This thesis is devoted to the construction, optimization and characterization of an experimental apparatus, capable of creating spinor condensates of ~1x10^5 atoms with a repetition rate of 10s, using an all-optical evaporation technique. I report a complete description of the experimental apparatus and techniques used in the experiment and a characterization of the BEC sample. We study the transmission of absorption imaging pictures through a coherent fiber bundle. We show that the fiber bundle introduces spurious noise in the picture mainly due to the strong core-to-core coupling. We demonstrate that we can retrieve exact quantitative information about the atomic system using this technique. We also explore the equilibration dynamics of ferromagnetic spin-1 system as a function of the initial magnetization of the sample and the external magnetic field. We show that the magnetization of the system is conserved despite of the presence of dissipative processes that are intrinsic to any experiment. We investigated the formation of the BEC in a spin-1 quantum gas in the presence of an external magnetic field. We report on the spontaneous magnetization of the condensate fraction during the evaporation process at low magnetic fields. We as well observe multi-step condensation, and found signatures of a possible interspecies Feshbach resonance.
This thesis is devoted to the construction, optimization and characterization of an experimental apparatus, capable of creating spinor condensates of ~1x10^5 atoms with a repetition rate of 10s, using an all-optical evaporation technique. I report a complete description of the experimental apparatus and techniques used in the experiment and a characterization of the BEC sample. We study the transmission of absorption imaging pictures through a coherent fiber bundle. We show that the fiber bundle introduces spurious noise in the picture mainly due to the strong core-to-core coupling. We demonstrate that we can retrieve exact quantitative information about the atomic system using this technique. We also explore the equilibration dynamics of ferromagnetic spin-1 system as a function of the initial magnetization of the sample and the external magnetic field. We show that the magnetization of the system is conserved despite of the presence of dissipative processes that are intrinsic to any experiment. We investigated the formation of the BEC in a spin-1 quantum gas in the presence of an external magnetic field. We report on the spontaneous magnetization of the condensate fraction during the evaporation process at low magnetic fields. We as well observe multi-step condensation, and found signatures of a possible interspecies Feshbach resonance.
Bose-Einstein condensation (BEC) is a phenomenon in which identical bosons occupy the same quantum state below a certain critical temperature. A hallmark of BEC is the coherence between particles every particle shares the same quantum wavefunction and phase. This coherence has been demonstrated for the external (motional) degrees of freedom of the atomic condensates by interfering two condensates. In this thesis, the coherence is shown to extend to the internal spin degrees of freedom of a spin-1 Bose gas evidenced by the observed coherent and reversible spin-changing collisions. The observed coherent dynamics are analogous to Josephson oscillations in weakly connected superconductors and represent a type of matter-wave four-wave mixing. Control of the coherent evolution of the system using magnetic fields is also demonstrated. The studies on spinor condensates begin by creating spinor condensates directly using all-optical approaches that were first developed in our laboratory. All-optical formation of Bose-Einstein condensates (BEC) in 1D optical lattice and single focus trap geometries are developed and presented. These techniques offer considerable flexibility and speed compared to magnetic trap approaches, and the trapping potential can be essentially spin-independent and are ideally suited for studying spinor condensates. Using condensates with well-defined initial non-equilibrium spin configuration, spin mixing of F = 1 and F = 2 spinor condensates of rubidium-87 atoms confined in an optical trap is observed. The equilibrium spin configuration in the F = 1 manifold confirms that 87Rb is ferromagnetic. The coherent spinor dynamics are demonstrated by initiating spin mixing deterministically with a non-stationary spin population configuration. Finally, the interplay between the coherent spin mixing and spatial dynamics in spin-1 condensates with ferromagnetic interactions is investigated.
