Lasers are increasingly being used to impart mechanical forces on materialparticles ranging from atoms to biological cells. I present here experimentsinvolving such forces, concentrating on the formation and excitation of Bose-Einstein condensates (BEC) in ultra-cold rubidium 87 vapours. Also reportedare experiments in laser locking using hyperfine spectroscopy of 87Rb andexperiments in the optical manipulation of particles much larger than atoms.The cooling and imaging techniques used in BEC experiments dependcrucially on lasers with frequencies stabilised relative to the hyperfine structureof atoms. This structure is typically probed using saturated absorptionspectroscopy or polarisation spectroscopy, but the reliability of both is compromisedby the narrowness of the spectral lines involved. An extension ofpolarisation spectroscopy is presented which overcomes this by measuringa combination of Stokes parameters, the ratio of which provides a usefulfeedback signal over many times the line-width.Particles up to the size of biological cells can also be trapped and relocatedusing tightly focused laser beams called optical tweezers. Slight displacementsof particles within tweezer traps are used here to measure opticalforces applied by a tapered optical fiber lens that delivers light to a trappedpolystyrene bead. The optical force on the bead was mapped as it was movedrelative to the taper, showing that the taper provides a long and well colli-mated force-field that might be used as an optical “conveyor belt”.A BEC is a macroscopic collection of particles which collectively displayquantum coherence. Condensates are created by trapping and cooling particlesto temperatures below the critical temperature Tc for Bose condensation.These experiments produce BECs of 87Rb atoms with an atom chip apparatusthat confines atoms to a cigar shaped region using a nearly harmonicmagnetic potential. One axis of this trap is much weaker than the others andis bisected by a Gaussian sheet of laser light, to which atoms are attracted.The beam thus provides a narrow potential well (dimple) of variable depthsuperimposed on the magnetic trap.This trap was used to form condensates by mechanical compression (whichraises Tc) rather than cooling. Thermodynamic properties of vapours wereobserved before and after the dimple potential was turned on and comparedto detailed theoretical calculations based on semi-classical Hartree-Fock (mean-field) theory, which is often presumed to be accurate for Bosegases. Good agreement was found when the dimple was switched on slowlyin a reversible process. Rapid turn-on however produced much less heating(and hence higher final condensate fractions) than predicted.The dynamics of condensation formation were studied by measuring theproperties of vapours at various times after suddenly switching on the dimple.This contrasts with experiments in which high energy atoms are rapidly removedfrom a cloud. Condensates form in such systems after a very differentkind of rethermilsation process.The dimple is also used to create overdensities which propagate throughthe BEC after the optical potential is suddenly turned off. Analytical calculationsof classical hydrodynamics and numerical simulations of the condensatebased on the Gross-Pitaevskii wave equation both show that the leading edgeof such pulses should form shock waves in finite time. The regime in whichthis occurs is experimentally accessible and data are shown which are compatiblewith theoretical predictions. However the existence of shock waves isnot demonstrated because the available imaging resolution is insufficient. Aproposal is made to repeat this experiment using much larger condensates,which calculations show should make shock waves apparent.
