 Okay, hello, thanks for inviting me here. So I'm the first experimentalist here. So you have to be a little bit brave. So there will be a lot of figures and not so much equations. And experimental physics is a little bit different. So we need actually big teams to do something. So me alone would do nothing. So we have from, it's on, so better. Okay, good. So I am from Stuttgart. I'm working in the group of Tillman-Fau. There's a group leader and we have two experiments I want to talk about. It's a ultra cold experiment one and ultra cold experiment two. And there's a large list of people we have here and there's also a large list of collaborations, mostly theory person. And I will not go into detail which person has done what. I just want to give you an overview. So the talk will be about root back attempts and you see here a simple picture of a root back attempt. It looks a little bit like the logo of the ICTP. But I asked already one guy, if this is an atom, is there anyone from ICTP who knows what this logo actually is? Well, this is my view of a root back attempt, a very simplified picture. And in the talk I will first talk about the properties of root back attempts and how they interact. And I will mostly refer to work of other people. Better now. Thanks. So, and then I will talk a little bit about root back molecules in the second part. So here are my favorite properties of root back attempts. So what we have here is for root back attempt is a highly excited atom in a high quantum number. So I have chosen here the 100 S state. And the size of this atom is already one micron. And to give you an idea how big one micron is. So if you have a 100 S state that we can produce in our lab, then this size really is comparable to something like bacteria and stuff like this. So this is a macroscopic object, but still it's in a pure quantum state. So these are really big objects. And due to this electron is far away from the nucleus, it's very weakly bounded by this. It has a hard time to find back to the ground state which leads to a long lifetime, which can be on the order of milliseconds. So this is a really long time for the experiments. Typically we do this on a microsecond scale. So this is a thousand times longer. So this is really basically staying there forever. And since this electron is so far away, it's very sensitive to what is happening in the environment. So if you apply a small electric field, this loosely bound electron will feel this immediately, which leads to a polarizability, a large polarizability which scales actually like with the seventh power of the main quantum number N. And for example, if you apply one volt per centimeter, it's a very small field, you already get a shift of six gigahertz. And if you compare this with the lifetime, so the lifetime is on the order of a millisecond, this corresponds to a line width of a kilohertz. So if you apply one volt per centimeter, you shift the line already a million times of the line width. So it's very easy to manipulate this root back states with electric fields. And why actually many people started the whole thing with root back atoms is the Thunderweiss interaction, which has this crazy skating with the 11th power. So this is extreme. And I will tell you what happens than if you have this extremely strong Thunderweiss interactions. So we had this slide already yesterday, where you can see that the root back atoms, so here's the 100 S state, that this interaction strength is a function of the radius. I mean, here you start with Thunderweiss and then you go to dipodipole. And of course, if you come up with a quantum number, at some point, of course, you will basically approach the Collore interaction. And so we are close to the Collore already, but not exactly. And the interesting thing about this, I mean, of course, they're very, very strong. At this, they're long range, long range I will define in a second. They're tunable, we just choose different states. We can have Thunderweiss, dipodipole with different strengths, switchable, because we can tune the interaction strength either by the electric field, so we can just go to a root back state and come back down. And they're anisotropic. I mean, if you have a dipodipole interaction, of course they're anisotropic, but you also can have like dipole, quadrupole, or whatever interactions. So we really can shape the azimutal interaction strength. And what do people want to use it for? And mostly, of course, I mean, it's for quantum computing and quantum simulation. I mean, there are really groups who seriously want to build a quantum computer out of this root back attempts, and they're doing quite well. And another thing is like that, people want to use these root back attempts and combine it with quantum degenerate gases to get some of the interaction into a quantum gas to have more control options. And a big field, which has started now, is like that you can use this root back attempts by the interaction as an effective optical non-linearity, which works down to the single photon level, and there have been single photon transistors and stuff like this already demonstrated. Since this is an experimental talk, I have to tell you how we do experiments. So typically, we start out with atom, for example, the rubidium, and since the root back state is energetically a little bit too far away, most people use a two photon excitation. So they split the wavelength from a UV photon into a two, and by this we can go from a S state with an intermediate P state back to a S state, and the S state is the most simple one, and then everybody basically starts with the S state because there you can understand everything. What you need in this vacuum