Abstract Techniques to manipulate the individual constituents of an ultracold mixture are key to investigating impurity physics. In this work, we confine a mixture of hyperfine ground states of 87Rb atoms in a double-well potential. The potential is produced by dressing the atoms with multiple radiofrequencies. The amplitude and phase of each frequency component of the dressing field are controlled to independently manipulate each species. Furthermore, we verify that our mixture of hyperfine states is collisionally stable, with no observable inelastic loss.
Abstract Techniques to manipulate the individual constituents of an ultracold mixture are key to investigating impurity physics. In this work, we confine a mixture of hyperfine ground states of 87Rb atoms in a double-well potential. The potential is produced by dressing the atoms with multiple radiofrequencies. The amplitude and phase of each frequency component of the dressing field are controlled to independently manipulate each species. Furthermore, we verify that our mixture of hyperfine states is collisionally stable, with no observable inelastic loss.
Abstract Methods to manipulate the individual constituents of an ultracold quantum gas mixture are essential tools for a number of applications, such as the direct quantum simulation of impurity physics. We investigate a scheme in which species-selective control is achieved using magnetic potentials dressed with multiple radiofrequencies, exploiting the different Landé g F -factors of the constituent atomic species. We describe a mixture dressed with two frequencies, where atoms are confined in harmonic potentials with a controllable degree of overlap between the two atomic species. This is then extended to a four radiofrequency scheme in which a double well potential for one species is overlaid with a single well for the other. The discussion is framed with parameters that are suitable for a 85 Rb and 87 Rb mixture, but is readily generalised to other combinations.
This thesis presents experimental results which utilise multiple-radiofrequency-dressed potentials as a method of confinement for cold atoms, and details progress towards investigations of the out-of-equilibrium dynamics of two-dimensional systems. This work has demonstrated the matter-wave interference of Bose gases released from a double-well potential. We investigate the scaling of the matter-wave inteference fringe wavelength, verify the coherence of the experimental procedure, and demonstrate the controllability of the multiple-radiofrequency-dressed potentials to load a prescribed fraction of atoms into each potential well. Methods presented elsewhere were also used to produce two-dimensional Bose gases in the Berezinskii-Kosterlitz-Thouless regime. We present a discussion of phase correlations in two-dimensional Bose gases, including their time-dependent behaviour as the system equilibrates following a quench. Radiofrequency-dressed potentials are intrinsically state-selective, as we demonstrate through manipulations of a mixture of Rb-87 atoms in different hyperfine states within a double-well potential. This can be used to implement species-selective potentials and is a promising architecture for future experiments concerning impurity physics. We find this mixture to be long-lived in the radiofrequency-dressed potential, in stark contrast to recent experiments on this apparatus, in which mixtures of radiofrequency-dressed Rb-87 and Rb-85 were found to suffer from very fast inelastic loss. Furthermore, we describe the application of machine learning methods to optimise the experimental sequence, in particular the stages of laser cooling and evaporative cooling which are necessary to achieve a quantum gas. We compare several machine learning methods, including Artificial Neural Networks and Gaussian Process regression, and determine experimental parameters which have the strongest influence on the desired outcome. We focus our efforts initially on producing Bose-Einstein condensates with very large atom numbers, although we later highlight the versatility of this method by improving other metrics, such as sequence repetition rate or the final temperature of the gas.
We have developed experimental techniques to confine two species of ultracold atoms in trapping potentials that are independently controlled by applied radiofrequency (rf) fields. Elementary species-selective manipulations are demonstrated for the isotopes 85Rb and 87Rb, such as changing the relative positions of the species. More detailed manipulations are possible, such as creating a double-well potential for one species that is overlapped with a single harmonic well of the other. We observed that both isotopes have long lifetimes in the rf-dressed potentials when separate; this is well known for 87Rb which has very favourable collision properties, but the stability of 85Rb in rf-dressed potentials had not previously been demonstrated. A large rate of inelastic inter-species collisions was observed when the clouds were brought together, and our results identify this as being due to two-body collisions between 85Rb and 87Rb. We explain the origin of these losses using a qualitative model. Our experimental data is compared to computations of the inelastic loss rate coefficients that have been carried out using coupled-channel calculations by theoretical chemists at Durham University. We have created double-well potentials for ultracold atoms by dressing with three rfs, as reported in our published work. Since this first demonstration we have eliminated sources of noise from the experimental apparatus using a systematic method, enabling a quantum degenerate gas of 87Rb to be loaded into a double-well potential with 6.7 micron separation between the wells. This is an order of magnitude improvement over our previously published work, and this separation is confirmed by matter-wave interference observed between atoms released from the two wells. In the near future, this apparatus will be able to split a quantum degenerate, two-dimensional gas, as has been performed for one-dimensional gases elsewhere. Moreover, many technical improvements have been implemented, allowing large amounts of data to be measured under highly reproducible conditions.
