This dissertation details the construction of a neutral ultracold atom system along with one completed experiment and one future experiment. This systems contains a two-stage slowing/cooling scheme with a Zeeman slower and a magneto-optical trap (MOT), as well as an in-vacuum optical cavity. The optical cavity was specially designed for precision measurement experiments. When near resonance atoms are present in the cavity, they cause a phase shift in the light. This enables use of the cavity light transmission as a minimally invasive feedback for information on the presence of atoms in the cavity. We look at this atom cavity interaction in two regimes: a deep lattice and a shallow lattice. Atoms confined to a deep optical lattice are constrained to thousands of individual wells. Any feedback on the lattice from motion of atoms in a single well is amplified by thousands due to this repetition. We exploit this deep lattice uniformity to measure atomic motion immediately after release from the deep lattice using another very shallow probe lattice. The feedback from the atomic motion on the probe lattice cavity transmission is used to measure the atomic sample’s original temperature without losing the atoms. We measure the sample's temperature in both the axial and radial direction, with the axial measurement taking $<10~\\mu$s with the ability to recapture $75\\%$ of the atoms back into the lattice. Atoms in a shallow lattice have wavefunctions spread out across multiple lattice cites. This enables them to undergo a process called Bloch oscillations, which requires a periodic potential plus a uniform force (gravity). I perform a simulation of the coupled atom and lattice distributions to optimize our experimental realization of the system. The atomic oscillations due to gravity imprint their oscillation frequency on the transmission of cavity light, which enables us to measure the force on the atoms which is directly proportional to the frequency. We plan on using this feature to perform a precision measurement on our atoms to look for ultralight dark matter. I estimate our limits on this type of dark matter, and discuss the future of this experiment.
This dissertation details the construction of a neutral ultracold atom system along with one completed experiment and one future experiment. This systems contains a two-stage slowing/cooling scheme with a Zeeman slower and a magneto-optical trap (MOT), as well as an in-vacuum optical cavity. The optical cavity was specially designed for precision measurement experiments. When near resonance atoms are present in the cavity, they cause a phase shift in the light. This enables use of the cavity light transmission as a minimally invasive feedback for information on the presence of atoms in the cavity. We look at this atom cavity interaction in two regimes: a deep lattice and a shallow lattice. Atoms confined to a deep optical lattice are constrained to thousands of individual wells. Any feedback on the lattice from motion of atoms in a single well is amplified by thousands due to this repetition. We exploit this deep lattice uniformity to measure atomic motion immediately after release from the deep lattice using another very shallow probe lattice. The feedback from the atomic motion on the probe lattice cavity transmission is used to measure the atomic sample’s original temperature without losing the atoms. We measure the sample's temperature in both the axial and radial direction, with the axial measurement taking <10mus with the ability to recapture 75 of the atoms back into the lattice. Atoms in a shallow lattice have wavefunctions spread out across multiple lattice cites. This enables them to undergo a process called Bloch oscillations, which requires a periodic potential plus a uniform force (gravity). I perform a simulation of the coupled atom and lattice distributions to optimize our experimental realization of the system. The atomic oscillations due to gravity imprint their oscillation frequency on the transmission of cavity light, which enables us to measure the force on the atoms which is directly proportional to the frequency. We plan on using this feature to perform a precision measurement on our atoms to look for ultralight dark matter. I estimate our limits on this type of dark matter, and discuss the future of this experiment.
We propose and demonstrate real-time subwavelength cavity QED measurements of the spatial distribution of atoms in an optical lattice. Atoms initially confined in one ``trap'' standing wave of an optical cavity mode are probed with a second ``probe'' standing wave. With frequencies offset by one free spectral range, the nodes of the trap fall on the antinodes of the probe in the $\ensuremath{\approx}{10}^{4}$ lattice sites around the center of the cavity. This lattice site independent atom-cavity coupling enables high sensitivity detection of atom dynamics even with atoms spread over many lattice sites. To demonstrate, we measure the temperature of $20--70\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{K}$ atom ensembles in $<10\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$ by monitoring their expansion by $\ensuremath{\approx}100$ nm after sudden release from the trap lattice. Atom-cavity coupling imprints the atom dynamics on the probe transmission. The technique will enable improved nondestructive detection of Bloch oscillations and other atom dynamics in optical lattices.