chamber is most people use an ultra cold sample of atoms like thermal clouds, but also both Einstein condensates or people going towards degenerate Fermi gases, and then you have these laser pulses to excite them, additional field plates because we need these field plates to control these repair atoms, but we also use them to apply a field pulse to rip out the electron and shoot the rip back atom then in the ion detection decor or the electron in the electron detection, and then after excitation pulse, we field ionize and then we get a signal, and if there's been a rip back, then we know that we were successful. So let's come to what I wanted to say is, experiments are now with ultra cold, rip back atoms have been done with rubidium, cesium, and strontium, so I would say worldwide we have about 25 groups or something like this working on cold rip back atoms, but in principle, whatever laser cooled atom or whatever atom you can have in the BEC, you also can excite the rip back atom, and so people started to think about exciting aerobium and dysprosium and stuff like this to rip back state, so it's becoming very popular. So here I have the thing which basically intrigued everybody to go into this field to do rip back physics, and what you see here is like you have two ground state atoms at a distance r, and if you bring two ground state atoms together, then there's basically no interaction. There's a van der Waals interaction which has something like 4,700 atomic units strength, which leads basically to the interaction to the mean field in the Bose-Einstein condensate. On the scale we talk here, we don't see this at all. If you excite one of the atoms to a rip back atom and bring these two guys together, the interaction is still extremely weak. But if you excite both atoms to a rip back state, then the van der Waals interaction is now on the order of 10 to the 19, yes? So this is like 16 orders of magnitude larger than for the ground state atoms. This is really visible. What happens now, if you bring two rip back atoms close together, they're getting shifted upwards. The van der Waals potential is in this case repulsive and you shift upwards, and if you do this far enough, you cannot excite them anymore. So up to a certain distance, so if the atoms in the distance here, you have only a very small shift and you're still within the line width of the laser and you can excite the two rip backs. But if you come to a critical distance which we call the blockade radius, from this point on, the laser has not enough intensity to excite the two rip backs and you will only excite one of those. And this length scale is for typical parameter experiment. If you take a 43s state or something like this and normal laser power is on the order of five microns. And this is actually a really long length scale because the distance in the Bose-Einstein condensate, for example, between the atoms is on the order of, let's say, below 100 nanometers. And then you have an effective interaction length of five microns. So this is much, much, much longer than next neighbor, next, next, next, next neighbor. So the picture is rather something like this that you have one atom in a sphere which you can excite the rip back and this is a red one and the blue one will not be excited due to this blockade effect. So you can have like hundreds or thousands of atom within one blockade volume which will not be excited. The effect is that you cannot say which atom in this blockade sphere will be excited. It can be either the first atom, the second or third one. So what we will have is an excitation from this ground state to excited state with only one rip back atom. You have N of this state. It's basically a W state. And the interesting thing is that the collective Rabi frequency here is enhanced by a square root of N. So this is what we call a super atom. You can see it, then why is it super? First of all, it's super because it oscillates much faster than an ordinary two-level system but it behaves like a simple two-level system because you only can excite to one excited state. So it's an effective two-level system made of many, many, many atoms. And you can do experiments. Here's an example from the group of Antoine Roves in Paris that have taken exactly one or two atom. If they do excitation of one rip back atom, that's a red line. So they have this situation here and you have a Rabi coupling to this one rip back atom and you just get the Rabi flopping and you get a certain Rabi frequency. If you take two atoms, what you see is the excitation probability is not going up to two. It stays roughly up to one but it oscillates with a square root of two faster. So you really can see that this interaction is taking place in the experiments. And if you use this, now this situation and a little bit more smart setup, you can build out of this a CNOT gate and this is a building block for a quantum computer. You can do this also with more atoms and there's a group of Immanuel Bloch in Munich. What they have is they have this quantum gas microscope so they can prepare square lattices, 2D lattices of atoms and they have done the experiment now up to 185 atoms up here. So what you see is here, if you have eight atoms then you get slow oscillations. If you take 131 though, this is a block of 131 atoms then you see the accordingly faster oscillations and of course the amplitude that's important stays again one. The damping here actually is not due to some funny interaction effects. It's basically they have to do the experiment such. They prepare the atomic sample, they have to do the Rüttberg excitation for a certain time and then they have to read out the system and they have to check if the atom is excited