Bloch Oscillations of Metastable Helium in an optical lattice

Abstract

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.