The main subject of this thesis is to demonstrate that helium is a promising candidate for matter wave interferometry, with the goal of providing an improved value for the fine-structure constant, α. In particular, fast and efficient momentum transfer via Bloch oscillations in an optical lattice is demonstrated, together with the enhanced sensitivity offered by an ion-detector based detection scheme that is unique to metastable helium (He*). The experiments are performed in a setup that was built to produce, slow down and trap ultracold (~0.2 μK) He-4 atoms. The approach to realising an ultracold sample of He* atoms consists of a sequence of trapping and cooling stages. The experiment starts at the plasma discharge source, where helium atoms are excited to the so-called metastable 2-3S1 state, with a lifetime of ~7800 seconds, 20 eV above the atomic ground state. The atomic beam exiting from the source is 2-D collimated by 1083 nm laser light, resonant to the closed 2-3S1 - 2-3P2 cooling transition. Next, the atoms are slowed down from about 1000- to 50 m/s in a two meter long Zeeman slower through continuous absorption of resonant 1083 nm light. The slowed atoms are captured in a Magneto-Optical Trap (MOT), that holds up to 5x10^8 atoms at a temperature of 1 mK. The MOT cloud is compressed, spin-polarised and transferred to a cloverleaf Magnetic Trap (MT), which provides both axial and radial trapping through a set of coils outside the vacuum. In the MT, 1D-Doppler cooling is applied to the atomic sample to reduce the temperature to about 150 μK. Following this step, the temperature is reduced further by forced evaporative radio-frequency cooling, resulting in the onset of a Bose-Einstein Condensate (BEC). The atoms are transferred from the MT to a crossed horizontal optical dipole trap (ODT), in which the sample is further evaporatively cooled to produce a BEC of 5x10^6 atoms. The BEC is then transferred from the ODT to either a horizontal or vertical optical lattice, consisting of two counter-propagating laser beams at 1557 nm. By detuning the frequency of one of the lattice beams, the standing wave can be accelerated. The trapped helium atoms can follow the accelerating lattice through a process called Bloch oscillations (BO's), which describes the behaviour of particles in a periodic potential that are subjected to a force. Specifically, in this scenario, particles' velocities increase in increments of two recoil velocities, instead of continuously. Bloch oscillations of cold atoms in periodic potentials formed by optical lattices were first studied in 1996. Importantly, BO’s provide a way to efficiently accelerate atoms by coherently transferring a well controlled, large number of photon momenta, which has been used to increase the resolution of recoil measurements with caesium and rubidium atoms. In this thesis, Bloch oscillations with a metastable helium BEC are demonstrated. Due to its light mass, helium can be efficiently accelerated to high velocities which makes it a candidate for experiments that benefit from splitting the atomic wave-function over large distances. Additionally, Rabi-oscillations in an optical lattice are shown, which serve to probe the depth of the lattice, and a measurement of gravity through Bloch oscillations in a static vertical optical lattice is demonstrated, with a statistical uncertainty of 4x10^-5. Finally, the first proof of principle demonstration of a pulsed light He* atom interferometer is shown. The measurements illustrate the high signal-to-noise that can be obtained using an MCP detector. Next to that, we have demonstrated a method for determining the polarizability of atoms at specific wavelengths, by observing the induced change of the characteristic Bloch oscillation frequency by introducing that wavelength to the system.
The work presented in this thesis is dedicated to studies of stabilization of Bloch oscillations of ultracold atoms in a one-dimensional vertical optical lattice under the influence of the gravitational force. The atoms simultaneously interact with both the lattice potential and a unidirectionally pumped optical ring cavity whose vertical arm is collinear with the optical lattice. In the proposed scheme, the atoms not only exchange photons between the optical lattice laser beams, but also collectively scatter light from the pump into the reverse cavity mode. The initial investigation of the system without the cavity allows understanding of the importance of the perfectly adiabatic atomic motion in the observation of stable Bloch oscillations and how easily the adiabaticity can be broken due to a very fast switch-on of the lattice or its amplitude and phase modulation. Under certain parameter regimes, adding the ring cavity to the system provides a surprisingly positive feedback on the atomic dynamics. It is found that, while acting back on the atoms, the cavity field establishes a mode-locking mechanism which assists adiabatic rapid passages between adjacent momentum states. Thus, the cavity-induced feedback mechanism enforces the adiabaticity of the process and reveals a regime where the Bloch oscillations are self-synchronized for long times. This stabilization technique is shown to steer the atoms to the lowest Bloch band preventing the problem of interband tunneling. A demonstration is also made of the ability of the system to stabilize the atomic Bloch oscillations against technical amplitude or phase noise and even suppress dephasing due to the atom-atom interactions. Furthermore, the response of the system to the atomic motion is generated in the form of perfectly detectable periodic bursts of light emitted into the counter-propagating cavity mode. Thus, the system offers a continuous and reliable non-destructive method to monitor the Bloch oscillations dynamics without perturbing their periodicity by detecting the scattered light transmitted through the cavity. All features studied in this work may be crucial for future improvements of the design of atomic gravimeters based on recording Bloch oscillations.
Ultracold neutral atoms confined in optical dipole traps have important applications in quantum computation and information processing, quantum simulators of interacting-many-body systems and atomic frequency metrology. While optical dipole traps are powerful tools for cold atom experiments, the energy level structures of the trapped atoms are shifted by the trapping field, and it is important to characterize these shifts in order to accurately manipulate and control the quantum state of the system.\n\nIn order to measure the light shifts, we have designed a system that allows us to reliably trap individual 87Rb atoms. A non-destructive detection technique is employed so that the trapped atoms can be continuously observed for over 100 seconds. Single atom spectroscopy, trap frequency measurements, and temperature measurements are performed on single atoms in a single focus trap and small number of atoms in a 1D optical lattice in order to characterize the trapping environment, the perturbed energy level structures, and the probe-induced heating.\n\nIn the second part of the thesis, we demonstrate deterministic delivery of an array of individual atoms to an optical cavity and selective addressability of individual atoms in a 1D optical conveyor, which serves as a potential candidate for scalable quantum information processing. The experiment is extended to a dual lattice system coupled to a single cavity with the capability of independent lattice control and addressability. The mutual interactions of atoms in different lattices mediated by a common cavity field are demonstrated. A semi-classical model in the many-atom regime based on the Jaynes-Cummings model is developed to describe the system that is in good qualitative agreement with the data. This work provides a foundation for developing multi-qubit quantum information experiments with a dual lattice cavity system.
