March 2018

Abstracts of the QSIT Lunch Seminar, Thursday, March 1, 2018

Manipulation of molecular hydrogen on a chip to study quantum effects in chemical reactions at low temperature

Katharina  Höveler – Molecular Physics and Spectroscopy (Merkt group), ETH Zurich

Quantum-mechanical effects start to influence the kinetics of chemical reactions when the collision energy approaches zero. The quantized angular momentum of the relative motion leads to a discretization of the centrifugal barriers associated with the different partial waves. The barrierless ion-molecule reaction H₂ + H₂⁺  → H₃⁺ + H  provides the opportunity to observe quantum effects at collision energies below k_B
∙ 1 K. In past experiments, the cross section could be measured down to collision energies of k_B
∙ 60 K, limited by the presence of stray electric field effects. To avoid these effects, we substitute the ionic reactant by the ionic core of a molecule in a high Rydberg state.
The Rydberg electron does not influence the reaction for states with a principal quantum number n ≥ 20 but provides electric neutrality. We exploit a curved Rydberg-Stark surface-electrode deflector to merge two supersonic beams. The collision energy is tuned by adjusting the temperature of the supersonic valves or by tuning the parameters of the Rydberg-Stark deceleration. We measure the cross section at collision energies from k_B
∙ 60 K down to temperatures below k_B ∙
1 K, where deviations from predictions based on the classical Langevin capture model are observed, primarily caused by the influence of the quadrupole moment of the quantized rotation of H₂.

Formation of a spin texture in a quantum gas coupled to a cavity

Nishant Dogra – Quantum Optics Group (Esslinger group), ETH Zurich

The phenomena that can be observed in quantum many-body systems are strongly governed by the nature of the interactions between its constituents. Hence, it is a long-term goal for experiments with quantum gases to realize and explore new types of interactions. Here, we observe cavity mediated spin-dependent interactions in an off-resonantly driven atomic Bose-Einstein condensate that is strongly coupled to an optical cavity. The cavity field mediates global-range interactions between all atoms. These interactions become spin-dependent due to the strong opto-magnetic response of the system originating from the vectorial polarizability of the multi-level atoms. Applying a driving field with adjustable polarization, we determine the opto-magnetic response of the system by studying a cavity induced self-organization phase transition. Using a condensate of two internal states coupled to an optical cavity, we realize a spin texture arising from dominant spin-spin long-range interactions.