I'll show you that the approach I am now pursuing aims on combining these systems so
that both approaches will profit from that.
So the first part is devoted to the tools and achievements we have made in bond in controlling
single atoms and then I'll show you how both worlds can profit from combining these approaches
and immersing single-neutral impurities into an ultracode gas.
So in the first part is the question how well can we prepare and detect these individual
atoms and so I had to cut somehow 15 minutes and now you'll get it through the tools we
have developed in bond and the first is we have also developed a method to detect single
atoms with sub-defraction resolution in optical lattice and for example if we take hundreds
of images of up to 30, 40, 100 atoms and look at the mutual distance and make an histogram
of how often we see a certain distance then we see with the methods we have developed
in a one-dimensional lattice we reproduce here in this histogram very well the optical
lattice even down to nearest neighbors and the diffraction here in this case is at the
point of four to five lattice sites so we go well beyond the diffraction and what you
see here at zero distance at the same lattice site that is what Stefan told us that's just
these light induced collisions as soon as you have two atoms at the same lattice site
you illuminate them with near resonant light they'll immediately get lost in our tight
traps.
So that's the first we see we can detect the atoms with high resolution but we can also
prepare them at certain lattice sites and that's just a demonstration on how we can
inscribe frequency information of for example microwave pulse into the position of atoms
if we fill our optical lattice with lots of atoms we'll get such a fluorescence image
here for example if we now apply a magnetic gradient field along this lattice and apply
a rectangular microwave pulse then we'll promote atoms to different spin states and the sync
function the sync spectrum of our pulse is inscribed now in the position distribution
of the atoms and if we now narrow for example this pulse width or the spectral width then
we can really address single atoms and for example this row of eight atoms has been just
written by this way applying eight microwave pulses at the respective positions we can
however also control quantum degrees of freedom and this is an example we have done where
we entangled the internal degree of freedom so spin state of the atom with its position
prepared coherence propositions of an atom being to the left and to the right and we
were able to realize the so-called quantum walk which shows a ballistic transport rather
than a diffusive as in the classical random walk and we were able to follow the evolution
from a quantum behavior and saw the quantum to classical transition here to a random walk
and as I said if you have any questions to any of these just ask in the end and I'll
be happy to tell you more about it we were also able to explore or to explore the emotional
states in the optical lattice in an optical lattice and to drive coherent transitions on
the carrier and sidebands between two spin states and the big thing that that we were able to do is
that we were able to engineer the coupling strength of these transitions and in ion traps usually the
carrier transition is the strongest and the sideband transitions are reduced by the lambda factor
and we're able in our system to engineer the lambda factor which is shown here and for example
we can make the carrier transition which is shown in green to have the same strength as
red or blue sidebands or to completely suppress the carrier and to use now the emotional state
the emotional vibrational states also as a qubit resource for any quantum physics experiment we
want to do so this is just to show you we have single particles under control we have high degree
of control over the external and internal degrees of freedom and we're able to coherently manipulate
them I want to show you one example some recent results on a single trapped atom interferometer
that we have done just to show you that this can be really something useful to explore some
quantum effects for example close to surfaces and the idea there is the following we have single
atoms trapped here at the site of an optical lattice which is indicated here at certain with
the dots the internal degrees are here encoded in colors as red and blue and we're able to prepare
the atom in a coherent position of these two internal states and we have developed a method
to entangle this internal degree of freedom with the position of the atoms and delocalize the atom
for the blue spin state to be on the left and the red spin state to be to the right and that
means now we have a coherent superposition of an atom being to the left and to the right
there they can acquire some phase difference then we can recombine them and read off this
phase difference in some ramsey type interferometer and that's very analog to a machine
interferometer where for example two light beams traveled along different paths and in the end you
read off the phase difference as the interference pattern being bright or dark. The big advantage
of this interferometer is that it's completely discrete so we you can translate any operation
into some block here either beam splitter operation into some microwave pulse the transport into some
shift operation and you can reassemble and reshuffle these operations at will and you
can design the sequence such as to investigate for example decoherence effects to explore some
other effects like have very strong delocalization distances and bring one part of the wave function
for example close to a surface and here you see already how far we can delocalize the system
that's several micrometers that's a real macroscopic distance for our purpose where for
example we can shine in a laser beam in the middle and just as a first example we have
so that's the way we detect the atoms for single atoms we look if the atom is in a certain spin
state and we get the answer either yes or no and just the time average over many realization then
reveals a typical interference fringe and from the phase shift of this fringe we can extract
the phase information we're interested in and as a first application we have looked at the
effect of acceleration which is equivalent for example to gravity so we delocalize the atom
while it is at rest then we accelerate the system and we want to measure now this effect of the
acceleration and to do that we have to combine now the wave packets here while they're drifting at a
