Towards quantum control over macroscopic distances : compact loading of rubidium atoms for a 2-dimensional magneto optical trap

• Main Author: Pong, Sze You
• Other Authors: Lan Shau-Yu
• Format: Final Year Project
• Language: English
• Published: 2019
• Subjects: DRNTU::Science::Physics::Atomic physics; DRNTU::Science::Physics::Optics and light
• Online Access: http://hdl.handle.net/10356/77098
• Institution: Nanyang Technological University

Manipulating external and internal degrees of freedom of quantum particles with light have been revolutionary over the last two decades. While such a great success has advanced quantum computing, quantum simulation, quantum metrology, etc., most of the quantum control schemes take place on the scale of millimetres to centimetres. Combining the laser cooling and trapping technology and photonic crystal fibres, this project intends to initiate the extension of the distance of quantum control and the geometry. The short-term goal of this project is to set up an experimental apparatus for loading ultra-cold atoms into a hollow-core photonic crystal fibre. The long-term goal is to load atoms into the hollow-core photonic crystal fibre and guide them over long distances. A 2-dimensional Magneto Optical Trap (2D MOT) consists of two perpendicular pairs of retroreflected laser beams that are linearly polarized and a magnetic field gradient along the third axis. Such a MOT is experimentally capable of cooling, capturing and trapping large quantities of atoms within vacuum at extremely low temperatures in the micro Kelvin range. This thesis presents the background information, setup of the experiment as well as measurement results and analysis. The physical properties of Rb-85 and information on laser cooling and magneto optical trapping are also covered. The impact due to spontaneous emission of rubidium-85(Rb-85) atoms in the vacuum system was studied, including the effects of laser beam strength and frequency on the trapped atoms. Throughout the experiment, the turbopump of the vacuum system must run continuously in order to provide a steady pressure within the chamber such that out-gassing can be prevented or ignored. Cooling and repump beams were configured and tuned using a combination of software and hardware applications. Two beam splitters were utilised to obtain the required coherence in frequency and wavelength since a higher intensity will not improve the performance of the MOT. Ultimately, four linearly polarized trapping beams derived from the same input source and power were collimated, coupled with 4.5mm beam shapers, and checked for drifting. The beams intersect at the centre of the trap cell and thus provide the initialisation for the experiment. The repump beam was also tuned to align and collimate accordingly with the cooling beam. The turbopump continuously keeps track of the partial pressure values within the vacuum chamber to ensure real time monitoring and prevent adsorption of air molecules onto internal chamber parts so that even spreading of Rb-85 atoms can occur naturally. Also, two pairs of magnetic coils were placed alongside the trap cell in an anti-Helmholtz configuration.