or not excited then it's over, they have to start over again. So they have to do this over and over again and of course you have some fluctuations in this placate of how many atoms you have in there and these fluctuations lead to different Rabi frequencies and this leads to this defacing. So this Rabi oscillations are a clear manifestation of the van der Waals interaction between these Rüttberg atoms which can take place. I mean there has been a recent experiment by Barry Dunning in Rice University where he went up to n equals 300 and the effective range where there's two atoms see each other is 100 microns or 200 microns which is an unbelievable distance for atomic physics not for cosmology but for our world this is I mean a Bose-Einstein condensate is typically 100 micrometer long so one atom from the one side sees the atom on the other side so you would go to n equals 300. Now I mean you can have a van der Waals interaction but of course you also can go to a resonant regime where you get dipole dipole interaction and what I have shown you here is like here's a situation where I have two S states and if you compute the van der Waals interaction then of course you have to think about the coupling to the neighboring states. Here the S state can go up to a P prime state or a P state down here and you see the distance between these two states is not equal. So if you put this in pair states so you put two atoms there so you have the SS state then there's a P, P prime state this is this state plus this state which is effectively we say detuned there's an energy defect in between these guys. So if you write down the coupling Hamiltonian then you have like the dipole the dipole potential from going up and from going down though there's D1, D2 divided by R cubed as Igor has already shown and there's this defect here so if you diagonalize this interaction Hamiltonian then you see you have on the one hand here the detuning and here the dipole-dipole interaction. If the detuning is very large then you're in the van der Waals regime you can expand this square root and then you get something which is like this thing squared so you get the one over R to the sixth dependency. If this thing is zero then you get the dipole-dipole interaction because the square root of this squared things just gives the D1, D2 divided by R to the cube. So you can tune if you have a handle and you have a handle you can apply electric fields I told you that these ripback states are very sensitive electric fields so if you apply electric field you can tune the detuning delta or this energy defect from a van der Waals regime to a regime where delta is equal zero to a dipole-dipole regime and you can do this for experimental cases on an infinite fast time scale so it's really nice that you can tune from the van regime to you can switch on and off the dipole-dipole interaction if you wish. And this has been done again in the group of Antoine Breuys in Paris where they have used a system now there's two D states and the D states one goes up to a P state the other one goes down to a F state and they can tune the distance between the atoms the experiment is done such they have here two microscopic traps so these are two focused laser beams and then each of these laser beams sits exactly one atom and you can manipulate this laser foci you can move them in space and what they have done they have measured the interaction strength the dipole-dipole interaction strength as a function of distance and you see here the coupling strength the dipole-dipole energy up to a distance of almost 15 microns they still have something on the order of one megahertz coupling strength so one megahertz is still very sufficient to do experiments because we have lifetimes of many hundred microseconds so you can do many, many, many Rabi floppings during this time or coherent operations so this is actually again I mean this is a quite large distance you have two atoms talking coherently to each other over this sort of 15 microns and now of course I mean if you have this two body first resonances you could think also going like having three, four or more order first resonances and they have been some experiment the plot is a little bit more difficult on the left it shows the energy scheme but what you should see here is like there are five atoms two go down three go up and if you do it in a smart enough way all energies compensate that this energy defect becomes zero and you can have a five-body first resonance it's not very visible this is this little peak here but if you read the paper it's convincing but the nice thing is like you can really make two-body, three-body, four-body, five-body I mean this was done in a cloud, in a thermal cloud if you would do this in a nicely arranged pattern as the other group in Paris is doing then this you can isolate this peak from everything because this is just a question of configurations you have in a thermal cloud so you have many, many options to play with these interactions and I think many body interactions might be perhaps interesting for someone here and now I want to come back to Rittback atoms so Rittback atoms look something like this in reality it's not just like this one electron flying on a Pluto-type orbit around the nucleus so we have here the wave functions and now I mean I was talking about on a length scale so this is the length scale of the Rittback atom this scales quadratically with the main quantum number and we have this blockade or interaction radius so let's say this on the order of 100 nanometers and we have something effective range of 10 micrometers so this is a factor 100 in this way where these two Rittback atoms still see each other but if