constant velocity and here you see the acquired phase difference between the two arms versus the
acceleration if we have delocalized the atom by four letter sides by 12 or by 20 letter sides
and you see that we can measure accelerations of several g with a precision of a few times 10 to
the minus 4 g and that is the first first shot we have now also some ideas to improve this even
further and to really turn this into a position interferometer to for example measure gravity
or kasimir effects on surfaces so you might have noticed everything i told you so far are purely
single particle effects the question is now i told you already why because the temperature of these
atoms still too high to really undergo coherent collisions and that is something we want now to
cure by immerse the single atoms into a quantum gas and the idea is so for for the perspective of
single atoms many body system can act as a as a bath and cool the single atoms and one question is
might there be a possibility to cool the motional degree of freedoms of the single atoms while
preserving the internal state coherence so that is something we want to look at but on the other
side single atoms and the high degree of control we have obtained over single atoms might be
a nice way to locally probe detect or manipulate a many body system and by that both approaches
that i introduced in the beginning might profit from this combination but it turns out it's more
than that if you increase the interaction between those two subsystems then it's rather you have to
rather describe the system in terms of new quasi particles which resemble so-called polarons in
solid states but you can enter regimes which are not accessible in solid state physics
so now i want to explain a little bit and to show you how far we have gotten to now combine
these two systems and to immerse the single atoms so the situation we're in everything takes place
again in a high vacuum system we have here a single atom mod at the position of these crossed
laser beams we have some coils to prepare a degenerate bozo gas and some optics for a
fluorescence detection of the single atom or absorption imaging of the many body system
to close up a little more here is the position of the single atom mod but for technical reasons
we have here a bose-einstein condensate in a so-called quick type trap which is located a few
millimeters away from the position of the single atom mod and i want to first introduce a little
bit the two subsystems the single atoms and the bose-einstein condensate to make clear how we take
our data and what information we can get from these systems so single atom mods so magneto
optical traps are clear and if you let the students play around enough you can see they
come up with nice ideas and obviously the atoms like what we do with them and they're dedicated
to our institute of applied physics at that time and these are images of on the average
thousand atoms at hundred micro kelvin however if we now make the magneto optical trap tighter
and tighter we come into a regime where the loading rate is extremely low and if we look
at the fluorescence versus time then we see discrete steps and these steps we can assign
to single atoms loaded into the single atom mod and if we do a statistics on these fluorescence
then you see we have here a histogram showing background counts so zero atoms how many single
atoms two or three and four atoms we have what is important for the moment is that these different
peaks are well separated so we can easily distinguish a single or two atom in our mod
and the loading statistics here is poissonion that means we cannot
load exactly one atom but we can load probably one atom and we can post select the events the
typical temperatures here are 30 micro kelvin and i'll tell you a little later what i mean by that
poissons and condensate is rather standard in our trap in the quick trap we could create
poissons and condensates of up to 10 to the 5 atoms of rubidium at temperatures around 100 nano kelvin
for our purpose however we chose a little different system so what we do is we pre-cool the atoms here
we shuffle them to the center of the single atom mod and there we use in the following thermal
clouds just above the condensation threshold at a phase space density of 0.2 not because we could
not do a Bose-Einstein condensate but because for the first experiments we want to get rid of any
quantum effects and study the pure classical cooling so to say of single atoms so now we
we realized that it's not easy to combine in three-dimensional space to objects with
which are 30 micrometers large so we had to find a way to really precisely locate the single atom
and the magneto optical trap so the single atom and the rubidium cloud and the way we do it is
shown here we use a cross dipole trap and we scan one of the beams of this cross dipole trap across
the single cesium mod and then we just see how many atoms do we load in this cross dipole trap
what you see here is the probability to load the atoms in the dipole trap and so that they
survive a certain loading time and you see we have a maximum loading efficiency here of of
80 to 90 percent and that is limited by by background collisions
now the situation changes if we add a rubidium cloud in the center of this dipole trap and what
you see is that at the wings we have the same behavior in the center however cesium atoms will
not survive and that is again an effect of these light induced collisions which here help us to
identify clearly where the rubidium cloud is located for our purpose it's important to notice
that here just a few 10 micrometers away from the center we load atoms very efficiently to the dipole
trap but at the same time they're in close vicinity to the rubidium cloud and now the first experiment
we wanted to look at was the elastic interaction between the single impurity and this rubidium
cloud and the cooling of the impurity and now the question is what is temperature if you look at a
single particle and temperature for our purpose is always just a different translation of kinetic
energy so we measure kinetic energy and translate this into a temperature scale and temperature of
a single particle is only defined in terms of a time average so we have to repeat this many times
and just then we get an energy distribution that we can assign a certain temperature
and that is not too different from the Bose-Einstein condensate experiments we have to do an ensemble
average and the first method to now extract the kinetic energy distribution is by adiabatic
