New Forms of Matter Near Absolute Zero Temperature
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00:00
Ketterle, WolfgangParticle physicsAtomismTemperaturePhase (matter)Lecture/Conference
00:30
AtomismPhase (matter)MinuteLastTemperatureNanotechnologyAbsolute zeroLecture/Conference
01:00
QuantumMaterialDensityCylinder blockAtomCrystallographic defectWavelengthClothing sizesSolidLiquidScale (map)RRS DiscoveryThermodynamic equilibriumFermionElectric power distributionTheodoliteMeteorologyDensityPhase (matter)AtomismVakuumphysikTypesettingRoll formingEnergy levelTemperatureMaterialNanotechnologySizingKopfstützeShort circuitWavelengthCosmic distance ladderForceField strengthSolidDomäne <Kristallographie>LiquidStrong interactionScale (map)Cylinder blockKey (engineering)StonewareAtmosphere of EarthField-effect transistorYearFuelWind farmTeleportationEffects unitBand gapKette <Zugmittel>WeekIceWill-o'-the-wispLecture/ConferenceComputer animationMeeting/Interview
06:10
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08:07
Astronomisches FensterLaserHochfrequenzübertragungOrder and disorder (physics)AtomismSpare partGround stationLecture/Conference
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AtomismVakuumphysikCooling towerNanotechnologyTemperatureDayLaserAnimal trappingCell (biology)WolkengattungBook designLecture/Conference
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VakuumphysikTemperatureWolkengattungNanotechnologyScale (map)MeasurementFlightLecture/Conference
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VakuumphysikLaserMicrowaveQuantensimulatorFundamental frequencyMaterialAssembly lineQuantumAtomCylinder blockAbsolute zeroLaserRutschungMaterialWork hardeningUltraPhase (matter)Order and disorder (physics)Orbital periodTemperatureHochfrequenzübertragungMicrowaveAtomismYearAstronomisches FensterMeasurementCartridge (firearms)SteckverbinderDayTypesettingLightPolradSpare partNanotechnologyOpticsMembrane potentialQuantumDichroismFlightCylinder headRoll formingAtom laserAdvanced Maryland Automatic Network Disk ArchiverField-effect transistorTone (linguistics)Photonic crystalOrbitPackaging and labelingCardboard (paper product)TheodoliteLecture/Conference
14:40
Fundamental frequencyQuantensimulatorAtomAssembly lineMaterialModel buildingMaterialPair productionPhase (matter)SolidElectronCylinder blockContainment buildingAtomismSizingDisplay deviceGasFuelKlassifikationsgesellschaftIndustrieelektronikWeekLiquid crystalMagnetic corePhysicistRoll formingSpeckle imagingIPadBook coverStonewareDayKoerzitivfeldstärkeFire apparatusSolidFermionLimiterLiquidCoulomb barrierYearComputer animationLecture/Conference
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SolidCondenser (heat transfer)Coherence (signal processing)Containment buildingCartridge (firearms)LaserRoll formingPhase (matter)ModulationDensityParticleLiquidSpaceflightWind farmLecture/Conference
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DipolJeepOpticsMagnetismMinuteJuli FernándezNanotechnologyVideoGround stationCommercial vehicleMaterialCocktail party effectLecture/ConferenceComputer animation
Transcript: English(auto-generated)
00:14
So, in my talk I want to give you an introduction on what can be done or what are we doing
00:23
with ultra-cold atoms at temperatures very close to absolute zero, and then I would like to illustrate it with a new form of matter which we have realized in our laboratory, and then in the last 10 minutes talk to you about some unpublished work, and I'm really excited
00:45
about that. So, a lot of people think the field of ultra-cold atoms is characterized by low temperature, nano Kelvin temperature, very very close to absolute zero. This is correct. However,
01:03
I want to make the point that what is much more important is that the systems we are preparing, the materials which we create from ultra-cold atoms is extremely dilute, and if you're very dilute the transition temperature to degeneracy is very very low and then you need low temperature,
01:24
but the more important thing is actually that we are very very dilute, because being very dilute means that forms of ultra-cold matter don't look like ordinary matter where the atoms are strongly interacting, they are pressed together and sometimes their impurities. In ultra-dilute
01:44
matter the system is enlarged by a factor of thousand, which means the density is thousand squared, a billion times lower than ordinary matter, and therefore the forms of matter I'm presenting to you, you should think about atoms, then there is a lot a lot of vacuum
02:06
