Atom interferometry allows for new precision limits in measurement and a key component of it is the beam-splitter. In this thesis we look at the properties of an all optical beam-splitter, which is created by the overlap of two Gaussian laser beams,which also function as waveguides for the Bose-Einstein condensate. For this mainly the split step Fourier method is used to model the propagation of a low density Bose-Einstein condensate of atoms. The two main areas of interest are the splitting and recombination properties. \nThe splitting is studied both in two and one dimensions. For the one-dimensional case both standing and propagating waves are used to find the ideal splitting conditions of a balanced splitting and transmission. The result of these methods are in general agreement with the exception that for the propagating wave some of the atoms can become localised inside the beam-splitter. For the propagation it was found that the splitting is not perfectly coherent as the outputs are slightly deformed. However, when looking at the two-dimensional case we see that the splitting is not even close to being coherent. This is because the beam-splitter excites the incoming wave into higher transverse modes of the waveguides. To compensate for this the depth of the waveguides was lowered and the width narrowed to reduce the potential kinetic energy that the atoms could acquire and to increase the separation between the eigenstates to lower the probability of excitation into higher modes, respectively. Nonetheless, these investigations improved the splitting balance but it is still not coherent. Another method investigated was the introduction of a third laser which acts as a filling to reduce the depth of the potential well where the lasers generate the beam-splitter. This approach does not improve the splitting. \nHence instead of recombining single mode waves we used a multimode approach. For this we found that the mirror position is crucial for certain parameters as the longitudinal momentum is not necessarily the same in the reflection and transmission waveguide. Our investigations have generated interference fringes in this model \nsystem with a fractional height up to 33%.
Atom interferometry allows for new precision limits in measurement and a key component of it is the beam-splitter. In this thesis we look at the properties of an all optical beam-splitter, which is created by the overlap of two Gaussian laser beams,which also function as waveguides for the Bose-Einstein condensate. For this mainly the split step Fourier method is used to model the propagation of a low density Bose-Einstein condensate of atoms. The two main areas of interest are the splitting and recombination properties. \nThe splitting is studied both in two and one dimensions. For the one-dimensional case both standing and propagating waves are used to find the ideal splitting conditions of a balanced splitting and transmission. The result of these methods are in general agreement with the exception that for the propagating wave some of the atoms can become localised inside the beam-splitter. For the propagation it was found that the splitting is not perfectly coherent as the outputs are slightly deformed. However, when looking at the two-dimensional case we see that the splitting is not even close to being coherent. This is because the beam-splitter excites the incoming wave into higher transverse modes of the waveguides. To compensate for this the depth of the waveguides was lowered and the width narrowed to reduce the potential kinetic energy that the atoms could acquire and to increase the separation between the eigenstates to lower the probability of excitation into higher modes, respectively. Nonetheless, these investigations improved the splitting balance but it is still not coherent. Another method investigated was the introduction of a third laser which acts as a filling to reduce the depth of the potential well where the lasers generate the beam-splitter. This approach does not improve the splitting. \nHence instead of recombining single mode waves we used a multimode approach. For this we found that the mirror position is crucial for certain parameters as the longitudinal momentum is not necessarily the same in the reflection and transmission waveguide. Our investigations have generated interference fringes in this model \nsystem with a fractional height up to 33%.
The goal of this thesis is to study a novel approach towards all-optical continuous waveguide interferometers based on quasi-Bragg waveguide beam splitters. The Bragg beam splitter for matter waves is comprised of a periodic lattice produced by partial or full interference between two Gaussian laser beams and are used as waveguides for ultra-cold atoms. In the experimental conditions, where both waveguides have wide and deep potentials, atoms are unable to split from a single lattice. Instead, several spatially separated regions of the optical lattice are required, fulfilling the Bragg condition. This thesis will present clear evidence of the detection of discrete quantised momentum states of the atoms, created from a process of ultra-cold atoms undergoing interference in the single arms of a Michelson interferometer. A detailed investigation of the different regimes of the splitter has been completed by varying the initial atom velocity and the amplitude of the optical lattice. The findings from the study have produced optimal parameters where the splitting ratio of the atoms between the two waveguides can be finely controlled by the amplitude of the lattice within certain limits. A Michelson interferometer configuration is used to test the coherence of the atoms by decelerating them after splitting in the lattice. The return velocity of atoms is set to be equal to the initial velocity and the two clouds are recombined using the original lattice. A sinusoidal oscillation of the atomic momentum between the two output ports of the beam splitter is visible. However, only interference in single Michelson interferometer arms has been observed. The reasons are discussed in detail along with plans to move towards an all-optical design.
We perform an experimental and theoretical study of a novel distributed quasi-Bragg splitter for cold atoms propagating in crossed optical waveguides. The atoms are guided by horizontal red-detuned laser beams which cross with an angle of roughly 90°. The lattice formed by the interference between the two waveguides is used as a quasi-Bragg splitter to continuously deflect the atomic flux from one waveguide into the other. In the limit of strong waveguide confinement and depending on the velocity of the cloud, three main regimes are observed corresponding (1) to the absence of reflection, (2) to partial reflection and (3) to full reflection into the second waveguide. In view of the application to atom interferometry, the condition to split the cloud into mainly two equally-populated fragments is only met in the highest velocity regime, where the fraction of reflected and transmitted atoms can be controlled by tuning the lattice height. A diagnostic of the momentum distribution shows that a quasi-Bragg splitter with the occupation of mainly two momentum states is achieved in this regime. This behaviour can be understood by considering the band structure associated with the potential in the crossing region and agrees with numerical simulations of the atomic dynamics.
