There is great interest in the use of ultracold polar molecules confined in op- tical lattices for quantum simulation of complex many-body problems. The objective of our work is to realise a system of ultracold fermionic 40K133Cs molecules in a 1D optical lattice consisting of an array of 2D pancake traps. Such a system will enable a wide range of future investigations, including the study of novel regimes of interlayer superfluidity [1]. To create polar molecules we will employ magnetoassociation on an interspecies Feshbach resonance followed by transfer to the rovibrational ground state using STIm- ulated Raman Adiabatic Passage (STIRAP). This thesis reports the progress towards this ambitious goal focusing on the initial laser cooling stage. We present a simple single cell vacuum system that can be used to locate the interspecies Feshbach resonances essential for magnetoassociation. We give a description of the laser setups for both species, including details of the laser spectroscopy used for frequency stabilisation. We then present results on the optimisation of the magneto optical trap (MOT) for each species (focusing on the most abundant 39K isotope). The effects of the intensities and detun- ings of both the cooling and repump light and the magnetic field gradient are investigated in order to maximise the number of trapped atoms. Finally, we present preliminary observations of simultaneously loading overlapped MOTs for both species.
There is great interest in the use of ultracold polar molecules confined in op- tical lattices for quantum simulation of complex many-body problems. The objective of our work is to realise a system of ultracold fermionic 40K133Cs molecules in a 1D optical lattice consisting of an array of 2D pancake traps. Such a system will enable a wide range of future investigations, including the study of novel regimes of interlayer superfluidity [1]. To create polar molecules we will employ magnetoassociation on an interspecies Feshbach resonance followed by transfer to the rovibrational ground state using STIm- ulated Raman Adiabatic Passage (STIRAP). This thesis reports the progress towards this ambitious goal focusing on the initial laser cooling stage. We present a simple single cell vacuum system that can be used to locate the interspecies Feshbach resonances essential for magnetoassociation. We give a description of the laser setups for both species, including details of the laser spectroscopy used for frequency stabilisation. We then present results on the optimisation of the magneto optical trap (MOT) for each species (focusing on the most abundant 39K isotope). The effects of the intensities and detun- ings of both the cooling and repump light and the magnetic field gradient are investigated in order to maximise the number of trapped atoms. Finally, we present preliminary observations of simultaneously loading overlapped MOTs for both species.
This thesis describes the advances made towards a dual-species magneto-optical trap (MOT) of Li and Rb for use in photoassociation spectroscopy, Feshbach resonance studies, and, as long-term aspirations, the formation of ultracold heteronuclear polar molecules. The initial discussion will focus on a brief theoretical overview of laser cooling and trapping and the production of ultracold molecules from a cold atom source. Subsequently, details of the experimental system, including those pertaining to the required laser light, the vacuum chamber, and the computer control system will be presented. Finally, preliminary optimization and characterization measurements showing the performance of a single species Li MOT are introduced. These measurements demonstrated the loading of over 8 x 107 Li atoms directly into a MOT without the need for a Zeeman slower.
Abstract We describe a three-dimensional (3D) magneto-optical trap (MOT) capable of simultaneously capturing 85Rb and 133Cs atoms. Unlike conventional setups, our system utilizes two separate laser systems that are combined before entering the vacuum chamber, enabling the simultaneous trapping of two different atomic species. We trapped 85Rb and 133Cs atoms using relatively low total power: 8 mW cooling and 4 mW repump for 85Rb, and 7.5 mW cooling and 1.5 mW repump for 133Cs. The number of trapped atoms was 1.6 × 108 for 85Rb and 1.4 × 108 for 133Cs. The optical depths were 3.71 for 85Rb and 3.45 for 133Cs. The temperature of trapped atoms was ∼200 μK for 85Rb and ∼200 μK for 133Cs. Our 3D MOT setup allows full horizontal optical access to the trapped atomic ensembles without spatial interference from the trapping or repump laser beams. Our vacuum system is also quite simple, avoiding much of the complexity typically encountered in similar dual-species systems. However, the red detuning of the cooling laser used for atomic trapping in our system is relatively small, leaving room for further optimization. This system offers a versatile platform for exploring complex phenomena in ultracold atom physics, such as Rydberg molecule formation and interspecies interactions.