in between and then there is the next atom. And this is really important and this is really what distinguishes our field, because if atoms are so isolated we know exactly the building blocks, we know the properties of atoms with atomic precision, and we also know the forces between
02:25
atoms very very precisely. Therefore if we put building blocks, the ultra-cold atoms together, we know exactly the ingredients, we know the building blocks, we know the Hamiltonian, and then we can focus on what is the physics behind the Hamiltonian, and of course we try to
02:44
visualize Hamiltonian, which are interesting enough because they have strong correlations and strong interactions built in. So this is really the decisive feature and this is also reflected in a hierarchy of length scales. In ordinary solids and liquids the distance between
03:03
atoms is the size of the atom, whereas in ultra-cold atoms we have a hierarchy that the size of the atom is the smallest length scale, smaller than the distance between atoms and smaller than the atomic deploy wavelengths. So this is just illustrating those hierarchies that we are very
03:23
cold and they are very dilute. And if the size of the atom is the smallest length scale, it doesn't, you can actually approximate inter-atomic interactions by a delta function. And this is an enormous simplification. Theories love that and that means in the materials we put
03:47
together we can really focus on the special phenomena in this material and not what are sort of the corrections due to complicated interactions at short range. And the realization with cold atoms literally means we have a system where pretty much with extremely high accuracy
04:09
we realize materials where it is not an approximation, it's still a complete description to use delta function type interactions. So let me maybe be a little bit more specific.
04:25
We characterize interactions in the ultra-cold domain usually with a parameter called the scattering length. But the scattering length which characterizes the strength of interaction can be much much larger than the size of the atom. Well you would say what is the size of
04:42
the atom? And for that purpose we usually pick the van der Waals length which tells us sort of what is the range, the van der Waals radius, the range of the inter-atomic forces. And we often have situations that the product of density and scattering length cube in a cube is one. This means very very strong interactions but at the same time
05:06
the product of n times the volume of the atom is much much smaller than one. And this now means well it's always nice to express a formula in simple words. This means atoms are
05:21
strongly correlated, are strongly interacting. This makes it interesting and an interesting problem in physics. Also the atoms are only occasionally touching each other. And that would mean we can now study interesting phenomena without having the uncertainty and the complication of what happens at close range. So when people ask me well everything is sort of
05:49
looks beautiful and cool with these cold atoms but what are really your contributions to the rest of physics? I came up with this following list. Because we know the Hamiltonian, we know
06:02
the ingredients of the system, we can now test many-body physics calculation at the percent level. And until recently the word precision many-body physics did not exist because there was no many-body physics system which could be controlled and studied with the precision which we have
06:23
right now because we don't have any uncertainties in what the atomic interactions are. I still regard our field as an exploratory field. We want to discover new materials, we want to create something in the laboratory which hasn't existed before, a new form of matter. So this of course in the tradition of discovering Bose-Einstein condensates
06:46
but there is more out in nature waiting to be discovered. And sometimes this is also something I want to emphasize. When we get into a class of materials with ultra-cold atoms let's say the BEC-BCS crossover, an insulator, spin Hamiltonians, topological materials.
07:09
Because we look at our system in the spatial domain, in momentum domain, we have different ways. I mean you can never do a time of light expansion of an electron gas in a semiconductor.
07:23
But so we have ways of looking at the system which are different and several times we were able to measure quantities like a Chern number, like entanglement which were thought to be theoretical constructs but could not be directly measured.