Lasers are increasingly being used to impart mechanical forces on materialparticles ranging from atoms to biological cells. I present here experimentsinvolving such forces, concentrating on the formation and excitation of Bose-Einstein condensates (BEC) in ultra-cold rubidium 87 vapours. Also reportedare experiments in laser locking using hyperfine spectroscopy of 87Rb andexperiments in the optical manipulation of particles much larger than atoms.The cooling and imaging techniques used in BEC experiments dependcrucially on lasers with frequencies stabilised relative to the hyperfine structureof atoms. This structure is typically probed using saturated absorptionspectroscopy or polarisation spectroscopy, but the reliability of both is compromisedby the narrowness of the spectral lines involved. An extension ofpolarisation spectroscopy is presented which overcomes this by measuringa combination of Stokes parameters, the ratio of which provides a usefulfeedback signal over many times the line-width.Particles up to the size of biological cells can also be trapped and relocatedusing tightly focused laser beams called optical tweezers. Slight displacementsof particles within tweezer traps are used here to measure opticalforces applied by a tapered optical fiber lens that delivers light to a trappedpolystyrene bead. The optical force on the bead was mapped as it was movedrelative to the taper, showing that the taper provides a long and well colli-mated force-field that might be used as an optical “conveyor belt”.A BEC is a macroscopic collection of particles which collectively displayquantum coherence. Condensates are created by trapping and cooling particlesto temperatures below the critical temperature Tc for Bose condensation.These experiments produce BECs of 87Rb atoms with an atom chip apparatusthat confines atoms to a cigar shaped region using a nearly harmonicmagnetic potential. One axis of this trap is much weaker than the others andis bisected by a Gaussian sheet of laser light, to which atoms are attracted.The beam thus provides a narrow potential well (dimple) of variable depthsuperimposed on the magnetic trap.This trap was used to form condensates by mechanical compression (whichraises Tc) rather than cooling. Thermodynamic properties of vapours wereobserved before and after the dimple potential was turned on and comparedto detailed theoretical calculations based on semi-classical Hartree-Fock (mean-field) theory, which is often presumed to be accurate for Bosegases. Good agreement was found when the dimple was switched on slowlyin a reversible process. Rapid turn-on however produced much less heating(and hence higher final condensate fractions) than predicted.The dynamics of condensation formation were studied by measuring theproperties of vapours at various times after suddenly switching on the dimple.This contrasts with experiments in which high energy atoms are rapidly removedfrom a cloud. Condensates form in such systems after a very differentkind of rethermilsation process.The dimple is also used to create overdensities which propagate throughthe BEC after the optical potential is suddenly turned off. Analytical calculationsof classical hydrodynamics and numerical simulations of the condensatebased on the Gross-Pitaevskii wave equation both show that the leading edgeof such pulses should form shock waves in finite time. The regime in whichthis occurs is experimentally accessible and data are shown which are compatiblewith theoretical predictions. However the existence of shock waves isnot demonstrated because the available imaging resolution is insufficient. Aproposal is made to repeat this experiment using much larger condensates,which calculations show should make shock waves apparent.
In 1925, Einstein predicted the condensation of bosons into the ground state of the system for low (but finite) temperatures. Several phenomena, including superfluidity and superconductivity have been associated with Bose-Einstein condensation, but these systems interact strongly with their environment and pure Bose-Einstein condensation could not be established. It took 70 years, in which time the laser was discovered, and laser cooling techniques to manipulate atoms in a dilute atomic gas, before Bose-Einstein condensation in dilute atomic gases could be demonstrated in 1995. In the first condensation experiments, BECs were created in a magnetic trap. Since in a magnetic trap not all mF states of the atom can be trapped simultaneously, thereby limiting the number of experiments that can be done, other ways of trapping and generating BECs were sought and found. In 2001, the first all-optical BEC was made, where the dipole force was used to trap atoms in the crossing of two far red detuned laser beams. In an optical dipole trap not only atoms in different internal states can be trapped, but also different atomic species simultaneously. In this thesis, the formation of an all-optical Bose-Einstein condensate with rubidium atoms is presented. Conventional all-optical BECs are usually created in high power CO2 laser dipole traps, or have complicated laser cooling schemes and complex dipole trap setups. In our simple and straightforward setup, we load rubidium atoms from a magneto-optical trap into a crossed optical dipole trap created by a single frequency Yb:YAG laser with a wavelength at 1030 nm. The small wavelength allows for a small diffraction limit, and permits us to use standard optical materials, thus making the experimental setup cost effective. Other attempts to achieve Bose-Einstein condensation in a multi-mode (frequency) fiber laser at 1064 nm failed, because the atom loss was quite high. It is assumed that the multi-mode character of the fiber laser induces Raman transitions in rubidium atoms, thereby heating them. We can trap about ∼ 5E7 atoms in a single beam dipole trap out of ∼ 5E9 atoms trapped in theMOT, and ∼ 350,000 atoms can be trapped in a crossed beam dipole trap due to the smaller trap volume. 