you increase the density I mean if you go to a Bose-Einstein condensate then the inter-particle distance becomes comparable to the size of this Rittback atom and what then happens is that ground state atoms happen to sit within the wave function of the Rittback atom and you might think this will kill the thing immediately but actually it does not actually it does something nice so the situation is something like this you have the ionic core, the electron, now orbiting again and there's this ground state atom in its way what happens of course the electron is bound to the core by the Coulomb interaction but there's a scattering between the electron and the ground state atom and actually this is a very small energy scattering so this actually happens in the S wave scattering regime so you have to do a quantum mechanical treatment of this thing and actually this scattering of the electron and this ground state atom leads to a binding mechanism which binds the green ground state atom to the Rittback atom and this makes a dimer the idea was brought up by Chris Green, Dickerson and Sadegpur already in 2000 where they have made a quantum mechanical description of the thing and actually these pictures became quite famous they're called trilobite molecules because they look a little bit like trilobites and of course we can produce this kind of trilobite molecules but I'm not showing you the trilobite I'm showing you the most simple version of it so let me just carry you through the idea how this Rittback molecules actually come to life so what we have is the interaction potential between the electron and the neutral atom and the neutral atom of course is a quadratic polarizability with the electric field and since the electric field drops quadratically with the distance of the electron you have an effective interaction potential which goes one over R to the four and in this one over R to the four potential you can do just an ordinary scattering theory like by partial wave decomposition and then you get a scattering length out of this which is momentum dependent it's A of K which is a scattering length and then you apply the Fermi pseudo potential to get the interaction between this electron and this ground set atom so it's just the delta function times the scattering length and the scattering length actually has a velocity dependency so we have a background scattering length of A naught which is actually negative in our case and then there's a linear correction with the velocity and higher order terms and the velocity here we are more or less classical we just take a classical energy so this is the energy where the electron starts in the Coulomb potential and then we just basically take the one over R Coulomb potential to get the kinetic energy at a certain point so we just use a classical velocity to get this momentum so this is a little approximation but it works quite nice. What you then do is you take the density the probability density of the electron and you take the scattering length which is negative at zero velocity and then you have this linear correction and it's of course not linear because the motion and one of our potential is not linear and you have to multiply these two things it's basically a mean field approximation so we have this scattering length time the density of the electron wave function which gives us a potential landscape here and this potential landscape it's a simple 1D problem you take this one and you use a 1D solver and then you will see that you can have bound states in this potential landscape there's something like ground state and something which we call the first excited state look like vibrational states so it's a very simple model yes we just say the interaction energy is proportional to the density of the electron you find at a certain position and the scattering length is velocity dependent and if you put this together you can do this very very simple picture. Now we want to see this guys what we do is we take again our two photons to do excitation and if you want to excite a Rittberg atom then we just go right on resonance here and then we see like here two lines because we have two spin components and then you see two atomic lines but if you want to see this Rittberg molecules you have to detune the upper laser you go down and your photo associate this state here this dimer state this state here exactly corresponds to this state here and then you see there are some excited states which corresponds to a state here and there's another state lying up here it's a little bit more complicated in this picture and the funny thing is like if you look at this position here and you go twice the energy you find another peak and this peak is actually a trimer state so if you increase the density according to you can have more and more ground state and sitting in the wave function and you can photo associate dimer, trimer, tetramer and higher order states. So the question is what happens if you increase the principle quantum number so I mean we have done this for let's say the regime of 30 or 40 but of course we have this knob so we use all knobs so one knob is the principle quantum number so what you see here is like this effective potentials for n equals 51 and of course if you increase the quantum number the potential is getting the Rittberg atom is getting bigger and bigger and of course the density of the electron is getting more and more shallow and the potentials are also getting less and less deep which means that the bounding energy for this ground state molecules will become smaller and smaller and if you do this experiment then you see something like this you have here the state, this is the 51 so you see nicely