07:42
So that's my introduction. This is what characterizes our systems and let me now talk to you about the ultra-cold atoms. Well I'm not telling you how we cool these atoms because that's the story by itself but I want to give everybody a feel how does it look in the laboratory
08:04
and what are we doing in principle. So this is the typical laboratory. We have vacuum chambers, this is where all the action happens and then in order to do all the cooling, the control, the observation, we have many many windows and we talk to the atoms with laser beams and
08:22
electromagnetic radiation. But the most important part of the whole experiment are the students. This is tabletop experiments. A small team of students, two or three students can build, design the whole experiment and run the experiment. Well building now, I've just started
08:41
to build a new experiment just two or three years ago and now it starts working. So to put all those elements together takes about two or three years. So it's still an effort but it's an effort which can very well happen during the PhD time of a student. Here we have a closer look.
09:01
So this is now the vacuum chamber right behind the window pretty much ready to be touched, not by hand but by laser beams. This is where the cold atoms are and I mentioned we need laser light and I've shown this picture for quite a while how colorful our laboratory is. Yellow is the
09:25
light is for special purposes but for 20 years there was something missing in my life. Blue light. But three years ago we started experiments on atomic dysprosium and now we have beautiful blue laser. Now I feel I have the whole spectrum in my research. So this is
09:45
sort of a pictorial picture. This is how our laboratory looks like. Let me now give you a tune picture. How should you think about our experiments? So just imagine with all the tricks which Bill Phillips, Steve Chu, Claude Kontanucci and others developed, we cool atoms to very very low
10:07
temperature and with evaporative cooling with Dan Kleppner, Tom Gratek, Eric Connell, Carl Wyman, myself. We have all the cooling techniques that at the end of the day we have let's say a million nano Kelvin atoms. They are in the middle of a vacuum chamber and they
10:25
are in an atom trap in a box made of magnetic or electric field. So this is our pristine system and now we want to look at them. We cannot touch them but we can shine laser light on it
10:40
and we get a spatial picture here of an elongated cloud or we can switch everything off the cloud expands and then we get a momentum space picture. Now this also illustrates how can we measure for instance temperature at very very low temperatures at nano Kelvin scale
11:03
because if we see an expanding cloud we just have to measure the radius divided by the time of light and with a very very simple formula we fulfill the absolute definition of temperature and actually this temperature measurement has now become even more direct
11:23
because in the new SI units temperature is no longer defined by the triple point of water, temperature is defined by defining the Boltzmann constant. So when we started to measure nano Kelvin temperatures with time of light we were probably already 10 or 20 years ahead of the SI
11:43
definition because all we did is we measured kinetic energy and now with the Boltzmann constant confined an energy measurement is what temperature is. Let me just go back, there was one slide which I jumped but now we want to create new forms of matter. So when we want to create
12:03
new forms of matter we do it by taking these ultra cold atoms and then shining sometimes five or ten laser beams or beams of electromagnetic radiation and microwaves on the atoms and this modifies the system. It can create a periodic lattice potential for the atoms, lasers can induce
12:22
spin-orbit coupling, lasers can coherently drive transitions between hyperfine states. So all these beams, these are the ways how we implement additional terms in a Hamiltonian and create now a Hamiltonian which is the Hamiltonian for an interesting many-body system.
12:43
So this is sort of what we do and when you're asking me the question how many laser beams can you send through one window the answer is probably infinite, it's just a question of human creativity. In order to accommodate all the different beams we use beam splitters,
13:00
we use dichroic beam splitters, sometimes we have pieces of optics which are automatically moved away from the first phase of the experiment and then other pieces of optics deliver new laser beams for the second part of the experiment. So there's a lot of experimental tricks and multiplexing going on.
