Summer internship at the Institut for Laserphysics -DESY Hamburg

Chiara Decaroli

27th May-9th August 2013

Abstract

The following document aims at giving a record of the internship I pursued in Hamburg,Germany during the summer 2013. The internship was carried out at the Institute forLaserphysics under the supervision of professor Andreas Hemmerick and PhD studentsArne Ewerbeck and Georg Wirth. The main subject of research for the group is the creationof Bose-Einstein condensates using techniques such as MOTs, laser cooling, evaporativecooling, optical lattices. At the time I joined the group, a new experiment was beingdeveloped, therefore my contribution for the months I spent in the group was mainlylaboratory work. In this report I will give an account of theoretical concepts behind theexperiment but I will mostly focus on describing the experimental work I partecipated in.In particular I will take the reader step by step through the process of setting up a vacuumsystem and building a MOT. There are unfortunately no results to present except for thegreat enrichment which I gained through this experience.

Introduction

My internship consisted mainly in assisting a PhD student in the early stages of the as-sembling of the experiment, therefore it was heavily experimental. However I researchedthe theory behind the experiment, which will be running in several months time, and Iwere shown some experiments already running in the same group. Due to the experimen-tal nature of my internship I had the opportunity of experiencing for the first time a reallaboratory environment, developing skills in laboratory techniques and pragmatic sense,which one cannot develop otherwise. I realized that, for me, playing in the lab is reallywhat gives the clearest and most concrete understanding of natural phenomena. The lab-oratory work requires you to be hands on all the time, with a clear and creative mindin order to find solutions quickly in case of emergencies. I also found determination andpatience to be very important skills for an experimentalist. Very often the experimental-ist comes up with an idea, testes it and it doesnt work. It takes time to figure out themost efficient way of finding a solution and the one which will give the best result in thelong term. So if on one hand a theoretician has sharper skills in abstraction and concentra-tion, on the other an experimentalist needs to have a good sense of strategy and foreseeing.

I will now describe the stages we had to follow in the setting up of the experiment, butbefore getting technical I will first give a brief introduction to the main topics covered inthe experiment. In doing so I hope to clarify many aspects of our work.

Theoretical background

BEC

A Bose Einstein condensate is a particular form of matter which consists of a gas of bosonscooled down to temperatures near absolute zero (0 K). At these conditions the bosonscondense into the lowest accessible quantum state. Theoretically proposed in the 1920s byS.N. Bose and Einstein, the first BEC was produced in 1995 cooling atoms of Rubidiumto 170 nanokelvin. The bosonic gas enters the condensate state when it reaches a specifictemperature called the critical temperature Tc.The most interesting aspect of BEC is that this state is characterized by properties shownby superfluid elements such as Helium 4. In fact, when Helium 4 was found out to becomesuperfluid below the temperature of 2.17K, it was thought that the reason for it had to dowith a partial Bose-Einstein condensation. Bose Einstein condensates are very fragile bothto achieve and to study. Once a Bose Einstein condensate is created, the original particlesbecome indistinguishable and therefore there is only a one unique matter comprehensiveof all the bosons. This matter is modeled as a giant matter wave as shown in the diagramabove.

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Figure 1: The stages of matter towards a BEC by cooling the temperature

Bosons vs Fermions

The property that characterizes bosons is that they can occupy the same quantum state,whereas Fermions obey the Pauli Exclusion Principle, according to which no more thanone fermion can occupy the same quantum state. As a result of the two different types ofstatistics, the Fermi-Dirac and the Bose-Einstein statistics, fermions are the building blocksof matter, whereas bosons are the sort of forces or glues that hold matter together. At aquantum mechanical level, bosons are particles which have integer spin whereas fermionshave half integer spin. If fermions are bounded together, they will make up bosons, in factadding half integers will result in having integers values for the spin, hence the resultantelement will display a bosonic behavior. Bosons on the other hand can only give otherbosons when added together as integers add up to integers.The spin of the particles isdetermined by the number of electrons, protons and neutron that they are made of, in factelectrons are spin particles, and fermions and neutrons and protons are made respectivelyof 3 quarks, which are fermions.The statistics for Bose gases were developed by N.S. Bosefor the special case of photons and then extended to massive particles by Einstein. It isbecause of the fact that Bosons can occupy the same quantum state that one can achievea Bose Einstein condensate.