here this dimer state and if you go already to 62 it moves in and it becomes smaller and smaller but the interesting thing is you see the dimer, trimer, tetramer, pentramer you can see all the states but then for some point I mean they move closer and closer together and you cannot see anything anymore you just get a hump on the side and the hump on the side is basically all the unresolved molecular lines and why do we see this hump? Because we leave the density constant if I come back to this picture I mean we have a certain probability to find ground set atoms in here but of course if the Rittberg atom is much bigger the same density you get much and many many more ground set atoms in there and the interesting thing is if you compute it you see that the center of gravity of this wing of the left, this molecular wing basically stays more or less constant because of the scaling I mean the density goes down but the volume goes accordingly up and these two things just compensate such that you get a constant what we call a density shift we were very excited about this until we learned that people have done this already 80 years ago so there's data from Rostock in 1934 we have exactly the same experiment with vapor lamps so they have taken vapor lamps and you see these Rittberg lines of sodium or what was this here actually sodium or potassium and then they added additional gas and what they see is that for the main quantum number in the beginning there are changes but after some time actually stays constant and actually the funny thing is that you get red shift or blue shifts depending on what kind of baffer gas you use so this is actually a little bit funny so if you would use some polarizability models you would only expect shifts in one direction but as we have learned the scattering length can be of course positive and negative and the funny thing is that Enrico Fermi developed his pseudo potential to explain data like this because I have seen this red and blue shifts and I couldn't explain it and they say okay we take effective potential, the pseudo potential then we have a positive and negative so this is the invention of the pseudo potential okay the last parameter I want to show you we can shift this of course we can increase the quantum number but we also can increase the density all the data I've shown us was a thermal cloud but we wanted to go to a really dense regime where we have of course both Einstein condensate and what we do is actually an experiment that we really excite only in the densest regime we have an excitation laser which we actually poke into this is a both Einstein condensate and we have a laser which we poke through the center of the BEC and what you see is that yeah it's hard to see but the ribbed atom the 111s state for example is almost the size of the BEC it's like a sphere like this and if you do this experiment now again as before as a function of one direction of detuning then you see that you have now a density shift before the density shift was only on the order of a megahertz now you get density shifts on the order of 40, 50 megahertz of course many many more atoms inside the density shift becomes longer the funny thing was here that our simple model you know what I have shown you like the multiplication of the scattering length time density of the ribbed wave function this would give a line shape something like this but this is certainly not true what you have to do is you have to not only include S wave and P wave scattering the interesting thing is that in this P wave scattering there's a shape resonant happening and this shape resonant does something ugly to the to the molecular lines so what you see here is the line of a 53s state and this ground set atom and this is now enlarged by a factor of 1000 though there's this little wiggles somewhere here in this region where we have this traditional bound states but due to our shape resonance exactly at this point where the electron has a right velocity you have the shape resonant and basically it drags down states from the hydrogen manifold and you get something like avoided crossing here and new funny states like this trilobite state so this one we have discussed already before or I've shown you this picture from this Hossin, Sadegpur and Grisgrin paper but they're also like something like butterfly states and they have all these nice names and all these states have been now already spectroscopy identified or excited but for this case now here in the BC let's have a look at this crossing here what you see is the blue line is this traditional potential I was showing you then here you have the C4R to the 4 potential of the ground set atom with the charged nucleus and then due to this scattering resonance you get here energies in this potential energy landscape which allows to have much larger energies on the red side but also on the blue side and if you look at the data there's some wing to the blue side but also this large wing to the red side and this wings and this wings here is due to atoms which happen to lie in this position or this position so we modeled the whole thing by just throwing randomly Poisson distributed atoms in according to the distance and to the density and then we get this nice agreement and this one I mean every of this if you put the atom here then this is for example like this kind of butterfly shape so if we only excite this guy here then we are pretty sure that we have excited some so this atoms if we excite here some roadblocks then they have looked like this before okay what you can do with this I mean as this density shift of course depends on the density and we can have a laser well focused in the BEC what we have done is