Structure and Applications of the Coupled A1Σ+ − b3Π0 Electronic System in RbCs

Albert Li Tao

Published Oct 15, 2026• 0 citations

Abstract

This thesis reports the development of theoretical and experimental tools to understand and control the coupled A1Σ+ − b3Π0 system in ultracold 87Rb133Cs molecules, with applications in magic trapping and direct imaging of molecules. Building on detailed modelling of molecular structure and light–matter interactions, the work establishes conditions for long-lived rotational coherence and explores pathways towards quantum simulation and computation with RbCs. High-resolution spectroscopy of lowest vibrational levels in the coupled complex is performed, yielding vibrational splittings, rotational and hyperfine coupling constants, transition dipole moments and natural linewidths. Spectroscopy of a highly excited vibrational level of the complex is also performed, with the polarisation-dependent transition strengths and polarisabilities measured. These results demonstrate strong anisotropy of the initial state and enables direct detection of molecules. We then develop and validate a comprehensive description of rotationally magic trapping in RbCs. A general theory of dipole polarisability is applied near the transition from the rovibrational ground state in the X1Σ+ potential to the lowest vibrational states of the b3Π0+ potential, predicting rotationally magic frequencies at which differential polarisabilities between rotational states vanishes. Experimentally, we build and characterise a new magic-wavelength trapping system based on a 1145 nm laser. Measurements of magic frequencies for spinstretched superpositions, combined with long coherence times in the magic trap, confirm the model. Dephasing due to dipole–dipole interactions are observed, while long-lived rotational coherence is demonstrated with non-dipolar states. Extending the theory and experiments to non-stretched states by diagonalising the full AC Stark Hamiltonian shows nonlinear Stark shifts driven by laser-induced hyperfine couplings and identifies local magic conditions where intensity gradients vanish, enabling tailored coherence protection across a wide range of internal state.

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