Exploring Dissipative and Coherent Spin Dynamics with Superradiant Quantum Gases
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2023
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Doctoral Thesis
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Abstract
Experiments integrating ultracold quantum gases and optical cavities provide a versatile platform for exploring emergent collective phenomena, ranging from symmetry-breaking phase transitions to out-of-equilibrium many-body dynamics. In this thesis, we report on a series of experiments employing a Rb-87 Bose-Einstein condensate (BEC) coupled to a high-finesse optical cavity, with the goal of investigating photon-mediated dissipative and coherent spin dynamics. In the dispersive regime of atom-light interactions, we engineer cavity-assisted Raman transitions that couple specific internal and external modes of a degenerate quantum gas. This gives rise to superradiant Raman scattering of cavity photons, a process that is collectively enhanced by the number of participating atoms.
In a first project, we couple two internal and external modes to realize an extended Dicke model with tunable coherent and dissipative interactions. The system undergoes a superradiant phase transition featuring spin-changing self-organization of the atoms. We experimentally access a dissipation-stabilized phase and a discontinuous superradiant transition in an extended region of phase bistability. The underlying mechanism is a collective decay of the hybrid light-matter excitations, which we resolve in real time by probing the cavity spectrum.
In a second set of experiments, we engineer dynamical tunneling in a synthetic lattice in momentum space. Collective hopping between discrete momentum modes of a two-component BEC is implemented via superradiant Raman scattering, resulting in directional lattice dynamics due to the inherent cavity losses. By performing frequency-resolved measurements of the leaking cavity field, we resolve the individual tunneling events both in real time and non-destructively. We further extend our observations to a regime exhibiting mutually stimulating hopping cascades.
In a third project, we demonstrate a mechanism for generating correlated atom pairs in well-defined spin and momentum modes. The pairs are created within tens of microseconds following the exchange of virtual cavity photons. We report on the first observation of coherent pair oscillations involving momentum modes, and achieve independent optical control of unitary pair processes and competing dissipative superradiant scattering. By characterizing the pair statistics and momentum-space correlations, we reveal beyond mean-field features and show their correlated nature.
Our results demonstrate a comprehensive approach for studying photon-mediated magnetic phenomena in quantum gases. Extending the implemented cavity-assisted spin interactions to Hubbard systems can facilitate experimental access to strongly correlated magnetic phases, as proposed and theoretically investigated in a dedicated project. Finally, the observed pair mechanism paves the way for quantum-enhanced matter-wave interferometry and quantum simulation experiments beyond conventional solid-state systems.
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Examiner : Esslinger, Tilman
Examiner : Zilberberg, Oded
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ETH Zurich
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03599 - Esslinger, Tilman / Esslinger, Tilman