experiment we just took the laser beam and was going with the laser beam through a BEC and the spectra and what you see if you for example so this is the distance so we move the laser from the BEC and if you have a large detuning like minus 55 megahertz and we know that we only can talk to a regime where we are very dense so you've got only in the center of the BEC we have a real big excitation and we only have a small detuning then you see that we only can excite in the wings in the center it's too dense we cannot excite and then you can use this as a density tomography for a BEC or if you want to do some excitation at a specific regime of a BEC you just dial in the right detuning and they can be sure that you're in the center of the BEC or on the wings okay so finally perhaps just the other knob we have is not the main quantum number but we also can change the angular momentum and I want just to show you more colorful pictures so we have also switched to D states so the S state molecule I have shown you before but the D state molecule of course you have more shapes to choose from and I just want to show you the spectroscopy of the MJ one half state so you have this dumbbell thing where you have like this torus around here and this big dumbbell so there's a large electron density here and a small electron density here so if you look at the potentials and you see that you have very deep potentials here and here and very shallow potentials here and if you do spectroscopy like this photo association of ripback molecules you can excite either states which are weakly bound so they're living in this low density regime here or you can excite states which are deeply bound which live in this big lobes up and down so what you can do with this you can basically produce aligned states of molecules so aligned means like that all molecules are either oriented like this or you can use anti-aligned states if you go to this torus here then all the molecules are like in a plane and the funny thing is what you could do which we have not done if you start to hybridize these molecules by bringing two different angular momentum states together by electric field you could even think about getting to aligned and orientated atoms such that the core the ripback core is sitting here and that the neutral atom is sitting always on top of it so you can really think about making aligned and orientated samples of molecules okay so this brings me to the end in this very quick talk the summary I have only two messages for you you can have thunder walls and dipole-dipole interactions in the system and there are long range in terms of that they can have that they can talk a one million atoms further downstream and you can have interactions there this is what we call long range and the other thing is that we can have this ripback molecules and I've shown you the D state because in the outlook I want to show you that what you could do so let's have a look here so I've shown you the D state and the interesting thing about the D state is we would like to use this as a tool to probe correlations in a quantum gas because if we have like for example excited this kind of molecule this is a dimer state and then one atom would sit here one atom sits here so we have a measure for the probability to find two atoms at a certain distance well this might be a very useful tool if you go to like some D-general Fermi gases you really can look in space what are the G2 or G3 I mean if you have to dimer, trimer, tetramer we can go to higher order correlation functions and perhaps learn what's going on and the interesting thing is here I mean the quantum gas microscope can look in a 2D system but it's very finite it's like 20 by 20 atoms or something like this this thing you can use in a bulk assemble because you can do it in the middle of the system and I think this has some potential another thing we would like to do is we want to this what I have not told you is that this Rydberg atoms I mean we have an interaction between the ground set atoms and the Rydberg wave function which of course means if I put a Rydberg atom into a BEC although the atoms of the BEC will start to roll into the Rydberg wave function because it's attractive potential and if you do a simulation of this is a BEC and we have placed in one of these D-state molecules and then you let the system evolve for some time then of course the superfluid fraction will flow into the Rydberg atom and if you stop at a certain time you can have an imprint of a Rydberg atom in a BEC and if you then do a face contrast imaging you should be able to make a picture of an atom which is the big goal of what we want to do and last thing here I mean what you were also thinking about I mean we would like to have a very highly excited say Rydberg state such that the electron lifts outside of the Bose-Einstein condensate so it acts like a faraday cage and then we have an ion sitting inside the BEC and then we can look at the coupling of ions with like a ground set atoms and do some kind of Polaroid physics so you see there are more things to do more long range or less long range and the last slide as I said everybody has his own long range meeting so we had a long range meeting which was mostly dedicated to dipolar systems so the dipolar BECs and the Rydberg systems but I've seen now what kind of other long range system are there and I hope that I will see some of you people next year in the Herrera seminar in Bad Honeff in Germany which will be in October 25 to 27 and we have no program, no speakers yet but we will start to organize this and I think we should have a large fraction of you guys here and you will hear from us okay so now this is really the end