## Viscosity of quantum fluids and Hall viscosity

The analogue of friction in fluid dynamics is provided by viscosity, which causes dissipation of the flow energy. However, if time reversal symmetry is broken (spontaneously or by an applied magnetic field), the viscosity tensor may have non-dissipative components, termed "Hall viscosity", in the same way that the resistivity tensor can include the non-dissipative Hall resistivity. The Hall viscosity was shown recently to be topologically protected in rotationally-invariant systems, and to be equal to half the particle density times the orbital angular momentum per particle. Hence, measuring its value in an experiment can help one decide between different proposals for explaining the physics underlying the more exotic quantum Hall states and related systems (e.g., p+ip superconductors). Knowing the Hall viscosity may thus tell us which of these systems is a candidate for the realization of topological quantum computation. However, until recently no realistic setup for the experimental extraction of the Hall viscosity of electrons in solid state systems has been proposed.

## Rabi-Kondo correlated state in a laser-driven quantum dot

The Kondo problem, addressing the physics of a magnetic impurity or a quantum dot with an odd number of electrons coupled to an electronic reservoir, is one of the basic problems of condensed matter physics. Despite its apparent simplicity and long history it continues to generate new surprises over and over again, leading to better understanding of diverse phenomena in many body physics, such as asymptotic freedom, quantum phase transitions, and non-Fermi liquid effects.

The group of Ataç İmamoğlu at ETH, Zurich, plans experiments aimed at creating and probing a novel dynamic version of the Kondo effect, where a strong laser light excites Rabi oscillations between a state of single-occupancy of a quantum dot, in which Kondo screening of the dot spin by the surrounding leads is active, and a trionic state, in which the Kondo effect is inactive.

Read more: Rabi-Kondo correlated state in a laser-driven quantum dot

## Quantum impurity physics with microwave photons

Superconducting nanostrucures are among the leading candidates for implementing scalable quantum computation, which can be used for efficient simulation of quantum many-body dynamics. While there has been significant progress in the last decade towards such "digital" quantum computation, a quantum computer large enough to outperform existing classical computers is still very far from reach. The situation is different for the less flexible "analogue" quantum computers, or "quantum simulators", where one imitates the physics of one quantum system through another equivalent one, and where coherence requirements are less restrictive. Ultracold atomic and molecular gases have long been seen as the leading candidates for implementing quantum simulators. Solid-state superconducting circuit realizations (of, e.g., Bose-Hubbard physics in different dimensionalities) have also been proposed, but none of the ideas brought forward is yet within the reach of present experimental capabilities.

## Electronic scattering in 2D topological insulators

In the last few years it was theoretically predicted, and experimentally verified, that some ordinary-looking band insulators may have mid-gap surface states, or edge modes, which are metallic (gapless), and moreover protected from localization by topology and symmetries. The resulting unique properties of these materials, dubbed "topological insulators", have attracted significant interest.

Probably the most elementary examples of such systems are two-dimensional semiconductor quantum wells made of heavy elements with strong spin-orbit interactions. The edges of these systems support gapless "helical" edge states, where electrons which move in different directions also carry opposite spin. The existence of the helical modes is dictated by the band topology, while time reversal symmetry does not allow for elastic backscattering of single edge electrons by impurities. However, recent measurements by several groups have revealed that the edge states actually display finite scattering rates.

Read more: Electronic scattering in 2D topological insulators