We have performed a principle-proof-experiment of a magneto-optical diffraction (MOD) technique that requires no energy level splitting by homogeneous magnetic field and a circularly polarized optical lattice, avoiding system errors in an interferometer based on the MOD. The principle for this new MOD is that asynchronized switching of quadrupole trap and Ioffe trap in a quadrupole-Ioffe-configuration trap can generate a residual magnetic force to drive a Bose-Einstein condensate (BEC) to move. We have observed asymmetric atomic diffraction resulting from the asymmetric distribution of the Bloch eigenstates involved in the diffraction process when the condensate is driven by such a force, and matter-wave self-imaging due to coherent population oscillation of the dominantly occupied Bloch eigenstates. We have classified the mechanisms that lead to symmetric or asymmetric diffraction, and found that our experiment presents a magnetic alternative to a moving optical lattice, with a great potential to achieve a very large momentum transfer ($>110 \hbar k$) to a BEC using well-developed magnetic trapping techniques.
We have performed a principle-proof-experiment of a magneto-optical diffraction (MOD) technique that requires no energy level splitting by homogeneous magnetic field and a circularly polarized optical lattice, avoiding system errors in an interferometer based on the MOD. The principle for this new MOD is that asynchronized switching of quadrupole trap and Ioffe trap in a quadrupole-Ioffe-configuration trap can generate a residual magnetic force to drive a Bose-Einstein condensate (BEC) to move. We have observed asymmetric atomic diffraction resulting from the asymmetric distribution of the Bloch eigenstates involved in the diffraction process when the condensate is driven by such a force, and matter-wave self-imaging due to coherent population oscillation of the dominantly occupied Bloch eigenstates. We have classified the mechanisms that lead to symmetric or asymmetric diffraction, and found that our experiment presents a magnetic alternative to a moving optical lattice, with a great potential to achieve a very large momentum transfer (>110ℏk) to a BEC using well-developed magnetic trapping techniques.
We have performed a principle-proof experiment of a magneto-optical diffraction (MOD) technique that requires no energy level splitting by a homogeneous magnetic field and a circularly polarized optical lattice, avoiding system errors in an interferometer based on the MOD. The principle for this MOD is that asynchronized switching of the quadrupole trap and the Ioffe trap in a quadrupole-Ioffe-configuration trap can generate a residual magnetic force to drive a Bose-Einstein condensate (BEC) to move. We have observed asymmetric atomic diffraction resulting from the asymmetric distribution of the Bloch eigenstates involved in the diffraction process when the condensate is driven by such a force, and matter-wave self-imaging due to coherent population oscillation of the dominantly occupied Bloch eigenstates. We have classified the mechanisms that lead to symmetric or asymmetric diffraction and found that our experiment presents a magnetic alternative to a moving optical lattice, with a great potential to achieve a very large momentum transfer ($>110\ensuremath{\hbar}k$) to a BEC using well-developed magnetic trapping techniques.
We have observed Bragg diffraction of a Bose-Einstein condensate of sodium atoms by a moving, periodic, optical potential. The coherent process of Bragg diffraction produced a splitting of the condensate with unidirectional momentum transfer. Using the momentum selectivity of the Bragg process, we separated a condensate component with a momentum width narrower than that of the original condensate. By repeatedly pulsing the optical potential while the atoms were trapped, we observed the trajectory of the split atomic wave packets in the confining magnetic potential.
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.
Matter wave interferometry offers a novel approach for high precision measurements, such as the determination of physical constants like the local gravity constant g or the fine-structure constant. Since its early demonstration, it has become an important tool in the fields of fundamental and applied physics. The present work covers the implementation of matter wave interferometers as well as the creation of novel guiding potentials for ultra-cold ensembles of atoms and Bose-Einstein condensates for this purpose. In addition, novel techniques for the manipulation of atoms with Bragg lattices are presented, serving as elements for interferometry. The measurements in this work are performed with a Bose-Einstein condensate of 25000 87 rubidium atoms created in a crossed optical dipole trap. The crossed optical dipole trap is loaded from a magneto-optical trap and allows a measurement every 25 s. This work introduces the novel technique of double Bragg diffraction as a tool for atom optics for the first time experimentally. The creation of beamsplitters and mirrors for advanced interferometric measurements is characterized. An in depth discussion on the momentum distribution of atomic clouds and its influence on double Bragg diffraction is given. Additionally experimental results for higher-order Bragg diffraction are explained and double Bragg diffraction is used to implement a full Ramsey-type interferometer. A second central result of this work is the implementation of novel guiding structures for ultra-cold atoms. These structures are created with conical refraction, an effect that occurs when light is guided along one of the optical axis of a bi-axial crystal. The conical refraction crystal used to operate the novel trapping geometries is a KGd(WO4)2 crystal that has been specifically cut orthogonal to one of the optical axis. Two regimes are discussed in detail: the creation of a toroidal matter wave guide and the implementation of a three-dimensional dark focus. Additional geometries accessible with conical refraction are introduced and possible applications are shown. The first regime characterized in detail is the creation of a toroidal wave guide for ultra-cold atoms and Bose-Einstein condensates. With the aid of a lightsheet potential atoms are trapped in a quasi two-dimensional ring geometry. Inside of the geometry atoms are accelerated, decelerated and held for extended storage times of up to two seconds. First attempts for the implementation of a Mach-Zehnder-type interferometer in a toroidal trap are presented. The second regime shown is the creation of a three-dimensional dark focus that is used to trap atoms in a repulsive confinement of light. The parameters of the dark focus are investigated in detail. Future application of a two-dimensional array of dark foci is shown by demonstrating the respective light field.
Quantum gases in optical lattices allow for fundamental studies in atomic and condensed-matter physics and the exploration of novel effects. After a brief introduction to the fundamentals of quantum gas experiments in optical lattices, we discuss two recent experiments focusing on driven matter waves in a one-dimensional optical lattice. The first experiment uses a tilted bichromatic optical lattice to investigate the interplay of disorder and collisional interactions in the accelerated transport of a Bose-Einstein condensate. Here, a screening effect is observed, in which the interactions effectively cancel the damping of Bloch oscillations induced by a (quasi-)disordered potential. This effect can be understood through a modification of the underlying band structure by the interactions. The second experiment studies the dynamics of a weakly trapped condensate resonantly coupled to the orbitals of a strongly confining state-selective lattice. We observe momentum distributions that correspond to matter wave diffraction from a periodic structure; however, the diffractive dynamics remain strongly linked to the internal-state Rabi oscillations. In the regime investigated, which we call the nonadiabatic regime, no diffracting potential can be defined. We show how only for much stronger coupling, the internal and external dynamics decouple, transitioning from nonadiabatic diffraction to the well-studied Kapitza-Dirac diffraction. We further investigate prospects for realizing dissipative spin models in our ultracold atomic gas experiment. To this end, we develop and test in the laboratory several possible implementations of effective spins with differential coupling to a superfluid background and examine their viability for a realization of the spin-boson model. | 94 pages