This thesis presents progress toward the production of ultracold CsYb molecules. To this end, an apparatus capable of producing magneto optical traps of Yb and Cs was designed, built and tested. Both atoms are produced in a dual species oven and both slowed to low speeds by a single Zeeman slower. From the Zeeman slower atoms are captured in a dual-species magneto-optical trap. To cool caesium the 852 nm D2 transition is addressed by two lasers for cooling and repump. For ytterbium the 399 nm 1S0 ⇒ 1P1 transition is addressed for the Zeeman slower and the 556 nm 1S0 ⇒ 3P1 transition is addressed for the magneto-optical trap. The 399 nm light is produced by two homebuilt diode lasers in an injection-seeding setup, which can produce up to 100 mW. The 556 nm light is produced from a commercial frequency doubled fiber laser, which can produce up to 260 mW. The Zeeman slower is characterised experimentally for both Cs and Yb, and the results compared to those of a numerical simulation of the slower for Yb. The velocity distribution exiting the slower is very sensitive to the exact magnetic field profile, the laser power and detuning of the laser light. The number of atoms loaded into the magneto-optical trap was investigated as a function of the magnetic field gradient, the laser power and the laser detuning. The capture velocity of the Yb MOT is small because the linewidth of the MOT transition is narrow, and so we investigated the influence of broadening the laser linewidth by adding multiple finely-spaced sidebands to the laser light. After optimisation the caesium MOT trapped 5.5 ×10^8 atoms at 125 ± 4 μK. The ytterbium MOT trapped 4.7 ×10^9 atoms at 81 ± 2 μK. Lastly we demonstrate that both MOTs can be produced in the same vacuum chamber simultaneously.
Ultracold mixture experiments hold the promise of providing new insights into many-body quantum systems as well as ultracold chemistry and few-body phenomena. The work presented in this thesis dealt with the construction of a new apparatus for the production and study of ultracold gases of fermionic lithium-6 and bosonic caesium-133. These isotopes offer a wide tunability in their interaction strength, both in inter-species and intra-species collisions, through magnetic Feshbach resonances. Additionally, the widely different resonance frequencies of lithium and caesium enables independent control of each of the species. With this apparatus, Bose-Einstein condensates (BEC) containing 10^4 lithium Feshbach molecules are routinely produced. The cooling system for caesium has been developed in parallel and important steps towards producing ultracold caesium gases have been made. An optical dipole trap has been loaded with 2x10^6 caesium atoms and evaporative cooling towards quantum degeneracy can now be pursued. Laser, vacuum, magnetic and control systems have been developed for the implementation of this experiment. Light produced with this laser system was used to laser cool atoms, create conservative dipole traps as well as to provide means of imaging atomic clouds. Additionally, a system to produce strong magnetic fields of up to 1400 G has been established in order to exploit the wide tunability in the atomic interactions. Software that was developed for the computerised control system facilitated the coordination of all the components involved in the experimental sequence. Measurements and calculations that showcase the functionality of relevant parts of the setup are presented in this thesis. In this experiment, lithium and caesium atoms are obtained from a novel type of Zeeman slower and are loaded into a magneto-optical trap (MOT). The system is capable of doing this independently for each of the atomic species as well as sequentially. After the MOT has been loaded with atoms, they are transferred into a conservative far-off-resonance optical dipole trap. By adjusting the interactions between atoms and lowering the depth of the dipole trap, efficient evaporative cooling of lithium was carried out from which a molecular BEC was obtained. Time-of-flight measurements were used to characterise the condensate and study its expansion dynamics.
Ultracold quantum gases have become a frequently used tool for the investigation of a variety of physical issues within the last decade, ranging from general questions in quantum physics up to special topics, e.g. quantum simulations of the physical origin of high temperature superconductivity in condensed matter physics. In this thesis, experiments are presented on the creation of an ultracold quantum gas of the fermionic potassium isotope, 40K, by all-optical means. For this purpose, a new experimental setup was designed, which was started from the scratch, with the long term goal of creating a supersolid in an optical lattice. One focus of this work lies in identifying light-assisted losses as a main restriction for achieving higher 40K atom numbers in a magneto-optical trap and in identifying light-assisted losses as a main reason for a low atomic transfer efficiency from a magneto-optical trap into an optical, quasi-electrostatic dipole trap (further called optical dipole trap) for 40K atoms. Naturally, another focus of this work lies in the application of different techniques to reduce these from light-assisted losses dependent restrictions. A two-dimensional magneto-optical trap, located in the prechamber of a vacuum system, creates a continuous beam of 40K atoms. The atoms of this beam are recaptured in the main chamber of the vacuum system by a spatial dark spot magneto-optical trap. Here, a dark magneto-optical trap is implemented instead of a standard magneto-optical trap in order to reduce light-assisted losses. Up to 