13:21
Okay, so you can actually regard our experiment as material research but it's material research which goes back to my childhood, I love to play with Legos and unfortunately in those days Legos have changed, you get all these sets and you build something with building blocks which
13:44
are very very special but in my days Legos were just kind of a box of bricks and I really had to use my imagination how to build very very complicated things out of very few building blocks and this is what we do in my research, our building blocks are atoms and just to make
14:01
a connection at the end of my talk I will talk to you about other Lego pieces molecules which are different. So maybe one piece just have round Lego pieces atoms around is maybe not enough but we have very few basic building blocks and with those we build together ultra cold
14:21
forms of matter and realizing Hamiltonians with basic building blocks I often think we are realizing new forms of matter in the laboratory but other people call it also a quantum simulation we solve a Hamiltonian which is known, we implement it experimentally and experimentally we
14:42
look at the phase diagram or solutions of the Schrodinger equation. Now many of the materials we want to study are complicated, are challenging I mean ultimately we want to learn something I often regard a simple system which we immediately understand just as a stepping stone
15:04
to calibrate our apparatus to make sure that everything is working but eventually we want to add more complexity and create a situation where even the best supercomputers cannot exactly solve the Hamiltonian then we are really learning something and we are showing theoretical
15:24
physicists the way how they can improve their approximations but what I'm just describing is the general approach in physics if you want to deal with complexity you want to build up the complexity step by step and hopefully be able to test every single step this is probably
15:45
this is done when you write a computer code if you write a computer code to calculate strongly correlated form of matter you would first switch the strong correlations out and check that your computer code is realizing the very very simple limits where you know that where
16:02
you know what the answer is so you want to build confidence now this is sort of often done in theoretical description but in experiments at least in many situations you don't have the material if you want to study an electron in an electronic system the theorists assume
16:23
non-interacting electrons but have you ever seen non-interacting electrons you cannot switch off the Coulomb interaction but when our building blocks are ultra cold atoms we actually have the ability to switch off the interaction to modify the interaction because we know our
16:43
building blocks and we know so many tricks in atomic physics how we can put atoms in different hyperfine states expose them to magnetic fields and such so we have a very very controlled way of building up complexity controlling every step and I should say it's still a pleasure for me
17:01
that we build a system we can study something which is so basic which is in the textbook and has never been but has never been studied before because we have the simple system we can do a study of Pauli blocking with non-interacting fermions this is known but it's wonderful to see that in its purest form in the laboratory but then we turn one knob
17:22
and we have a strongly correlated Fermi system and nobody knows how to describe it okay so let me give you as one example super solidity we have recently a few years ago observed for the first time a form of matter
17:41
which combines two aspects namely it is a solid and it is super fluid at the same time well of course you know that the classification of materials into solids liquids and gases is oversimplified physics is much much richer and for instance with liquid crystal displays
18:06
we all know that liquid properties and solid properties can be confined it can be combined and they can also be combined in in other systems so this is the example of
18:20
cats which have actually sometimes the property of a solid they are rigid and then they are liquid you can put them into a into a container and they softly fill out the whole container so that if but so to combine liquid and solid this is sort of pretty well known but
18:42
an interesting challenge was can you get something which is solid and super fluid at the same time and this cartoon sort of illustrates to you that maybe it's not so obvious because in a solid let's assume a crystalline solid every atom is indistinguishable but it is localized at a
19:07
crystal site and therefore you can distinguish it by location if you have a solid of a fermionic isotope and the solid of a bosonic isotope there is no dramatic difference because the quantum statistic doesn't really matter because you can distinguish the atoms by location
19:26
whereas in a Bose-Einstein condensate or a superfluid it requires all the atoms to be delocalized and form one matter wave be coherent with each other so how can you combine
19:41
the property of a solid where particles are distinguishable with the property of a superfluid well the cartoon answer is in that way because what I'm using here is what is the true definition of a solid a solid is something which well the theory says which breaks
20:01
translation invariance of space but I would say it's simply a solid is something which spontaneously has shape and form whereas the liquid has always the shape of the container and so what we were able to create is we were creating a superfluid which had a shape in this case a density modulation a stripe phase but those stripes were not imprinted with laser beams
20:26