Fermionic gas

This is a type of condensate which is similar to the Bose condensate except that in thiscase is it made of fermions instead of bosons. It is a superfluid phase of matter which arises

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Figure 2: The density of matter in space towards the Bose Einstein Condensate

when the temperature of the system is near absolute temperature. In this case, however,the distinguishability of the fermions is maintained due to Pauli Exclusion Principle.

Laser system

The term laser means Light amplification by stimulated emission of radiation. Lasers arehence devices which produce light thanks to an amplification process. There are threedifferent processes by which laser fields interact with atoms: stimulated absorption, stimu-lated emission and spontaneous emission. In the stimulated absorption, an atom in is theground state, absorbs a photon and is transferred to the excited state. In the stimulatedemission an atom is in the excited state, emits a photon with the same frequency anddirection as the laser field, and ends up in the ground state, in the spontaneous emissionthe atoms is as well in the excited state but this time emits a photon in a random direction,however it still ends up in the ground state. A laser system is made of different parts andthe process of stimulated emission is the one that governs their functioning. In order toobtain a stimulated emission process, it is necessary to achieve a population inversion, inwhich most of the atoms are occupying the excited state. These excited state atoms willtend to drop down to the ground level emitting photons of equivalent energy, hence pro-ducing coherent light. The coherence of the emitted light can be either spacial or temporal.

The design of a laser is fairly simple and consists of 2 parallel mirrors, one of them is 100%reflective whereas the other only 99%. The 1% of transmitted light is the output of thelaser. The beams which get through the output have been reflected back and forth severaltimes and are highly collimated. Since the light comes from a specific atomic transition, itis monochromatic.

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Figure 3: A schematic of a laser system

Laser cooling

From a basic understanding of the thermal kinetics of a gas we learn that the kinetic en-ergy associated with a gas equals a specific amount of energy, specifically 3/2 KT, whereK is the Boltzmanns constant. By solving this equation for the velocity of the atoms,we obtain a value of 300m/s. The fact that the speed is so high causes a broadening ofthe spectral lines and makes measurements more inaccurate, for this reason it is useful todevelop techniques to slow the atoms, or cool them. Refrigerators have been used to coolthe atoms to 100m/s but this value is still too high. The technique of laser cooling provesto be more efficient.

An explanation for how laser cooling works is the following. A photon from a laser hitsa moving atom. The atom absorbs the photon and thermal excitations occur within theatom. The atom emits then a photon with higher energy than the incoming photon, due tothe thermal excitations. Another way of looking at this phenomenon is from the point ofview of momentum conservation. When an atom is travelling towards a laser and absorbsa photon from it, the atom will be slowed down by the amount of momentum the incomingphoton has. This is shown eloquently in the picture below.

One of the main techniques used in laser cooling is the Doppler Cooling. In Dopplercooling a laser light is manipulated in order to shine light which is red-detuned and willtherefore be absorbed by the atoms if the atoms are moving towards the laser.

Magneto Optical Trapping

This is a technique whose aim is to reduce the speed of neutralatoms which have beentrapped at its center. Reducing the speed of an atom is equivalent to reducing its kineticenergy, hence it thermal energy, thus to cooling down the atom. MOT can slow downatoms travelling at a hundred meters per second to tens of centimeters per second. MOT

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Figure 4: How does laser cooling works

uses laser cooling and a specific set up of magnetic coils. A MOT can be 1, 2 or 3 dimen-sional. In each case, for each direction two lasers are shone in opposite directions. Whathappens is that the lasers push the cold atoms in the middle of the MOT where the atomsare trapped by a magnetic field created by the magnetic coils. When atoms come froma left to right direction, the laser beam is shone in a right to left direction. At a specificwavelength of the laser beam, the beam will bounce back from the atom with more energythan its original one, therefore it will take away some of the atoms momentum. The resultof this is that the atom is slowed down, or cooled. The MOT uses a combination of 3 pairsof counter-propagating laser beams, all slightly red-detuned from the atomic transition.

