This thesis presents work performed to obtain an optical mask for conducting matter-wave interfer-ometry experiments with ultra-cold rubidium-85 atoms. The optical mask is essentially an absorptive diffraction grating made of light, and it can imprint a periodic density pattern on a cloud of atoms with a period of half the wavelength of the light used. The mask has an analogous effect to quickly passing a diffraction grating through the cloud, removing atoms located at the nodes of the grating (though the optical mask depumps atoms to a different hyperfine ground state rather than actually removes them). The mask should be useful in performing precision measurements of physical constants such as the fine-structure constant α, and acceleration due to gravity g. The thesis briefly expounds major historical developments in atom interferometry and laser cooling. Prerequisites for an optical mask include a functioning magneto-optical trap, and the ability to perform polarisation-gradient cooling on trapped atoms. The theory of these two techniques is reviewed and the principle of the the optical mask is explained. The experimental apparatus that was constructed to realise the optical mask is described, and technical developments made along the way are presented. Finally, aspects of the ex-periment relevant to the optical mask are characterised. The experiment can reliably produce samples of cold atoms at temperatures of around 10 µK, and can use the cold atoms as an optical frequency reference accurate to ∼ 1 MHz. Clear evidence that the cloud of atoms is being density-modulated by the optical mask is presented. Improvements required to make the experiment into a fully-functioning interferometer are also discussed. i ii
This thesis presents work performed to obtain an optical mask for conducting matter-wave interfer-ometry experiments with ultra-cold rubidium-85 atoms. The optical mask is essentially an absorptive diffraction grating made of light, and it can imprint a periodic density pattern on a cloud of atoms with a period of half the wavelength of the light used. The mask has an analogous effect to quickly passing a diffraction grating through the cloud, removing atoms located at the nodes of the grating (though the optical mask depumps atoms to a different hyperfine ground state rather than actually removes them). The mask should be useful in performing precision measurements of physical constants such as the fine-structure constant α, and acceleration due to gravity g. The thesis briefly expounds major historical developments in atom interferometry and laser cooling. Prerequisites for an optical mask include a functioning magneto-optical trap, and the ability to perform polarisation-gradient cooling on trapped atoms. The theory of these two techniques is reviewed and the principle of the the optical mask is explained. The experimental apparatus that was constructed to realise the optical mask is described, and technical developments made along the way are presented. Finally, aspects of the ex-periment relevant to the optical mask are characterised. The experiment can reliably produce samples of cold atoms at temperatures of around 10 µK, and can use the cold atoms as an optical frequency reference accurate to ∼ 1 MHz. Clear evidence that the cloud of atoms is being density-modulated by the optical mask is presented. Improvements required to make the experiment into a fully-functioning interferometer are also discussed. i ii
This thesis details the experimental investigation of a new type of atom interferometer using rubidium-85 atoms in the unexplored near-resonant domain. A cold cloud of atoms, all prepared in the same hyperfine ground state, are subjected to temporally periodic pulses of near-resonant standing waves of light. The standing wave pulses are made to act like an absorption grating where only atoms located around the low intensity region about the nodes remain in the initial ground state, the rest are pumped into a dark hyperfine ground state. The output of the atom interferometer is a measure of the fraction of atoms remaining in the initial ground state after N standing wave pulses for different times between the pulses. An increased survival rate is observed for certain times between pulses due to the occurrence of a coherence echo and the matter wave Talbot effect. This feature allows us to use our atom interferometer to make measurements of the Talbot time which is an important parameter in determinations of the fine structure constant alpha.\n\nWe provide a theoretical model to describe the relevant physics behind our atom interferometer that compares well with our empirical results. Design and implementation of the apparatus are discussed along with characterisation of parameters such as pulse duration, pulse number, and frequency. Finally we include a demonstration of how, in principle, our atom interferometer could be used to make precision measurements of the Talbot time along with some of the necessary steps to bring it in line with current leading measurements.
Atom optics has been a highly active field of research with many scientific breakthroughs over the past two decades, largely due to successful advances in laser technology, microfabrication techniques, and the development of laser cooling and trapping of neutral atoms. This dissertation details several atom optics experiments with the motivation to develop tools and techniques for precision atom wave interferometry. It provides background information about atom optics and the fundamentals behind laser cooling and trapping, including basic techniques for cold gas thermometry and absorptive detection of atoms. A brief overview of magnetic trapping and guiding in tight wire-based traps is also provided before the experimental details are presented. ❧ We developed a novel laser source of 780 nm light using frequency-doubled 1560 nm fiber amplifier. This laser system provided up to a Watt of tunable frequency stabilized light for two Rb laser cooling and trapping experiments. One system generates Bose-Einstein condensates in an optical trap while the second is based on atom chip magnetic traps. The atom chip system, detailed in this thesis, was designed and built to develop the tools necessary for transport and loading large numbers of cold atoms and explore the potential for guided atom interferometry. Techniques and results from this experiment are presented, including an efficient magnetic transport and loading method to deliver cold atom to atom chip traps. We also developed a modeling tool for the magnetic fields formed by coiled wire geometries, as well as planar wire patterns. These models helped us design traps and determine adiabatic transportation of cold atoms between macro-scale traps and micro-traps formed on atom chips. Having achieved near unity transfer efficiency, we demonstrated that this approach promises to be a consistent method for loading large numbers of atoms into micro-traps. Furthermore, we discuss an in situ imaging technique to investigate magnetic field contours of the traps and the dynamics of atoms within those confining potentials. We also controlled the propagation along the atom chip guides by accelerating atoms with longitudinal magnetic gradients, and investigated an atom focusing scheme. ❧ While the atom chip wire guides perform a role analogous to optical fibers guiding light waves, ""free space"" cold atoms offer great opportunity for precision interferometry. We describe a second on-going atom optics experiment that measures gravity gradients using a pair of atom fountain interferometers separated by one meter. We have demonstrated Gravity Gradiometer resolution down to 4x10⁻⁹ g/m using a 40 kg test mass. The atomic physics subsystem is described in detail, including the vacuum, cold atom source, optics, magnetic coils and shields, and vibration isolation and compensation. The system is designed to be a compact, robust, transportable instrument, taking strides towards future gravity gradient measurements in the field. In the realm of space applications, there has been interest for micro-gravity science experiments aboard the International Space Station, along with instrument development for gravity mapping of Earth and planetary bodies with satellite-based instruments. Furthermore, there are ground-based applications for gravity imaging of local density distributions, precision measurement of gravity, as well as proposals for redefining the kilogram, detecting gravitational waves and determining the Gravitational constant.