they were spontaneously formed and this spontaneous symmetry breaking of translational symmetry this is the defining feature of a solid and when we do it with a Bose-Einstein condensate we also have superfluidity so the ingredient which turns a Bose-Einstein condensate into a
20:49
super solid is spin orbit coupling you all know spin orbit coupling from atomic physics it provides the fine structure in atoms spin orbit coupling is very important in condensed
21:02
matter physics we had the talk by Duncan Haldane where he emphasized how spin orbit coupling gives rise to materials with interesting topological properties and now we want to add spin orbit coupling to a Bose-Einstein condensate now if you have a Bose-Einstein condensate with spin orbit coupling it has a rich phase diagram
21:24
and I only want to focus here and give you an explanation how spin orbit coupling turns a Bose-Einstein condensate into a superfluid so what is spin orbit coupling spin orbit coupling is the coupling of spin and motion and this happens due to relativistic effect in a
21:47
solid but for cold atoms we can implement it remember with laser beams this is our tool we shine laser beams on the atoms and we implement interesting phenomena and that the spin
22:01
orbit coupling can be implemented at least in its simplest form and in one dimension by a two photon Raman transition if you take an atom in one momentum state and you flip the spin with the two photon transition you also transfer momentum so these two photon Raman transition flips the spin and changes the momentum spin and momentum spin orbit spin orbit
22:26
are coupled this is spin orbit coupling so with that preparation I can now tell you how spin orbit coupling modifies the properties of a Bose-Einstein condensate so what I'm doing here is I'm showing you the wave function of the Bose-Einstein condensate
22:42
in a cartoon here is the Bose-Einstein condensate in a blue state zero momentum state in a certain spin state and now this is a momentum space picture so we flip the spin we go from blue to red but at the same time we change the momentum by the recoil momentum of the photon
23:02
so in other words this spin orbit coupling puts sort of a little bit extra wave function into this place but now we have a condensate which is in spin up and spin down so therefore the Bose-Einstein condensate in two spin states with spin orbit coupling has those four contributions in its wave function
23:24
okay but now red and red is the same internal state two different momenta there is now a spatial interference and the periodicity of the spatial interference is nothing else than the deploy the pattern is at the deploy wavelengths related to the
23:42
equal momentum so now we have in the spatial domain a modulated density and this breaks translation symmetry and fulfills the definition of a solid so this is almost I come to that in a moment this is a very trivial explanation for super solid
24:03
but it fulfills everything people want to see there have been dozens of papers of spin orbit coupled Bose-Einstein condensate it breaks two symmetry sketch and translation it has goldstone modes everything you want to see in a spontaneously symmetry broken system it is there so how do we
24:22
detect it well I mentioned already Bose-Einstein condensate a couple of laser beams to induce spin orbit coupling and now we have to detect it and we detect this periodicity again with the only tool we have laser beams so you can observe that you have a periodic structure by
24:41
break scattering if you shine laser light at a specific angle and you fulfill the break condition then your light is literally reflected and this shows that we have a periodic structure so in our experiment when we look through a lens the atoms scatter light really scattering
25:00
so the whole lens is filled but if you fulfill the break condition that we have in addition to the Rayleigh scattering literally a break reflected laser beam which appears as a laser beam as a point like as a very collimated beam and here is our final experimental result we couldn't use a lot of light this is why the signal to noise ratio is not amazing but here
25:26
you clearly see the angular distribution collected by our lens but when we add spin orbit coupling there is right in the middle the break reflected beam showing that we had created a super solid phase now let me just make one or two comments about this kind of research first
25:49
there were other ways to implement super solid behavior and Bose-Einstein condensates which I don't have time to explain one was by the ETH group using optical cavities
26:03
and those implementations are both sort of interesting in the way that they use laser beams to change the wave function of the condensate and the modulation which we see is related to the wavelengths of the laser of something which bring in externally but nevertheless
26:23
it is a Hamiltonian of a super solid but more recently using strongly dipolar quantum gases groups in Florence, Stuttgart and Innsbruck realized another implementation of a super solid where this the super solid structures did not come from laser beams but they came from
26:43
dipolar interactions let me now use that example to make one comment what are we doing here in our experiment well I said we are creating a new form of matter but what we created was a super solid which I could explain to you in one slide as an
27:04
interference within a wave function of a Bose-Einstein condensate this is trivial and I had discussions with condensed matter theorists who were so surprised that the spin orbit coupled Bose-Einstein condensate can be actually described in perturbation theory and it shows super solid
27:24
properties but this is sometimes what we want to do we want to show this with our approach we want to show the simplest possible way to realize a phenomenon we want to realize what are the simplest ingredients necessary to see super solid behavior and when later when people
27:44
