In this thesis I report the realization of a large spacing optical lattice exploiting the beat-note of two retro-reflected laser beams with slightly different wavelengths i.e 1064 nm and 1013 nm. The resulting potential, in addition to a very fast modulation, exhibits a slow periodicity that provides an optical potential for the atoms that behaves like an effective lattice with 10 micrometers of separation between lattice sites. \nI studied theoretically the band structure of the new lattice and I performed numerical simulations in order to obtain predictions of the energy gap as a function of the lattice depth in different configurations. \nRegarding my experimental contribution, I have complitely designed and assembled the setup for the stabilization of the intensity and the frequency of the laser sources. In particular, for the stabilization of the relative frequencies of the two lasers, I develope a technique that allows to lock the two frequencies to the same optical cavity. \nThanks to the high control of the residual forces on the sample and the capability to cancel the interatomic interactions using magnetic Feshbach resonaces, we have managed to observe spatial Bloch oscillations of condensates, measuring the atom number in each well. The observed dynamics exhibits a coherence up to 1 s that correponds to a sensitivity of the order of 5 times10^-5 g with a spatial resolution of ten microns. We characterized experimentally the frequency and the amplitude of the oscillations for different value of the force and the results show a very good agreement with theoretical simulations. \nTo our knowledge the sensitivity acquired in this work is the higher reported in litterature in the measurement of small forces (<< g) using a trapped atom interferometer. The main limitations to the coherence time and the precision of the measurements are related to the limited control we have in the way we tune the external force, i.e, optically. \nThe simplicity of the Bloch oscillation interferometer and the creation of a large spacing periodic potential open interesting perspective in the measurements of small and local forces with trapped condensates and can be useful for the investigation of Casimir-Polder forces or deviations of the \ngravitational law at short distances. \nIn the first half of my PhD I participated to another project: we studied the behaviour of an ultra-cold mixture of two different internal states of potassium. In particular regimes of interaction the cloud tends to collapse but, at high density, many-body effects occur and stabilize the mixture to a fixed volume. In free space it doesn't collapse or expand and it behaves like a liquid droplet. We have characterized the time evolution and the equilibrium properties of the droplets and we have peformed experimental studies on the collisional dynamics between two droplets. Good agreement with theoretical analysis confirm our control on the system and open interesting perspective for the studies of their excitation spectrum and superfluid behaviour.
In this thesis I report the realization of a large spacing optical lattice exploiting the beat-note of two retro-reflected laser beams with slightly different wavelengths i.e 1064 nm and 1013 nm. The resulting potential, in addition to a very fast modulation, exhibits a slow periodicity that provides an optical potential for the atoms that behaves like an effective lattice with 10 micrometers of separation between lattice sites. \nI studied theoretically the band structure of the new lattice and I performed numerical simulations in order to obtain predictions of the energy gap as a function of the lattice depth in different configurations. \nRegarding my experimental contribution, I have complitely designed and assembled the setup for the stabilization of the intensity and the frequency of the laser sources. In particular, for the stabilization of the relative frequencies of the two lasers, I develope a technique that allows to lock the two frequencies to the same optical cavity. \nThanks to the high control of the residual forces on the sample and the capability to cancel the interatomic interactions using magnetic Feshbach resonaces, we have managed to observe spatial Bloch oscillations of condensates, measuring the atom number in each well. The observed dynamics exhibits a coherence up to 1 s that correponds to a sensitivity of the order of 5 times10^-5 g with a spatial resolution of ten microns. We characterized experimentally the frequency and the amplitude of the oscillations for different value of the force and the results show a very good agreement with theoretical simulations. \nTo our knowledge the sensitivity acquired in this work is the higher reported in litterature in the measurement of small forces (<< g) using a trapped atom interferometer. The main limitations to the coherence time and the precision of the measurements are related to the limited control we have in the way we tune the external force, i.e, optically. \nThe simplicity of the Bloch oscillation interferometer and the creation of a large spacing periodic potential open interesting perspective in the measurements of small and local forces with trapped condensates and can be useful for the investigation of Casimir-Polder forces or deviations of the \ngravitational law at short distances. \nIn the first half of my PhD I participated to another project: we studied the behaviour of an ultra-cold mixture of two different internal states of potassium. In particular regimes of interaction the cloud tends to collapse but, at high density, many-body effects occur and stabilize the mixture to a fixed volume. In free space it doesn't collapse or expand and it behaves like a liquid droplet. We have characterized the time evolution and the equilibrium properties of the droplets and we have peformed experimental studies on the collisional dynamics between two droplets. Good agreement with theoretical analysis confirm our control on the system and open interesting perspective for the studies of their excitation spectrum and superfluid behaviour.