1x109 atoms at temperatures around 240 μK are trapped in the dark magneto-optical trap. This is a factor of ten higher compared to atom numbers of a 40K standard magneto-optical trap. The light-assisted losses in the 40K magneto-optical trap were measured as a function of the total trapping intensity and as a function of the repumping intensity. In both cases, the smallest observed loss coefficient was β ≈ 2.5x10-10 cm3/s. Compared to the smallest values of β of all other alkali elements (apart from the bosonic potassium isotopes), the light-assisted losses with 40K atoms are approximately two orders of magnitude larger. Up to 6x105 atoms were transferred from the spatial dark magneto-optical trap into the quasi-electrostatic dipole trap, which was created by using the trapping potential induced by radiation emitted by a CO2 laser operating at a wavelength near 10.6 μm. In order to reach the here reported atom numbers in the optical dipole trap, its trapping volume had to be increased in comparison to a simple dipole trap arrangement as induced with a single focused dipole trapping beam. Simultaneously, the strong confinement, as present in a tigthly focused single beam trap and as required for a subsequent evaporative cooling, had to be maintained. For this purpose, the trapping laser beam of the optical dipole trap was retroreflected in such a way that an overall optical dipole trap with a nearly doubled waist and a slightly deeper potential depth was created. By this trapping arrangement, the number of atoms confined in the optical dipole trap could be nearly doubled compared to a simple dipole trapping arrangement consisting of a single tightly focused trapping beam. The atomic transfer process itself, from the magneto-optical trap into the optical dipole trap, was organized in two steps. In a first step, the dark magneto-optical trap was transformed into a compressed magneto-optical trap by increasing the field gradient and bringing the cooling laser closer to resonance as compared to the detuning of the usual magneto-optical trap. In contrast to other experiments, the repumping laser of the magneto-optical trap was detuned to the blue of the used repumping transition. As a consequence, atoms were optically pumped into the upper hyperfine ground state |S1/2, F=7/2〉, realizing a temporal dark, compressed magneto-optical trap in analogy to the operation of a temporal dark magneto-optical trap. By this approach, light-assisted losses were reduced. In the second step of the transfer process, the light fields of the magneto-optical trap and the optical dipole trap were alternately switched on and off. In this way, a spatially inhomogeneous ac Stark shift of the cooling transitions due to the spatially inhomogeneous intensity distribution of the dipole trapping beam was avoided and again light-assisted losses were reduced during the loading phase of atoms from the compressed magneto-optical trap into the optical dipole trap. The loading rate was measured to be approximately 2.8x108 atoms/s. The light-assisted losses during loading of the optical dipole trap were determined to be β ≈ 4.8x10-10 cm3/s. In the final step of the experiment, the temperature of the atomic ensemble confined in the optical dipole trap was further reduced by evaporative cooling. After an evaporation time of about 30 s, temperatures of 140 nK were reached at typical atom numbers of 12000. The corresponding Fermi temperature is TF = 110 nK yielding a ratio T/TF ≈ 1.3 which indicates that the fermionic potassium atoms are cooled close to the onset of quantum degeneracy. In the future, an additional trapping and evaporative cooling step within a magnetic quadrupole trap should allow for an improved loading efficiency into the far detuned optical dipole trap and finally allow for a subsequent evaporative cooling to a quantum degenerate Fermi gas.
This thesis reports the observation of interspecies Feshbach resonances in an ultracold mixture of Rb and Cs atoms. A versatile combined magnetic and optical potential has been designed and constructed which is capable of bringing both 87Rb and 133Cs to degeneracy, and reaching high phase-space density in 85Rb. High phase-space density mixtures are the first step required in the production of ultracold polar molecules, the topic of much current research. The apparatus capitalises on the efficient capture of atoms by a magnetic trap from a magneto-optical trap, and the efficient sympathetic cooling of Cs by Rb therein. Upon transfer to the crossed optical dipole trap condensates in excess of 1×10687Rb atoms and approximately 1×105133Cs atoms are produced after direct evaporation and gravito-magnetic tilting of the potential. The observation of six interspecies 87Rb-133Cs Feshbach resonances are reported, three of which had only been predicted theoretically, allowing testing and development of the theoretical model. Furthermore, the extrapolation of this model has predicted numerous Feshbach resonances between 85Rb and 133Cs, none of which have been experimentally observed prior to this work. The versatile nature of this apparatus is discussed, including the application of the current system to cooling of 85Rb. Initial experiments observed seven interspecies resonances, including a broad s-wave resonance at a magnetic field of (644±2) G which is in excellent agreement with the theoretical prediction. Further work has revealed that fourteen Feshbach resonances exist in the 0-700 G magnetic field range between 85Rb and 133Cs atoms in the ∣2,+2⟩ and ∣3,+3⟩ states, respectively. Several of these resonances would be ideal for magneto-association of RbCs molecules, prior to transfer to the rovibrational ground-state.