lowering the trap depth so at a temperature if we just realize a single experiment the atom will
have a certain initial energy and now we adiabatically lower the trap depth and at some point
at a certain energy of the trap depth the particle can escape and we can now map out
how this behaves this survival probability versus the energy either of the final trap depth or
we can calculate back what the initial energy was and that is shown here that gives us the
cumulative Maxwell-Boltzmann distribution and it shows for example that 60 percent of the atoms
have an energy which is smaller than roughly 50 micro kelvin and I should stress this is
a real energy scale just divided by kB to give a decent number here in terms of micro kelvin
and a fit of this distribution shows us that the atoms have a temperature on the order of 27 micro
kelvin that is one way to look at this we have a different method to determine the kinetic energy
and that is we just look we just switch off the trap have the system evolve for a certain time
and then we look if the atom could escape from our trap volume or not and this we can do for various
release times and then look again at the survival probability versus the release time and by some
numerical simulations we can then also assign this a temperature of 30 micro kelvin and this release
recapture method has been pioneered in Matthias Wiedemuller's group and also in Philippe Cange's
group in Palaiso so that is the way we extract temperature information on the system and now
the question is what happens in our case with a single impurity in a quantum gas so the first
thing is we have to complicate the sequence a little more initially we load the atom in an
optical lattice and adiabatically lower the optical lattice then after a certain time we switch on
again the lattice depth and do a release recapture method and then we see from this method we get
temperature of five micro kelvin and this cooling is just solely due to the adiabatic
expansion of this optical lattice and that is the way we had or we had to do this because
it's not easy to combine an object with a which is 30 micro kelvin hot with a bosein or with an
ultra cold gas at a few hundred nano kelvin with a five micro kelvin that is possible and we can
now look what happens if we put an ultra cold rubidium cloud here at the center so again we
adiabatically lower the optical lattice we have the single atom oscillate through the trap with
the ultra cold cloud we remove now the ultra cold cloud and in this system we again measure the
temperature by the release recapture and what we see is that already at an interaction time of a
few 10 milliseconds the survival probability is prolonged for short release times and then falls
off more steeper or more steep and if we interact for 130 milliseconds this goes further and further
here at temperature of 500 nano kelvin and we can now map out the temperature of the cesium atom
versus the interaction time and what you see is first it goes down and agrees within our
error bars with the temperature of the ultra cold gas and i have to say that is measured by a very
different method so that's measured by time of light velocimetry of the rubidium gas and this
here is measured by the release recapture method and the fact that they coincide tell us we can
use really single atoms as temperature probes for gas or for another object an interesting
question is at this point here the single atoms were equilibrated with the light field of the
magneto optical trap so in thermal equilibrium with something else at this point here also the
single atoms were in equilibrium with the ultra cold gas but it's an open question what the
distribution the energy distribution of the single atoms is at these points and that's
something we could not get the measure that's something we want to look at
another nice feature of the single atoms is that we can track this thermalization curve at every
point so usually in large systems it's only possible to track this if they have been already
equilibrated the two subsystems but we are able to really extract information of the interaction
by looking at the equilibration dynamics assigning the system a constant cross section
which is basically given only by the scattering length and rendering here the density and relative
velocity time dependent we can then calculate this really to see the full thermalization
distribution i'll show you that in a second i just want to point out that's not the first
experiment where single objects have been immersed in an ultra-cold gas there have been
single ions immersed in the Bose-Einstein condensate either in Michel Köll's group or in
Janusseger Denchlag's group there they have single ions at a temperature at a final
temperature of one milli kelvin but to my knowledge that's the first time that a real
object really equilibrates with a ultra-cold gas
so as i want to show you quickly how we can really map out the thermalization of the system
and here is a movie where we of the calculation that we did and what you see here is the density
distributions of the ultra-cold rubidium cloud and the single cesium atom versus the temperature
and here you see the different time dependent quantities in the system and what counts now for
the thermalization is this dashed line which is essentially what i showed you before the integral
over the densities and the relative velocity and it turns out that although the density overlap
increases dramatically in the end as you see here it is a little bit lowered by the gravitational
sag and that's just the fact that the single atom sags a little further in our potential due to
gravity than the ultra-cold atom cloud the rubidium cloud and moreover the relative velocity
is strongly decreased but still we get an overall increasing interaction cross-section which tells
us that we're in the so-called running way regime of cooling so and from this we can really determine
the scattering length and so we could extract a value of the rubidium cesium scattering length of
450 nano Bohr radii and i give here an upper value because we have several issues that well
make us no lower value than we really have and the first that you immediately see is if we release
our single atom from the trap it could oscillate in an orbit that for example doesn't see the rubidium
cloud so the interaction time is effectively or the interaction is effectively lower than we think
and there are other technical issues but this agrees very well with a recent calculation
of the group of Houdy Grimm and Paul Julien which calculated a value of 650 Bohr radii.