got my explanation and said but this is trivial I said actually thank you this is a compliment because if something you shouldn't do trivial research but if you do research and afterwards after you've explained it to people people say but this is trivial you should be proud of it
28:02
now those guys did the much harder part you need strong dipolar interaction this is a complicated system but we showed that the characteristics of a super solid can already be realized using Bose-Einstein condensates in a very very special and simple way anyway I have only a few minutes left and this takes me now from atomic quantum matter
28:26
to molecules why do we need molecules well here's a shopping list precision measurement called chemistry and such but in my research I really want more lego pieces lego pieces which have long-range interaction which are anisotropic and this is why we want molecules
28:45
as building blocks for quantum simulator now there has been a quest in our field for more than a decade to get colder denser more quantum degenerate molecules it's really
29:01
become very very big it's not just ultra cold atoms there are so many groups now working on ultra cold molecules and people have used many approaches buffer gas cooling electric magnetic slowing laser cooling assembly from cold atoms but ultimately the goal of all this cooling is to get in the end an ensemble which can be cooled close to absolute zero and we know that
29:25
from atoms the ultimate cooling laser cooling is great bill I agree but the ultimate cooling is always done by collisions collisional cooling evaporative cooling and sympathetic cooling has no cooling limit and therefore this is always needed to take us to the to deep degeneracy
29:43
both einstein condensates and pharmacies now what happens is there has been a decade-long search for a system where collisional cooling is possible and I just give you here some examples however nobody has found a system where the molecules collide without doing reactions
30:05
so the ratio of good to bad collisions was not sufficient to do collisional cooling and then people thought wow here's an idea if we assemble molecules from cold atoms we can now create sodium potassium sodium rubidium we can make diatomic molecules which cannot react
30:22
because the energetics is such that they can only do exoergic reactions and at low temperature this doesn't happen but then and this tells you there are surprises when people did those experiment the molecules were not stable they didn't survive collisions
30:41
and this has now led to the discussion in our field about sticky collisions about long-lived complexes where not just two molecules but three molecules stick together and then nasty things happen so let me take this search for ultra cold molecules to make the following statement
31:05
many people outside our field maybe even inside our field think we are really in control we have atoms we are controlling them with atomic precision and such and then we build piece by piece interesting forms of matter we are not controlling nature we are constantly looking for places where
31:26
nature allows us to play with her we are looking for and we are looking for molecules where we can study interesting physics in a in a limited Hilbert space where interesting physics happens but we do not want the coupling to this dissipated field but space molecular
31:44
reactions and all that so anyway so so far nobody had seen collisional cooling and thermalization and just a few weeks ago we observed it we observed it in a place where nobody expected it we observed it with a sodium lithium experiment with sodium lithium molecules
32:03
which are reactive but they didn't react and what may have helped us i don't have time to really now go through all that what helped us is that we realized that there may be a chance to control reactions to control the bad things
32:25
by completely spin polarizing it so we put the sodium lithium and sodium atoms in a state where all the spins all the nuclear spins all the electron spins were aligned and then we were hoping that the conservation of spin would prevent the system from doing
32:44
reactions people told us that this may not work when molecules collide the spin is rearranged spin is not conserved spin angular momentum is transferred into orbital angular momentum and i told my group we just have to try it and we were completely surprised when we
33:04
looked at collisions of sodium lithium with sodium and we found here in red when we used this fully spin-aligned state there was a long lifetime and for the other spin state there is a short lifetime so here the lifetime is long that means many many good collisions without reaction
33:24
and indeed and i should maybe not go through the details we clearly saw now that we had strong collisional cooling we could reduce the temperature we got more than an order of magnitude in phase space density so so for the first time we have a system of molecules
33:41
which can be collisionally cooled so now we maybe we found a place where now nature allows us to play with her to play now games with very very cold molecules we're not yet there we have to cool further but it looks extremely promising okay so let me conclude that i illustrated
34:04
to you the quest for new materials but new materials in a system where it is extremely dilute a billion times more dilute than ordinary materials but i should also embed my talk into the bigger picture of quantum science there are so many discussions research also funding
34:24
opportunities in quantum science and quantum technology and what i illustrated to you was maybe the most basic aspect of those quantum technologies namely fundamental research on quantum simulations so with this i want to give credit to for whom credit is due and i thank you for
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your attention