Figure 5: A schematic of a magneto optical trap

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Figure 6: A diagram showing the lasers shining at the MOT

If an atom is moving toward or away from a laser beam, the frequency that it ”sees”is up- or down-shifted, respectively, due to the Doppler Effect. Thus, red-detuned laserbeams will be shifted into resonance for atoms moving toward them. Those atoms willthus preferentially absorb photons from the laser beam opposing its direction of motionand will get net momentum kicks that slow it down and push it back toward the center.This can effectively cool atoms down to several hundred micro-Kelvin and confines them toa volume that is given by the overlap volume of the laser beams. As mentioned before themagnetic field is added to confine the atoms even in a more compact space. A 3-dimensionalMOT creates a magnetic quadrupole at whose center resides a point where the field is zero.

Dark Spot Magneto Optical Trap

Normal MOTs have a limitation in the density of the atoms that are cooled down. Thislimit is set by different factors: firstly the collisions between ground and excited stateatoms. In this case some of the excitation energy may be transformed into kinetic energywith a trap loss. Secondly the repulsive force between the atoms which arises because ofthe reabsorption of scattered photons. To overcome these limits a Dark Spot Mot canbe developed, which confines the atoms in a dark hyperfine ground state which has nointeraction with the trapping light, is developed. In practice, a system with a laser shonethrough a glass which has a dark spot in its middle has to be created. Lenses can be usedto enlarge the laser beam in order to remove a central cross section to create a cylindricaldark tunnel in which the atoms will tend to fall. In this way the density of said atoms willincrease greatly.

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Figure 7: The field lines from the magnetic coils

Vacuum system

In order to have ultracold gas an ultralow vacuum is necessary. Ultra low vacuums fallroughly in the range E-10 to E-12 mBar. In this situation the mean free path of a moleculeis between 1 and 100000 kilometers. This means that before a single molecule encountersanother it has to travel this distance, therefore there really is very little left inside thesystem. In order to achieve this vacuum one needs to make use of pumps which will pumpout all the water molecules and dirt molecules left inside. There are a number of differenttypes of pumps whose functioning is different and which target specific types of molecules,for instance a turbo pump, an ion pump, a titanium sublimation pump and a neg pump.

An Ion pump is a pump which is used in Ultra High Vacuum systems and which cancool to pressures of E-11 mBar under ideal conditions. It works by ionizing the gas in-side the system and applying a strong electric potential which attracts and captures the gas.

A turbo pump or vacuum pump removes molecules from a sealed volume in order to achievea vacuum. A turbo molecular pump consists of an alternate series of rotor and stator pairs.The molecules first go through the rotor which transfers momentum to them pushing themtowards the holes of the stator, from the stator the molecules enter the next layer of rotorsand the process continues in an alternate fashion until the molecules are lead outwards.The vacuum achieved by this type of pump varies from E-5 to E-11 mBar.

A titanium sublimation pump (TSP) is a type of pump which has some titanium filaments.These filaments are heated up by making a current go through them (usually around 40

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Amps). When the current flows through, the titanium reaches its point of sublimation anda thin coating of titanium is laid on the inside of the pump. As titanium is a very reactiveelement, the gas molecules left inside will react with it and solidify or stick to the titanium,reducing the overall pressure. However after a while the titanium wont be clean any moreas all the gas molecules will be attached to it, so it will need to be heated again to achievebetter pressures. This process can be done in cycles until the requested pressure is achieved.

In order to achieve a vacuum as the one in our lab, of circa E-11 mBar these pumpsare used together at different times and in cycles. The process of achieving an ultralowvacuum and a stable pressure is one that requires some time. A few weeks are needed forthe system to reach stability. Once the pressure has stabilized, the Bake-out process isinitialized. This consists in heating up the whole vacuum system to around 100 degrees.The aim of the bake out is to make the leftover water molecules evaporate and to driveoff the gas molecules still within the vacuum so that they can be more easily pumped out.The bake out is achieved using heating plates placed at strategic points of the systems andthe temperature is monitored during the process. After the bake-out, again the titaniumsublimation pump and the ion pump are activated.

This concludes the theoretical section of this report. I will proceed now to describedthe different stages of the experiment which I collaborated in.