Another thing i would like to point your attention to is the fact that during the cooling we hardly
lose any cesium atoms that means we can so here is the survival of cesium atoms during the cooling
if i go back you see until the collaboration we need like 60-70 milliseconds and we start at 80
percent at 60-70 milliseconds we still have 70 percent so we lose on approximately 10 percent
but we can use this cooling really to prepare ultra-cold single atoms
we can use the probes as i told you for temperature for density of a many-body
system and i'll tell you later how we want to increase our control to really employ this
as a standard tool and importantly we really immerse the single atoms in the ultra-cold gas
despite the strong repulsive interactions between rubidium and cesium and that is important because
if you combine large bosons and condensates of rubidium and cesium you have usually face
separation due to the strong repulsive interaction. Okay there is one thing we wanted to look at in the
following and that is the question of coherence of the single impurity as it is cooled but this
could not be done due to some vacuum problem but one thing we could look at is the inelastic
collisions of the single impurity with the ultra-cold gas and usually for an experimentalist
that is a bad effect or a dirt effect to look at losses. In this case it very nicely shows the
power that we have with our system and we can look at every event at every single event with
single atom resolution and at the end i'll tell you right away the results are not surprising
because we have so few cesium atoms the result will be there is no cesium-cesium interaction
but the tools we develop here are very sensitive tools to show the onset of cesium-cesium
correlations and at the end we could extract a very precise number for the three-body loss
coefficient for rubidium and cesium. The idea is the following if you look for longer times
and look at the survival rate of rubidium and cesium then you see again rubidium is completely
unaffected by this interaction with the single cesium atoms that's again no surprise you have
hundred thousand rubidium atoms you have a few cesium atoms getting lost from time to time
so the rubidium cloud is completely unimpressed. For cesium that's different the cesium survival
probability decays exponentially with time and for the internal states that we chose here
f equals one for rubidium and f equals three for cesium these are the absolute ground states for
the respective hyperfine manifolds and so the only loss mechanism that could be active is molecule
formation that means three-body recombination that is known and in large mixture experiments
that's the same however there it's not clear what is the mechanism is it two rubidium atoms forming
a molecule with one cesium atom or vice versa in our case that's easy to look at because we can
for example just put in a single cesium atom so we have post-selected this here and now looked at
the case for a single cesium atom two three and four cesium atoms and we can assign then the
different loss mechanisms the different three-body recombination mechanisms that could be active
and here what we see is that all these curves are described by the same average decay curve
as in the average data set and this already suggests that since that's the same for a single
cesium atom only two rubidium atoms form a molecule with hung with a single cesium atom
as I said no surprise however we can look closer at here the case of two atoms for example and
look at every single loss event and see how many atoms got lost and that is shown here
here is again the fluorescence trace along the sequence that we have so here we know we have two
cesium atoms loaded into our optical or into our system then we let them interact for a certain
time and we see how many atoms do we have left in this case all atoms survive after the interaction
in this case a single atom gets lost and in this case no atoms survive so both atoms get lost
and again we can assign the different loss mechanisms and then we can see how is the
distribution the statistics of these losses versus the total survival probability or equivalently
we could here also draw some interaction time scale and then we plot these data points the
different statistics probabilities of relative occurrence and make a statistics of non-interacting
cesium atoms and we see it perfectly fits which is again no surprise but this method tells us very
nicely as soon as we have two-body correlation so increased two-body interactions then we would
easily see this from a deviation of this curve but from our system is so simple so to say that
we can very easily extract a coefficient for the three-body loss in this case and that's to my
knowledge the first case where such a three-body loss coefficient could be extracted for this
system and i heard rumors that there are some calculations around which lie in the same range
out of magnitude so i want to show you a little bit the limitations of the system and what we want
to improve in the future because i have to admit that the apparatus we realized this in
was never meant or made for the things we did there and the first limitation is very clear if
