November 2016

Abstracts of the QSIT Lunch Seminar, Thursday, November 3, 2016

Supersolid formation in a quantum gas breaking continuous translational symmetry

Julian Léonard - Quantum Optics (Esslinger group), ETH Zurich,

A supersolid is a paradoxical state: it is at the same time a rigid solid and a perfect fluid, as it features both crystalline order and superfluidity. Despite having been proposed more than 50 years ago, its realization has remained elusive. We have now realized a supersolid in a superfluid quantum gas whose continuous translational symmetry is broken by coupling it with the modes of two optical cavities. The phase transition is driven by infinitely long-ranged interactions between the atoms that are mediated by photons in the two cavities. When leaking from the cavities, these photons provide us with real-time access to the crystal position, thereby allowing for studies on the process of symmetry breaking and the dynamics of the supersolid phase.

Cavity dispersive shift induced by Rydberg atoms

Sebastien Garcia - Quantum Device Lab (Wallraff group), ETH Zurich

Cavity quantum electrodynamics provide a very powerful way to measure light states with atoms or atoms states with light via quantum non-demolition measurements. We present an experiment where the transmission of a weak probe through a microwave cavity allows to measure the dispersive shift induced by Rydberg atoms. The system is quantitatively described by the dispersive Tavis-Cummings Hamiltonian which describes the coupling of a point-like  ensemble of identical two-level atoms to a single resonator mode.  We measured the dispersive shift scaling with the number of atoms and the detuning between the cavity resonance and the atomic transition. The dispersive shift measurement provides a non-destructive measurement of the atom number.
This constitutes a first step for our hybrid cavity quantum electrodynamics experiment. Indeed, we aim at coupling Rydberg atoms to superconducting qubits to take advantage of long coherence time and optical transition of the former, and strong coupling constant and scalability of the latter.