The experiment

When I joined the group the turbo pump had already been activated. However the crucialpart of the experiment, the glass cell, still had to be mounted. A previous attempt atmounting a glass cell resulted in the break of the glass cell itself. It is of fundamentalimportance that the glass cell is handled with extreme care and mounted in a very delicateand precise way. The pressure must be equalised at both sides of the appendix in order forthe glass not to break and for the vacuum inside to be ultralow. Below, a schematic of theexperiment is provided, which shows the position of the glass cell and the pumps describedin the earlier section. In order to mount the glass cell, at least three people are required inthe laboratory. The process lasts for about 8 hours in which no breaks can be taken. It isan iterative process in which once the glass cell is positioned in between the pumps it has tobe fixed to them and the appendices must adhere perfectly, therefore the spacing betweenthe points of matching must be continuously tightened until it is equally tight on each side.

Once the glass cell has been placed, the next stage of the experiment is the activationof the pumps. The turbo pump is left on during the process. First, the ion pump is ac-tivated, then the titanium sublimation pumps are activated. With them also an iterativeprocess is required until the pressure stabilised. Around a week is necessary for the pressureto fall down and for the system to reach a situation of equilibrium. Hence from this pointthe pumps can be actuvated again until the pressure is good enough for the experiment.

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Here we aimed at an ultralow vacuum, hence at lows 10−11 mBar.

While waiting for the pressure to fall down, we focused on some electronics needed forthe optical system. I soldered a photodiode and an Acusto Optical modulator and startedchecking the circuits. I also was shown the effect of doppler cooling and of saturated ab-sorption spectroscopy. Moreover I coupled an optical fibre, a job of precision but most ofall patience in which one tries to let as much power from the laser as possible through anoptical fibre. In this way it’s possible of course to have the optical system not necessarilyclose by to the glass cell and the pumps since the laser light can be transported via theoptical fibre.

Once the pressure stabilised, we proceeded to the next stage: looking for leaks in thevacuum. This was done by using a special leek searching machine which detected helium.helium was sprayed from outside the system and the machine from inside would detect theamount of helium that leaked inside. With the machine a few leaks were founds and closed.Hence the bake-out was started. Baking out is a techinical term which means taking thetemperature up around 100 deg Celcius. In this was the last water molecules left inside thevacuum can evaporate. Various temperature detectors are placed at strategic points of thesystem and the whole system is covered in aluminium foil and in glass wool for insulation.The temperature is highered by using heating plates and this must be done slowly, at afew degrees per hour, hence the whole process takes around a week time.

After the bake out one last activation of the pumps brings the pressure down to around2*10−11. At this point the vacuum system is ready and it is necessary to focus on the mag-neto optical traps. The MOTs need to be built in the laboratory. They have been designedfor this specific experiment and have been build in the DESY workshop. The copper bandsmust be glued on the coils and baked. Three people are required for this: one places the gluon the copper band, one turns the coil and one counts the number of rounds made by thecopper bands and keeps the band under tension. There are two 2-dimensional MOTs andone 3-dimensional MOT to build. This process of ”coiling” also takes about one week time.

When all the coils are ready and tested, the water cooling system can be created. This isneeded to preventing the coils from overheating, in fact high currents will be put throughthe coils which will make their tempreature rise very fast and very high. Water runs insidethe coils so that the temperature is lowered. A machine called water water cooler is usedto cool the water that runs through the coils. The machine uses clean water kept at lowtemperature to lower the temperature of the water going through the coils. A system ofpipes needs to be build in order for the water to go smoothly from the water water coolingdevice to the MOTs.

With the setting up of the water system my internship comes to an end. The next stepswill be to set up the optical system and the lasers and the optical fibres. At that pointRubidium 87 atoms will be injected in the vacuum and a Bose Einstein condensate will

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be studied. Hence Potassium 40 atoms will be fired in the vacuum from the opposide endand a fermionic and bosonic ultracold quantum gas will be achieved and studied.Below Iprovide some pictures of the experiment: the glass cell and the pumps, the MOTs and theoptical system.

Figure 8: schematic of the experiment

Conclusions

This experience was for me the first real laboratory experience. Even though I had somelaboratory experience from my university courses in fact I never had the chance to spend

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every day all day in the lab. This really made me realise what working in a laboratoryis like and made me appreciate all the aspects involved. I developed practical sense andskills in building and assembling apparatus and electronics, so overall the aim of my ”own”experiment were very successful and extremely precious.

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Figure 9: picture of the glass cell and the pumps

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Figure 10: developing the optical system

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Figure 11: the Magneto Optical trap

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