Selectively Addressing Microwave Hyperfine Transitions in Rb-87 BECs

Abstract

Many-particle entanglement and interference are fundamental concepts of quantum mechanics. They have been demonstrated in seminal quantum optics experiments, for example, the EinsteinPodolsky-Rosen paradox [1, 2] and the Hong-Ou-Mandel effect [3]. However, these effects are not restricted to photons; they can also be demonstrated with massive particles. At the quantum-enhanced atom interferometer project QAI of the Quantum Atom Optics Group in Hannover, we are examining non-classical states of massive bosons in Bose-Einstein condensates, which are an important system to study many-body quantum effects. Where state changes and couplings in optics are realized with mirrors and beam splitters, these operations are performed with radio frequency and microwave transitions between hyperfine states in atom optics. Here, a significant challenge in high-fidelity state preparation and analysis is the undesired simultaneous addressing of hyperfine transitions with the same first-order frequency dependence. Currently, the preparation of the atoms is performed using microwave signals emitted by antennas that can only generate linearly polarized microwave signals, which address both transitions with the same coupling strength. As a result, this restricts the ability to perform state engineering of the atoms with respect to polarization. However, this limitation can be overcome if the entire power of the electromagnetic field is concentrated in a single circular polarization. This thesis investigates the enhancement of microwave signal generation for transition-selective preparation of atoms in hyperfine levels, focusing on suppressing undesired transitions. The research involved analyzing and characterizing newly designed antennas capable of generating circular polarized signals and employing multiple antennas to superimpose their signals at the atomic position to achieve the desired signal characteristics. Characterization was performed using a specially designed test setup to measure both power and field polarization. Following the integration of the antennas into the main apparatus, measurements were conducted with atoms. The results demonstrate successful polarization preparation through superimposed microwave signals, with no driving of the undesired transition observed. Measurements provide an upper estimate of the suppression of the residual unwanted transferred fraction of approximately 0.45 %. These results promise to advance the high-fidelity generation and analysis of many-particle entangled states of neutral atoms and eniiiable novel schemes for entanglement-enhanced interferometry at the Heisenberg limit.