The key elements for atom interferometers are atomic beam splitters and recombiners. An atomic beam splitter separates a single atomic wave function into a superposition state corresponding to two center of mass wavepackets that propagate in different spatial directions and a recombiner reunites the atomic beams. We report here on the investigation of 1) an atomic beam splitter with large separation angle based on the optical Stern Gerlach effect, 2) an atomic mirror based on the reflection of atoms by different types of evanescent waves (a simple evanescent wave and a surface plasmon wave), 3) an atomic beam splitter realized by the partial spatial reflection of an atom by an evanescent wave and 4) diffraction of atoms by a standing evanescent wave.
Bose–Einstein condensate (BEC)-based atom interferometry exploits low temperatures and long coherence lengths to facilitate high-precision measurements. Progress in atom interferometry promises improvements in navigational devices like gyroscopes and accelerometers, as well as applications in fundamental physics such as accurate determination of physical constants. Previous work demonstrates that beam splitters and mirrors for coherent manipulation of dilute BEC momentum in atom interferometers can be implemented with sequences of non-resonant standing-wave light pulses. While previous work focuses on the optimization of the optical pulses’ amplitude and duration to produce high-order momentum states with high fidelity, we explore how varying the shape of the optical pulses affects optimal beam-splitter performance, as well as the effect of pulse shape on the sensitivity of optimized parameters in achieving high fidelity in high-momentum states. In simulations of two-pulse beam splitters utilizing optimized square, triangle, and sinc-squared pulse shapes applied to dilute BECs, we, in some cases, reduce parameter sensitivity by an order of magnitude while maintaining fidelity.
Bose-Einstein condensates are coherent matter waves, produced by cooling gaseous atomic clouds to ultra-low temperatures. For applications in atom interferometry and precision measurements, Bose-condensed sources present an intriguing alternative to thermal atoms. Although the current sensitivity achievable with interferometers using coherent atoms is not comparable to thermal beam machines (mainly due to the lower flux), there are promising ways to utilise the potential of Bose-condensed sources for atom interferometry. Among those is the low momentum width of Bose-Einstein condensates, which can generally be well controlled and is advantageous for increased interferometric sensitivities by implementing large momentum transfer beam splitters. As part of this thesis, experimental and theoretical investigations are presented to investigate the potential of Bose-Einstein condensates for such applications. We shall present the quantum projection noise limited performance of a Ramsey interferometer operating on the atomic clock transition of a freely expanding cloud of Bose-condensed rubidium 87 atoms. The results include Ramsey fringes of high visibility, not measurably affected by atomic interaction-induced dephasing effects. The achievement and detection of the quantum projection noise limit rely critically on the precision and accuracy of both the imaging setup and the coupling scheme in the interferometric beam splitters. The stabilisation of the beam splitters via an optical Sagnac interferometer is the basis for the quantum projection noise limited performance of the interferometer presented. For an increase of bandwidth and flux in atom interferometric measurements, it is advantageous to use a continuous atomic beam. A truly continuous coherent atom source has not been realised to date, and we present results on a pumping mechanism in this thesis, as a decisive step towards a continuous atom laser. By the investigation of different momentum resonances, we find that the pumping scheme relies on a Raman superradiance-like process. Finally, the thesis demonstrates two interaction measurements in rubidium. The strong mean field interactions due to the high densities in Bose-Einstein condensates are used to probe the potential of a rubidium 87 condensate with an atom laser. The measurement allows a determination of the scattering length between the two atomic states involved. In addition to this two-body scattering scheme, we present a measurement of three-body loss coefficients, extracted from loss curves in rubidium 85 Bose-Einstein condensates. The measurement provides new upper bounds on the three-body loss coefficients at the scattering lengths considered.
Bright solitons in a quasi-1D Bose–Einstein condensate can be used to enhance precision in matter-wave interferometry, due to their inherent robustness and support against dispersion. Such a soliton interferometer typically relies on a potential barrier used to split a single soliton into two smaller coherent solitons which can then be recombined on the same barrier. In this thesis we examine two extensions to this scheme. Firstly, we investigate a binary BEC system consisting of two bright solitons which are coupled through a mutual nonlinear interaction term. We derive a set of conditions under which the two components can be separated on a potential barrier and use numerical simulations to probe the regimes beyond which this mathematical treatment is applicable. We then use the numerical simulations to look at the effect of the nonlinear coupling on the dynamics of the binary solitons interacting with the barrier. We also look at the interference behaviour found by doubling the simulation time in either a ring trap or a harmonic trapping potential (to ensure recombination on the barrier); as well as the case where the solitons start spatially separated on either side of the barrier in order to find conditions under which the solitons will combine on the barrier. We find a good agreement between the analytical predictions and the results of simulations. Beyond the regions of parameter space where the predictions are expected to hold, we find complex transmission and interference behaviour as a result of nonlinear effects. The second part of this thesis consists of an examination of the prospect of using a subwavelength barrier scheme in a soliton interferometry experiment. This involves using two resonant coupling beams in a Λ-system with a spatially varying intensity. Under certain conditions, this can be used to form an effective potential barrier with a width which is not diffraction-limited. We look at suitable parameter regimes for such a barrier to split and recombine solitons in an interferometer and probe the effects of possible complications such as misalignment in the beams and different scattering lengths in the different states. We simulate the soliton interferometer using the full three component GPE as well as the single component analogue with the effective potential in order to characterise the soliton behaviour and the dependence of the interferometer sensitivity on the system parameters. We find a trend towards an idealised soliton interferometer with a decreasing value of the parameter, w, controlling the barrier characteristics. Also, we demonstrate an agreement between the relevant three component GPE and the analogous single component GPE, in the limit of strong coupling fields.