Time-varying fields are widely used to extend the accessible range of trapping potentials for ultracold atoms. This work explores two very different examples of such fields, in the radiofrequency and optical regimes, whose interactions with trapped atoms can both be described in terms of the dressed atom picture. Forming the basis of this work are radiofrequency dressed adiabatic potentials based on macroscopic trapping coils. Atoms are confined at the south pole of the resultant oblate spheroidal trapping surfaces. This work describes the extension of these potentials by two different methods: the application of multiple dressing radiofrequencies, and addition of a rapidly-scanned optical dipole trap. This is the first experimental demonstration of a multiple-radiofrequency dressed adiabatic potential, explored using ultracold 87Rb atoms confined in a highly configurable double well. Due to the independent generation of each constituent dressing frequency, the depth of each trapping well and the height of the barrier are easily manipulated, enabling precise and reliable transfer of atoms between the available trapping geometries. Experimental work includes an exploration of the potential-shaping capabilities of the three-radiofrequency system, and characterisation of the potential landscape using radiofrequency spectroscopy with good agreement to the eigenvalues numerically calculated using Floquet theory. This initial exploration of multiple-radiofrequency techniques lays the groundwork for applications in studying double well physics in a two-dimensional system, and independent state or species selective manipulation of trapped atoms. The potential shaping capabilities of this method can also be extended by applying additional trapping frequencies. In a supplementary line of experimental work, an optical dipole trapping system has been constructed, and the trapping beam aligned to the lower surface of the radiofrequency dressed trapping shell in order to sculpt the radial confinement. Beam shaping is achieved using an acousto-optic deflector, which can be used to produce either a composite array of static deflected beams, a rapidly-scanned painted potential, or some combination of the two approaches. The development and extension of the experimental apparatus required to implement these enhanced dressed state potentials is explored, and the challenges of their experimental implementation considered.
This thesis presents experimental work to investigate the properties of ultracold rubidium atoms in multiple-radiofrequency dressed potentials and detailed theoretical calculations to understand this system. The use of multiple frequencies rather than a single dressing frequency increases the complexity of the spectrum, and we present theoretical calculations that predict all possible transitions as well as the corresponding strengths. Extensive measurements for dressing with a single and multiple frequencies have verified these predictions, observing transitions up to tenth order in the probe field. We observe previously unknown transitions even for the single-frequency case and have uncovered further transitions which we explain by the non-linearity of the Zeeman effect even at our small field strengths. These results are used to understand the absorption spectrum as the potential transforms from a single to a double well in order to split a Bose-Einstein condensate. This is important for applications since spurious noise leads to atom loss if it is resonant with a transition between eigenstates. We present a new method of splitting, replacing adiabatic ramps with a projection, that reduces the duration of the ramps as well as the spectrum of resonances throughout the ramps, thus reducing losses. The new experimental techniques will allow observation of the thermalisation of a two-dimensional quantum system using ultracold gases. We discuss how multiple-radiofrequency dressed potentials can be used to confine ultracold atoms in two dimensions, and subsequently split the trapped cloud into two parallel sheets. After a varying hold time, the atoms can be released and overlapped to produce interference fringes. Analysis of the fringes determines how the relative phase between the two sheets thermalises. Information can be accumulated to determine the probability distribution of the integrated contrast, which can then be compared to the equilibrium distribution to determine if and how the relative phase thermalises. This promises to answer key questions in non-equilibrium physics.
We present the first experimental demonstration of a multiple radio-frequency dressed potential for the configurable magnetic confinement of ultracold atoms. We load cold 87Rb atoms into a double-well potential with an adjustable barrier height, formed by three radio-frequencies applied to atoms in a static quadrupole magnetic field. Our multiple radio-frequency approach gives precise control over the double-well characteristics, including the depth of individual wells and the height of the barrier, and enables reliable transfer of atoms between the available trapping geometries. We characterize the multiple radio-frequency dressed system using radio-frequency spectroscopy, finding good agreement with the eigenvalues numerically calculated using Floquet theory. This method creates trapping potentials that can be reconfigured by changing the amplitudes, polarizations, and frequencies of the applied dressing fields and easily extended with additional dressing frequencies.