70% of the atoms in the dipole trap is optically pumped into one mF state. Quantum degeneracy is reached by evaporatively cooling the atoms in the crossed dipole trap by ramping down the laser power with three linear ramps. We can independently change the power of each beam by an AOM. This allows us to use one beam as an atom waveguide for future experiments. After evaporation, we typically have about 10,000 atoms at a temperature below the critical temperature. We have proved Bose-Einstein condensation by using the anisotropic expansion of a quantum degenerate gas trapped in an anisotropic potential. The aspect ratio of our atom cloud changed during a time of flight from 0.7 to 1.2 in 10 ms, thus proving that we have reached quantum degeneracy. We have about 5,000 condensed atoms in our optical dipole trap at a temperature less than 100 nK. The remaining atoms are thermal. Bose-Einstein condensation is obtained within 8 s, and we can repeat the experiment every 30 s. It should be mentioned that the Bose-Einstein experiment was moved from the ”Leibniz Universität Hannover” to the ”Technische Universität Darmstadt”, and had to be completely rebuilt. All-optical Bose-Einstein condensation was reached within one year after the move. Our Bose-Einstein condensation setup presents an ideal starting point for using our condensates in combination with miniaturized atom optical setups based on our novel microfabricated optical elements. With our microlenses, we can create a number of possible dipole trap configurations, such as the dipole trap array or the cylindrical microlens array. Using microlenses in miniaturized atom optical setups opens a completely new field of coherent atom optics. Also because the tight confinement of the microtraps allows us to load a 3D BEC, a 1D BEC, or a Tonks-Girardeau gas in the micropotentials depending on the density.
In this thesis I discuss work that has been carried out on the dynamics of a Bose Einstein condensate of Rb 87 produced near an atom chip. A Bose Einstein Condensate (BEC) is a quantum state of matter where a single quantum state becomes occupied by a macroscopic number of identical Bosons. In our case this is achieved by cooling a system of trapped identical rubidium 87 atoms to its ground state. To reach temperatures of condensation we initially laser cool atoms from room temperature, before loading them into a magnetic trap. The magnetic trap is produced through a combination of uniform magnetic fields from coils outside our vacuum chamber and currents running through wires on an atom chip. The atom chip is a microfabricated device, produced by a coating a silicon chip with a thin layer of gold and etching wires into it. Together, these fields create a magnetic field minimum 120μm from the surface of the chip which can be used to confine low field seeking hyperfine states of the atom in an elongated harmonic trap. Once the atoms are confined in the magnetic trap we used force evaporative cooling out to reach the phase space densities required for Bose Einstein condensation. The BEC is used to investigate the relative dynamics between the fraction of the atoms in the condensate to those not in the condensate. Our atom chip provided a suitable environment to investigate this due to fragmentation of the magnetic potential close to the chip. Small imperfections in the wires on our atom chip mean that the trapping potential isn’t smooth. Small regions of higher trapping frequency - or fragments - are formed. Due to the small size of these fragments it is possible to find a position where a condensate can form in the fragment, and see a potential of high frequency, whereas a non-condensed atom will see a lower frequency potential. We exploit this to set the condensed fraction moving relative to the non condensed part and investigate the subsequent damping of their motion relative to each other.
Research on degenerate quantum gases of dilute atomic gases has become a well established field in modern physics and revealed many astonishing phenomena. The first experiments on Bose and Fermi gases at low temperatures aimed to prove the predictions of quantum mechanics on a macroscopic scale. Since then we make use of Bose and Fermi gases and mixtures of any combination to model other many body systems. A property of Bose Einstein condensates is superfluidity which we want to investigate further in toroidal potentials with a new apparatus which can produce an all-optical BEC. This means trapping thermal atoms in an optical dipole trap and cooling the sample by evaporation into the quantum degenerate regime. This new technique was first successfully demonstrated by M.D. Barrett et al. in 2001 and promises to be faster and simpler than the well established and common way of evaporation in magnetic traps. A new apparatus was built by the author and we report on the experimental details of the trapping of 87Rb atoms from a hot vapour and cooling them in a magneto-optical trap, and further evaporation to quantum degeneracy in a crossed optical dipole trap operating at a wavelength of l = 1064nm, with accurate control of the power in both beams via a feedback loop. The evaporation of neutral atoms of 87Rb in far red detuned optical dipole traps using linearly polarised laser light is spin independent, and mixed spinor condensates of the F=1 manifold can be formed. As an empirical technique we found by applying a magnetic gradient field during the final evaporation that we can selectively populate mF spin states or prepare mixtures. This intriguing mechanism was found earlier as well by M. S. Chang, but is yet not fully understood and subject of our future research. We can now routinely prepare an almost pure condensate containing up to 7000 atoms in the condensed phase purely in the mF = 0 spin state. This thesis contains the theoretical framework of Bose-Einstein condensation with in mean-field theory, the concepts of cooling of atoms in magneto-optical traps, the trapping and evaporation in optical dipole traps and the experimental realisation. In the future this experiment will be extended by a fast scanning dipole trap to apply arbitrary time averaged potentials of more complex geometries, such as toroidal traps to study the effects of superfluidity in a periodic potential. The production of toroidal traps and cold atoms in such a trap are outlined in this thesis.
