October 2015

Abstracts of the QSIT Lunch Seminar, Thursday, October 1, 2015

Resonant and inelastic Andreev tunneling observed on a carbon nanotube quantum dot

Jörg Gramich, Nanoelectronics group, University of Basel

In quantum dots (QDs) with superconducting and normal metal contacts electronic transport is mediated by both Cooper pairs from the superconductor and single electrons in the QD. This interplay between single electron transport and the superconductors pairing interaction recently enabled the observation of Andreev bound states and Majorana fermions, or nonlocal processes like Cooper pair splitting, a potential source of spin-entangled electrons. However, the most fundamental and not yet experimentally identified process in such a system is resonant Andreev tunneling (AT), in which the two electrons of a Cooper pair tunnel coherently through a single QD level. In addition, phonons or other bosons can theoretically lead to inelastic replicas of AT resonances at finite bias, in which energy is exchanged between a bosonic bath and the electrons. Our recent advances in coupling large gap superconductors to carbon nanotubes (CNTs) allow us to observe and identify the theoretically predicted resonant and inelastic AT resonances in transport spectroscopy measurements on a CNT QD contacted by a superconducting Nb and a normal metal contact.

Incompressible polaritons in a flat band

Matteo Biondi, Quantum Condensed and Coherent Systems group, ETH Zurich

We study the interplay of geometric frustration and interactions in a nonequilibrium photonic lattice system exhibiting a polariton flat band as described by a variant of the Jaynes-Cummings-Hubbard model. We show how to engineer strong photonic correlations in such a driven, dissipative system by quenching the kinetic energy through frustration. This produces an incompressible state of photons characterized by short-ranged crystalline order with period doubling. The latter manifests itself in strong spatial correlations, i.e., on-site and nearest-neighbor antibunching combined with extended density-wave oscillations at larger distances. We propose a state-of-the-art circuit QED realization of our system, which is tunable in situ.