Enhancing fiber atom interferometer by in-fiber laser cooling

Abstract

Quantum science promises technological advances in many areas, such as sensing, computing, and communications. Among various platforms for such tasks, isolated atomic vapours are ideal as their properties are universal and decoupled from noisy environments. In particular, cold-atoms interferometric quantum sensors, developed over the last three decades, have impacted inertial sensing and fundamental science frontiers in various aspects. While large-scale free-space interferometers have shown unprecedented sensitivity in measuring large-scale gravitational effects, such as Earth gravity, the apparatus that is used to house atoms typically has a cross-section of tens of centimetres, set by diffraction of the laser beams that are used to interact with atoms. The diffraction of light and sizable sensor heads limit the broad applications of quantum sensors. In this thesis, I present coherent manipulation of quantum states of atoms in a diffraction-free waveguide provided by the hollow-core photonic crystal fibre. The compact design of the apparatus permits bringing atoms close to source fields for sensing and shows excellent promise in precision measurement. The quantum laboratory realized in the hollow-core fibre is usually conducted after trapping and loading an ensemble of cold atoms. The subject of this work is an atom loading simulation and an in-fibre atom interferometer experiment.

In the atom loading simulation, I employ the Monte Carlo method to study the dynamics of the loading process. First, the loading efficiency, geometry and temperature of the ensemble are visualized by calculating the trajectory of cold atoms released from a Magneto-optical trap (MOT). Secondly, I study the mode interference that causes heating and a loss of atoms during the process. The result could be used to design and optimize the loading process of cold atoms into a hollow-core fibre for cold atoms experiments

Experimentally, I demonstrate an inertia-sensitive atom interferometer optically guided inside a 22-cm-long inhibited coupling hollow-core photonic crystal fibre. Compared to the previous in-fibre atom interferometer, the sensitivity is improved by three orders of magnitude. The improvement arises from the realization of in-fibre Λ-enhanced grey molasses and delta-kick cooling to cool atoms from 32 μK to below 1 μK in 4 ms. The cooler temperature allows the atoms to be guided in a shallow optical dipole trap to reduce the decoherence. After optimizing the parameters for cooing and trapping, the coherence time of the inertia-sensitive in-fibre atom interferometer is improved from hundreds of microseconds to 20 ms with only 245 μW of the dipole beam.