We report the experimental realization of a new kind of optical lattice for ultracold atoms where arbitrarily large separation between the sites can be achieved without renouncing to the stability of ordinary lattices. Two collinear lasers, with slightly different commensurate wavelengths and retroreflected on a mirror, generate a superlattice potential with a periodic "beat-note" profile where the regions with large amplitude modulation provide the effective potential minima for the atoms. To prove the analogy with a standard large spacing optical lattice we study Bloch oscillations of a Bose Einstein condensate with negligible interactions in the presence of a small force. The observed dynamics between sites separated by ten microns for times exceeding one second proves the high stability of the potential. This novel lattice is the ideal candidate for the coherent manipulation of atomic samples at large spatial separations and might find direct application in atom-based technologies like trapped-atom interferometers and quantum simulators.
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.
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.
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.
Experiments on atomic Bose-Einstein condensates inside quasi-one-dimensional optical lattices and related developments in the realization of atom lasers are currently at the frontiers in atomic physics. We give a short review of theoretical progress in evaluating the coherent transport of matter in such configurations, which has been based on an adaptation of the Wannier function representation of quasi-particle states in periodic potentials and on the use of advanced numerical methods for the solution of the time-dependent Gross-Pitaevskii equation. We present various methods by which the band structure of the elementary excitations of a periodic condensate may be probed and describe in terms of Bloch oscillations the coherent emission of matter pulses from a condensate inside an optical lattice under the force of gravity. A harmonic force can be applied to a condensate inside a magnetic trap by a rapid displacement of the center of the trap and a transition of the condensate from superfluid to dissipative behavior is driven by superposing an optical lattice. Finally, in the superfluid regime the combination of a constant force and a harmonic force in the presence of an optical lattice drives Josephson-type oscillations of the condensate, leading to observable resonances and multimode behavior.
This dissertation describes our experimental investigations of quantum transport and atomic motion control using optical potentials.The system we study consists of ultracold sodium atoms under the influence of light forces.First, we introduce the dynamics of neutral atoms in a periodic optical potential.The system resembles the textbook problem of an electron inside the crystalline lattice, and we review the main characteristics of the interaction for the atom optics case.In particular, atoms trapped in a lattice subject to large accelerations undergo Landau-Zener tunneling, process which makes the system unstable.The number of atoms trapped in the potential shows the characteristic exponential decay over time.However, deviations from this law are predicted by quantum mechanics.We use the experimental access to the non-exponential time to demonstrate the Quantum Zeno and Anti-Zeno effects.These effects show the influence of frequent observations on the decay rate of a quantum unstable system.The second part of the dissertation introduces a new system we plan to study, namely, the quantum interaction between sodium atoms in the ground ix state and a conductive surface.We are interested in the measurement of the Casimir-Polder potential with a precision of better than 1%.In order to do this, we have chosen to launch the atoms towards the surfaces at very small incident velocities (a few mm/s), and measure the influence of the interaction on the reflection probability.Atoms reflect from the purely attractive potential due to quantum reflection, an effect with no classical analogy.The experimental observation of quantum reflection requires atomic distributions with temperatures below 1 µK.For this purpose, we have produced and studied a Bose-Einstein condensate (BEC) with sodium atoms.The region where the BEC is created is separated spatially form the surfaces by a distance of 10 cm, vertically.In order to bring the atoms close to the surfaces prior to their launching, we have developed an optical elevator.The elevator uses a moving optical lattice in the regime where tunneling is negligible.Results of the macroscopic optical transport technique, and current progress towards a measurement of the Casimir-Polder interaction, are reported.