you look or if you if you remember that the Bose-Einstein condensate delivers an ensemble
average of 100 000 atoms every minute that's fine and people can live with it single atom
experiment experiments rely on time averages and usually in the quantum walk and whatever
experiment i showed you before you get a single result every second and in order to get a time
average you take 100 of these images and so 100 seconds for a data point that's fine if you combine
the systems however the statistics is given by the time average and by the single atom
the cycle time is given by the rubidium atom and now you can also a little bit feel the the pain
of the students in the lab that had to take the data because one of these stabilization curves
is easily a day and so we have to do something about that and the idea is to make a Bose-Einstein
condensate every few seconds and so this that's not not a mystery anymore people have done this
before you can make the mod loading time very fast you make an all optical Bose-Einstein condensate
and we will also employ additional Raman sideband cooling and by this we want to ease a little bit
the life of the PhD students and get more statistics in shorter time another drawback
that you have realized is we have no optical resolution but we have developed the methods
i showed you before to do really single site resolution of few atoms this we want to implement
and finally we have so far no selective control over the two subsystems but what we would like
to or what we dream of is to really be able to move the single impurity within the many-body system
to locate at a certain point or to even dynamically move it through the system
and the idea to do that is so-called species selective potentials and here is a graph that
shows the idea and it shows the dipole potential versus trapped laser wavelength for the species
rubidium and caesium that we use and you see for usual wavelengths that we use YAG wavelengths over
here we are far retituned from caesium or rubidium D lines and that means the laser beam will exert
an attractive potential onto both species however if you for example sit down here between the
rubidium D lines you're retituned with respect to the D2 line but you're blue detuned with respect
to the D1 line so you get a repulsive and an attractive potential and if you hit the right spot
then you'll be here at a zero crossing of the dipole potential and strictly rubidium will see
no potential from this laser beam whereas caesium will see some potential and we want to thereby
create a potential that can move and trap the caesium atom without any effect on the rubidium
system well before we could do that that's how an experiment looks if it got completely
disassembled and packed up into boxes that's something you usually hope not to see because
it means a lot of work but we had to do it because i was moving to kaisa sloughton so we
packed up everything in bond onto a truck moved it to kaisa sloughton last october we had the idea
of building a new apparatus that is shown here and this consists of a two-dimensional magneto
optical trap for fast knot loading here glass cell where we have good optical resolution to detect
single atoms and i have a little movie that shows how far we we got so far so that is the mysterious
building of a two-dimensional magneto optical trap and for the people working in theory vacuum
is always a big issue because it's kind of voodoo and you have to bake out and see if everything
works out in the end but here that's the two-dimensional mod already here through the big
windows who will shine in laser beams to trap lots of atoms and here that's the well the pig
that is baked out here at the moment and that's the main experimental chamber that is now that
is now attached to the 2d mod and the vacuum system essentially is ready oops so and i see
i cut a lot but as i said all of what i might have cut i have in slides and back up i hope i
have convinced you that we have extremely good control over single particles now optical lattice
in terms of position in terms of preparation and in terms of quantum degrees of freedom
we're able to overlap single atom traps with a ultra-cold gas close to the condensation
threshold and it's absolutely no problem for us to go further to bring the rubidium system
to condensation we've observed the thermalization of a single impurity into a in a ultra-cold gas
and we see that the temperature is coincide within our error bars and we've looked at the
inelastic losses so the three-body recombination event by event and atom by atom and thereby we've
established a method to look very closely at the microscopically at the events that are going on
there so the last thing i would like to do is to introduce the team to you so let me point out here
that's nikolas spreadman he's the guy who basically made all the experiments work and he's the one
that suffered most from our vacuum leaks just a few weeks before the move so that very very
potentially nice experiments could not been done and then the rest of the team is the new team in
kaiserslautern and they do their best to make our dream come true and i would like to thank
you for your intention and i'm happy to answer all your questions