A system comprised of a small number of neutral atoms coupled to the mode of a high finesse optical resonator is a model system to study light-matter interaction at the quantum level and to explore fundamental effects like the influence of the measurement process on the atomic state. Promising proposals to generate entangled states in such systems are based on the simultaneous coupling of two atoms to one resonator mode. In chapter 1 of this thesis I shortly summarise the experimental tools to capture, trap, and transport single atoms using a magneto-optical trap and an optical conveyor belt. I discuss theoretical foundations to describe our system and present numerical simulations, the results of which enter the analysis of our experimental data. The average atom-cavity coupling strength is deduced from the measured cavity transmission and compared to a model taking atomic motion into account. The dynamics of the hyperfine spin state of one and two coupled atoms is the focus of chapter 2. I describe a nondestructive cavity-based state detection method, quantify the state detection fidelity, and identify optimum experimental parameters. This method is then used to record random telegraph signals, exhibiting quantum jumps on a timescale of milliseconds. Telegraph signals for two atoms are analysed employing Bayesian statistics, yielding additional information on the evolution of the hyperfine states. A good localisation of the atom is necessary to achieve stable coupling to the cavity mode. In chapter 3, I discuss two different aspects of how the motion of the atom can be controlled. Firstly an intracavity dipole trap is characterised and it is shown that it results in improved confinement of the atom. Secondly I examine the dependence of cavity-cooling forces on our experimental parameters and compare two different cooling regimes.
Ultracold atoms in an optical lattice Bloch oscillate when subject to a constant force. In the first work presented in this thesis we have theoretically studied the scenario where the optical lattice potential is provided by the electric field inside an optical cavity. The coherent atom-light interaction in a cavity gives rise to a backaction effect on the light field which can modify the intracavity field amplitude and phase. In our first treatment of this problem we model the cavity light field and atoms by classical fields and solve the coupled atom-light equations of motion. As a result, we find that the amplitude and phase of the transmitted light field is modulated at the Bloch frequency. Remarkably, the Bloch frequency itself is not modified by the backaction. Thus the transmitted light field can be used to observe the oscillations continuously, allowing high-precision measurement with small clouds of atoms. In the second problem presented in this thesis, we explore the band structure of the steady state solutions of the atom-cavity system. A crucial first step towards determining the band structure is the identification of an energy functional that describes the coupled atom-light system. Although, we do not include direct atom-atom interactions in our models, the coupling of the atoms to the single mode light field of the cavity introduces an effective mutual interaction which is correctly taken into account by the energy functional we introduce. Corresponding to each point in the band there exists a steady state light field associated with an average cavity photon number. The dispersive nonlinear atom-light interaction can lead to bistable solutions for this intracavity photon number. For parameters where the atom-cavity system exhibits bistability, the atomic band structure develops loop structures akin to the ones predicted for Bose-Einstein condensates in ordinary (non-cavity) optical lattices. However, in our case the nonlinearity derives from the cavity backaction rather than from direct interatomic interactions. We find both bi- and tri-stable regimes associated with the lowest band, and show that the multistability we observe can be analysed in terms of swallowtail catastrophes. Dynamic and energetic stability of the meanfield solutions is also studied, and we show that the bistable solutions have, as expected, one unstable and two stable branches. The presence of loops in the band structure can lead to a breakdown in adiabaticity during Bloch oscillations as the entire band is sampled during the dynamics. We therefore use the insight gleaned from this work in choosing parameters for the Bloch oscillation measurement proposal presented in the rest of the thesis. In the third work presented in the thesis, we go beyond the mean field description and consider effects of the quantised nature of the light and atomic fields. The cavity light field is always in contact with external electromagnetic fields through the partially transmissive mirrors. This coupling to the external modes enters as quantum noise in the dynamics of the intracavity field and can also be viewed as a manifestation of quantum measurement backaction corresponding to the continuous observation of the transmitted light field. We solve the Heisenberg-Langevin equations for linearized fluctuations about the atomic and optical meanfields and examine how this influences the signal-to-noise ratio of a measurement of external forces using this system. In particular, we investigate the effects of changing the number of atoms, the intracavity lattice depth, and the atom-light coupling strength, and show how resonances between the Bloch oscillation dynamics and the quasiparticle spectrum have a strong influence on the signal-to-noise ratio as well as heating effects. One of the hurdles we overcome along the way is the proper treatment of fluctuations about time-dependent meanfields in the context of cold atom cavity-QED.