Atom chips are an excellent tool for studying ultracold degenerate quantum gases, due to the high degree of controllability afforded by the precise potentials generated from the current-carrying microfabricated wires on the chip surface. The geometries of the trapping potentials are inherently capable of realising extreme aspect ratios, and therefore creating model systems with effectively reduced dimensionality, particularly the theoretically-tractable one-dimensional Bose gas. In addition, the temporal tunability makes it possible to impart non-adiabatic changes on the trapping potentials, allowing experimental investigation of samples which have been brought out of equilibrium - a situation which is not fully theoretically understood. This thesis describes the implementation, development and characterisation of an experimental system for producing the first Bose-Einstein condensates of atomic rubidium 87 gas trapped on the surface of an atom chip in Nottingham. Such an apparatus is very complex and requires careful characterisation in order to run in a stable and reliable way. Details of the experimental setup are thoroughly outlined, including the vacuum system, lasers, electronics, computer control and timing, and the optical imaging system. A newly installed compact two-dimensional magneto-optical trap provides an loading rate of 5e7 atoms per second for loading a three-dimensional mirror-magneto-optical trap with 1.5e8 atoms, at a temperature of 300uK within 10s. The cloud is then sub-Doppler cooled to 50uK, and spin-polarised with 96% purity into the |F=2,mF=+2> ground state within 5ms, in preparation for loading a purely magnetic trap. A millimeter sized copper Z-shaped conductor located beneath the atom chip surface creates a Ioffe-Pritchard magnetic trap, into which the laser cooled cloud is loaded with 70% efficiency, and can be held with a vacuum-limited lifetime of 40s. Evaporative cooling then pre-cools the sample to below 20uK within 10s, to allow the subsequent loading into potentials created by the atom chip with 100% efficiency. A final evaporation stage then cools the cloud below the phase transition temperature of 800nK, resulting finally in pure BECs with 105 atoms confined using the atom chip. Key measurements of various properties of the trapped condensates are presented, which are important in order to characterise the system fully, and to compare with theoretical expectations. In particular, included are the variation of condensate fraction with temperature, the BEC expansion dynamics, and the condensate lifetime in the trap, for example. Finally, it is demonstrated how BECs can be produced on the atom chip without the use of external macroscopic coils, achieved by using novel, integrated sheet structures located beneath the chip surface - unique to this experimental system - to create the necessary bias fields.
