Physikalisches Kolloquium: Transport through open quantum systems – insights from geometry and quantum-information

Veranstaltungseinladung als pdf-Download.

Kurzfassung: 

I will discuss two recent advances in the description of open quantum systems using density operators focusing on strongly interacting nanostructures.
(1) I will discuss a geometric approach to pumping of various physical quantities in strong nonequilibrium transport situations. In this problem a simple gauge freedom emerges: the possibility of physically recalibrating the meter registering the transported quantity (charge, spin, energy, etc.). Our gauge-invariant formalism leads to a simple expression for the geometric pumping curvature in the weak coupling limit. These suggest a geometric non- equilibrium spectroscopy that complements standard steady-state transport measurements. (2) I will show how the full perturbative expansion for the time-evolution of an open system can be cast into a form that identifies technical expressions (diagrams, projections, etc.) with physical measurements. This allows us to rigorously implement Kraus' theorem from quantum-information theory on the Keldysh contour of nonequilibrium statistical physics, enabling a novel exchange of ideas and techniques.

An example of practical relevance is that we can give explicit diagrammatic rules for preserving – or improving – the (complete) positivity of the reduced evolution in approximation schemes based on physical principles of measurement. We highlight a nontrivial competition of these rules with the – equally fundamental – constraints imposed by probability normalization. The approach covers any evolution of a system initially not entangled with a reservoir, as we illustrate for the weak-coupling limit – where we recover Gorini-Kossakowski-Sudarshan- Lindblad form – and for explicitly solvable cases with strong coupling for which our more general form is obtained. 

Biographie: Maarten Wegewijs obtained his PhD in physics in Delft (2001) and after a postdoc stay in Aachen became a Helmholtz Young Investigator at the Forschungszentrum Jülich (2007- 2013), focusing on single-molecule transport. He is presently a researcher at the Forschungszentrum Jülich and apl-Professor at the RWTH Aachen University researching charge, spin and heat transport through nanostructures. 

Die Programmübersicht des Physikalischen Kolloquiums im Sommersemester 2017 ist hier als pdf-Download verfügbar.