The subject of this thesis is the use of BECs for atom interferometry. The standard way atom interferometry is today performed is by interrogating free falling samples of atoms. The employed samples are cold (but not condensed) to have high coherence, and dilute, not to interact significantly with each other. This technique represents nowadays an almost mature field of research in which the achievable interferometric sensitivity is bounded by the atomic shot noise. Until a few years ago the employment of BECs in such devices was strongly limited by the effect of the interactions between the condensed atoms. This obstacle is today removable exploiting interaction tuning techniques. The use of BECs would be advantageous for atom interferometry inasmuch they represents the matter analogue of the optical laser providing the maximum coherence allowed by quantum mechanics. Moreover, non-linear dynamic can be exploited in order to prepare entangled states of the system. The realization of entangled samples can lead to sub-shot noise sensitivity of the interferometers. At today very nice proof-of-principle experiments have been realized in this direction but a competitive device is still missing. This thesis work is inserted in a long term project whose goal is the realization of such a device. The basic operational idea of the project starts with the preparation of a BEC in a double well potential. By the effect of strong interactions the atomic system can be driven into an entangled state. Once the entangled state is prepared, interactions can be †switched off†and the interferometric sequence performed. This thesis begins with the description of the apparatus for the production of tunable BECs to be used in the interferometer. We chose to work with 39K atoms because this atomic species presents many convenient Feshabch resonances at easily accessible magnetic field values. The cooling of this particular atomic species presents many difficulties, both for the laser and evaporative cooling processes. For this reason, this was the last alkaline atom to be condensed. Its condensation up to now was only possible by employing sympathetic cooling with another species. In this thesis our solutions to the various cooling issues is reported. In particular we realized sub-Doppler cooling for the first time for this species and we achieved condensation via evaporation in an optical dipole trap taking advantage of a Feshbach resonance. In the last part of this work, are presented original calculations for the effects of thermal fluctuations on the coherence of a BEC in a double well, discussing the interplay between thermal fluctuations and interactions in this system. Estimations and feasibility studies regarding the double well trap to be realized are also reported.
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.
This thesis reports on the construction of an apparatus capable of producing a molecular Bose-Einstein-Condensate (BEC) of 6Li2. To create a condensate fermionic lithium is first evaporated in an oven, slowed by a Zeeman slower and captured in a magneto-optical-trap (MOT). A scheme for creating time-averaged arbitrary optical potentials for optimized transfer of the atoms from the MOT to a high-power, far detuned optical dipole trap was implemented, as well as magnetic field coils to tune the interactions of the atoms via Feshbach resonances. The first experiments with the new machine include the characterization of the optical dipole trap and evaporative cooling of 6Li.
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.
Optical trapping as a viable means of exploring the physics of ultracold dilute atomic gases has revealed a new spectrum of physical phenomena. In particular, macroscopic and sudden occupation of the ground state below a critical temperature—-a phenomenon known as Bose-Einstein condensation—-has become an even richer system for the study of quantum mechanics, ultracold collisions, and many-body physics in general. Optical trapping liberates the spin degree of the BEC, making the order parameter vectorial (‘spinor BEC’), as opposed to the scalar order of traditional magnetically trapped condensates. The work described within is divided into two main efforts. The first encompasses the all-optical creation of a Bose-Einstein condensate in rubidium vapor. An all-optical path to spinor BEC (as opposed to transfer to an optical trap from a magnetic-trap condensate) was desired both for the simplicity of the experimental setup and also for the potential gains in speed of creation; evaporative cooling, the only known path to dilute-gas condensation, works only as efficiently as the rate of elastic collisions in the gas, a rate that starts out much higher in optical traps. The first all-optical BEC was formed elsewhere in 2001; the years following saw many groups worldwide seeking to create their own version. Our own all-optical spinor BEC, made with a single-beam dipole trap formed by a focused CO₂ laser, is described here, with particular attention paid to trap loading, measurement of trap parameters, and the use of a novel 780 nm high-power laser system. The second part describes initial experiments performed with the nascent condensate. The spinor properties of the condensate are documented, and a measurement is made of the density-dependent rate of spin mixing in the condensate. In addition, we demonstrate a novel dual-beam atom laser formed by outcoupling oppositely polarized components of the condensate, whose populations have been coherently evolved through spin dynamics. We drive coherent spin-mixing evolution through adiabatic compression of the initially weak trap. Such dual beams, nominally number-correlated through the angular momentum-conserving collision m=0 + m=0 ⇌ m=+1 + m=-1 have been proposed as tools to explore entanglement and squeezing in Bose-Einstein condensates.