This thesis describes the construction and characterisation of a standard six-beam configuration magneto-optical trap (MOT) used to cool and trap neutral 85Rb atoms. This system is capable of producing a cloud of ~108 atoms whose average temperature is in the sub-Doppler range. The fabrication process for the production of subwavelength tapered optical nanofibres (TONFs) is also described, as well as their guiding properties. These have many proposed applications in the field of cold atoms. However, significant technical progress must be made before many of these applications can be realised. The objective of the research work was to combine the TONF with the cold atom setup to develop tapered optical nanofibres as a tool that can be used in cold atom experiments. This thesis presents a novel technique to measure the characteristics of a MOT by monitoring the spontaneous emission from the trapped atoms coupled into the guided mode of the TONF. This has allowed the measurement of cloud profile, loading times and decay times. This serves as a demonstration that TONFs can indeed play an important role in cold atom physics. The work presented here also describes a theoretical model for an atom diffraction grating based on oscillating magnetic fields produced by an array of current carrying wires.
An optical mask is a standing wave of light in resonance with an open atomic transition. The mask depletes the population of the initial internal state everywhere except in a narrow vicinity of the nodes. We have experimentally explored the optical mask as a tool for creating a periodic pattern in the density of cold rubidium atoms (in a particular hyperfine ground state) and also as an instrument for imaging the density in situ. The instrumental width of the imaging technique is a small fraction of the optical wavelength and is limited due to an effect of the dipole force rather than the initial velocity spread.
Abstract : An atom interferometer is constructed in which the atom waves are physically separated prior to being recombined. Fabricated transmission gratings are used as optical elements for the matter waves. Atom interferometers should be useful in studies of atomic properties, tests of basic quantum physics, for metrology, as rotation sensors, and perhaps ultimately as devices to make ultra- small structures using atom holograms. During the last year our atom interferometer has evolved from a rough plan to an essentially complete device. At present all the major components of the interferometer have been built, and tested at least once. The system will be given its first real try in the next year. Our interferometer consists of three 0.2 microns - period diffraction gratings equally spaced approx. 0.65 m apart in our atomic beam machine. The maximum separation of the beams will be approx. 60 microns. The first two gratings separate and redirect the atomic beam forming a standing wave interference pattern in the atomic flux at the third grating, which acts like a mask to sample this pattern. A principle technical obstacle is the mechanical vibrations of our machine which will blur the interference pattern.
This thesis describes recent experiments conducted on a Bose-Einstein condensate in an optical waveguide. This optical potential confines the atoms against gravity in the vertical dimension, guiding them to freely propagate in the horizontal. Being supported against gravity enables long expansion times of hundreds of milliseconds, facilitating techniques such as delta-kick cooling. In an ideal atom interferometer, the beamsplitters would impart a large momentum splitting between the interfering states in order to increase the signal. However, their effective use requires both an even narrower momentum width source and good vibration isolation. Three such techniques were investigated in this thesis: reflection from a repulsive light potential barrier, Bragg transitions from an optical lattice (which are effectively bouncing atoms off a moving grating), and Bloch-acceleration by loading the atoms into such an optical lattice and then accelerating the combined system. It is found that a combination of both Bragg and Bloch provides the most promising route to truly large momentum transfer in a system which is sensitive only to acceleration. Lastly, large-momentum transfer techniques can be used to effectively increase the way in which the output signal scales with time, creating interferometers which generate the same sensitivity faster (increasing the bandwidth of a sensor), or generate a much better sensitivity in the same time. We form a dual condensate of both rubidium-85 an rubidium-87 via sympathetic cooling. The collisional properties of Rubidium-85 can be modified by applying an external magnetic field, at an easy-to-experimentally-reach value of between 150-170G. It turns out that this combination of atomic isotopes is ideal for an interferometric test of the weak equivalence principle, one of the underpinnings of General Relativity. This thesis also presents the results of the first Bose-condensed version of such a dual-species interferometer. By removing any 87Rb left after condensation, we have created a pure 85Rb BEC. Using this we can now explore how the inter-atomic interactions affect the phase shift of a condensed atom interferometer, as we have complete control over the interaction strength. A condensate with no inter-particle interactions should behave like an ideal particle. With a small attractive interaction between atoms, it is possible to create a self-trapped cloud of atoms which propagates dispersionlessly. This cloud is known as a soliton, and it is predicted to have even more interesting quantum mechanical properties. For example it is predicted that by colliding two such solitons, an entangled state can be generated. Our results indicate that the dispersionless character of the soliton out-performs all other interaction strengths in an atom interferometer, including even the non-interacting cloud.