November 2015
Abstracts of the QSIT Lunch Seminar, Thursday, November 5, 2015
Quantum phases emerging from competing short- and long-range interactions in an optical lattice
Renate Landig, Quantum Optics Group, ETH Zurich
The competition between interactions acting on different length scales lies at the core of a variety of processes leading to structure formation in nature. Examples range from the folding mechanisms of proteins to the appearance of stripe phases in quantum matter. Theoretical characterization of such emerging structures is often exceedingly challenging even if simple toy models are used. A complementary approach to gain insights into complex phenomena has been advanced for quantum matter, where simulation experiments with ultracold atoms are carried out. However these experiments are mostly limited to short-range collisional interactions. Recently observed perturbative effects of long-range interactions were too weak to reach novel quantum phases. Here we experimentally realize a bosonic lattice model with competing short- and infinite-range interactions, and observe the appearance of four distinct phases - a superfluid, a supersolid, a Mott insulator and a charge density wave. Our system is based on an atomic quantum gas trapped in an optical lattice inside a high finesse optical cavity. The strength of the short-ranged on-site interactions is controlled by means of the optical lattice depth. The infinite-range interaction potential is mediated by a vacuum mode of the cavity and is independently controlled by tuning the cavity resonance. When probing the phase transition between the Mott insulator and the charge density wave in real-time, we discovered a behaviour characteristic of a first order phase transition. Our measurements have accessed a regime for quantum simulation of many-body systems, where the physics is determined by the intricate competition between two different types of interactions and the zero point motion of the particles.
Microwave emission from hybridized charge states in a semiconductor charge qubit
Anna Stockklauser, Quantum Device Lab, ETH Zurich
We investigate a hybrid circuit quantum electrodynamics (QED) architecture, in which a double quantum dot charge qubit couples to a nearby microwave cavity. We discuss experiments exploring microwave emission from the voltage-biased GaAs double dot. A superconducting coplanar waveguide resonator serves as a tool to study details of the quantum dot level structure that cannot be accessed in transport measurements. We detect radiation emitted in inelastic electron tunneling processes between the
dots and the leads and in interdot transitions resonant with the cavity. This resonant emission manifests itself in a strongly enhanced emission signal at the two resonance conditions. We show how the dependence of the emission signal on the quantum dot level configuration is related to the hybridization of the dot wavefunctions.