The possibility to mould and control pure quantum systems has been offered by the experimental observation of Bose-Einstein condensation, a unique phase of matter when macroscopic quantities of a gas occupy the lowest quantum state. Techniques for creating these degenerate gases vary from laboratory to laboratory; each offers an unique test bed for studying quantum physics on a macroscopic scale. This thesis reports on the experimental design, construction and performance of an apparatus to create two-component 87Rb and 41K condensates for studies of non-equilibrium dynamics. This thesis is broken down as follows. In Chapter 1, a brief overview of the history and status of Bose Einstein condensates is presented, to affirm the motivation behind our work. Chapter 2 then presents the fundamental theory and background information to understand how and why our experiment was built. Chapter 3 describes the bulk of the work undertaken at the beginning of this thesis. In particular it describes the design choices, construction and performance of the vacuum, and laser and magnetic coil systems that are the key structural elements of the apparatus. In particular the vacuum system is a two-component differentially pumped system designed to optimise the number and lifetime of trapped atoms. Another integral element of the vacuum chamber is the science cell that enables high optical access, and close physical, access to an atomic cloud. As a result, high resolution imaging, ≃ 980 nm resolution at 780 nm is expected with a commercial microscope objective lens. The laser system is carefully de- signed to best combine and deliver the seven different optical frequencies required to simultaneously trap and manipulate 87Rb and 41K atoms. The magnetic coil system also represents an integral component of the apparatus, responsible for trapping and transferring atoms in a quadrupole field. One pair of coils is able to have their current direction reversed in order to generate bias fields. This allows access to Feshbach resonances between the two species, once they have been condensed. The second part of this thesis, Chapter 4, describes the performance of the apparatus when used to simultaneously trap 41K and 87Rb in the 3D magneto- optical trap (MOT) and produce a condensate of 87Rb atoms. In particular 1 × 109 rubidium atoms are routinely trapped in the 3D-MOT and transferred to the science chamber. In the hybrid trap the atoms are evaporatively cooled via microwave radiation, later to be used to sympathetically cool 41K, and loaded into a hybrid optical dipole and magnetic trap. At this stage ≃ 5 × 106 atoms are evaporatively cooled, by lowering the trap depth of the optical dipole beam, until a condensate of 87Rb is formed, containing 1.5 × 105 atoms. Additionally we are able to produce 3D MOTs of 41K simultaneously with the 87Rb 3D MOTs.
This thesis reports on the design, setup, and operation of a new atom trap apparatus, which is developed to confine few rubidium atoms in ultrahigh vacuum by means of taylored and dynamically variable light potentials (dipole traps), thus making the atoms available for spectroscopy and controlled manipulations. To maintain low background pressure, atoms of a vapour cell are transferred into a cold atomic beam by laser cooling techniques, and accumulated by a magneto-optic trap (MOT) in a separate part of the vacuum system. The laser cooled atoms are then transferred into dipole traps made of focused far-off-resonant laser fields in single or crossed-beam geometry, which are superimposed with the center of the MOT. Gaussian as well as hollow Laguerre-Gaussian (LG01) beam profiles are used with red-detuned and blue-detuned light. Microfabricated dielectric phase objects allow the efficient and robust mode conversion of Gaussian into Laguerre-Gaussian laser beams. Trap geometries can easily be changed due to the highly flexible experimental setup. The dipole trap laser beams have powers up to several hundred milliwatts and are focused to around 10 m. At a detuning of several tens of nanometers from the atomic resonance, the resulting traps are a few millikelvin deep, and the trapped atoms exhibit motional frequencies near 30 kHz, trap light scattering rates of a few hundred photons per second per atom, and storage times of several seconds. The number of dipole-trapped atoms ranges from more than ten thousand to below ten. The dipole-trapped atoms are detected either by a photon counting system with efficient straylight discrimination, or by recapture into the MOT, which is imaged onto a sensitive photodiode and a CCD-camera. Due to the strong AC-Stark shift imposed by the high-intensity trapping light, energy-selective resonant excitation and detection of the stored atoms is possible. The measured energy distribution is consistent with a harmonic potential shape and allows for the determination of temperatures and heating rates. From initial measurements, the thermal energy is found to be about 10 % of the trap depth. Using a crossed-beam geometry with red-detuned laser light, efficient transfer of atoms between the beams is observed. Optimum transfer occurs when the two beams cross at a radial offset, which can qualitatively be understood when the particle energy and geometrical properties of the two-beam trapping potential are considered. Numerical simulations reproduce the general features of the measured transfer efficiency vs. radial beam offset. This experiment is a first step towards dynamically variable potentials. Atoms have been radially confined in a blue-detuned hollow beam, and their decay time has been determined. This configuration is currently extended to a three-dimensionally confining blue-detuned dipole trap. For advanced laser cooling, state manipulation and spectroscopy of trapped atoms, a system of two diode lasers has been set up which are phase-coherently locked with a difference frequency near 6.834 GHz, to drive Raman transitions between the hyperfine-split ground states of 87Rb atoms. Resonances of such a transition have been recorded in a buffer gas loaded rubidium vapour cell with linewidths below 100 Hz.