Department of Physics • University of Cologne

Antrittsvorlesung.

Reciprocity is a fundamental symmetry in physics; in optics it can be understood as the principle of ‘if I can see you, you can see me’. Breaking this symmetry results in asymmetric information transfer between two systems, i.e., the transmission amplitudes change under the exchange of source and detector. In the optimal situation, information transfer only occurs in one direction, which is a highly valuable feature for quantum information processing, where one aims to read out a quantum system while protecting the signal source. The violation of the symmetry of reciprocity requires rather special conditions, and one may ask the question if there is a general way to break reciprocity between two systems. This is indeed possible, we find that one can construct nonreciprocal interactions by combining the appropriate coherent and dissipative dynamics. Our nonreciprocity concept was experimentally confirmed within an optomechanical array setup in a collaboration with Oscar Painter’s group at Caltech. Furthermore, a number of related experiments can essentially be mapped to our approach as well. Overall, the nonreciprocity protocol is a powerful tool for realize directional interactions between two quantum systems and its full potential has yet to be explored. In this talk I give an introduction of the basic concept on how to engineer nonreciprocal interactions and devices.

The input-output relations of a biological system summarize its functional characteristics. Therefore, answering many fundamental questions on biomolecular machines ultimately reduces to understanding their input-output relations. Enzymes, that catalyze biochemical reactions, are among the simplest machines for which both input and output are chemical in nature. Many enzymatic reactions follow the "Michaelis-Menten" relations between input concentrations and output concentration of the molecular species involved in the reaction. We begin with a graph theoretic analysis of the input-output relation of a single enzyme. Input-output response of a graph can be characterized by the steady-state concentrations of the vertices. We present the conditions that must be satisfied by the structure of the graph (network) of states for the validity of the Michaelis-Menten input-output relation. This analysis sets the stage for understanding, at the next level of complexity, the input-output relation of a molecular machine, that is a macromolecular complex, and then at an even broader context of a group of interacting machines. I'll present an overview of our recent results on these input-output relations at multiple scales.

Geometric phases in condensed matter play a central role in topological transport phenomena such as the quantum, spin and anomalous Hall effect (AHE). In contrast to the quantum Hall effect - which is characterized by a topological invariant and robust against perturbations - the AHE depends on the Berry curvature of occupied bands at the Fermi level and is therefore highly sensitive to subtle changes in the band structure. We demonstrate that the Berry curvature can be manipulated by interface engineering of the correlated itinerant ferromagnet and antiferromagnets. Using theoretical calculations, we show that the tunability of the AHE at interfaces arises from the competition between two topologically non-trivial bands. D. J. Groenendijk, C. Autieri et al. Berry phase engineering at oxide interfaces, https://arxiv.org/abs/1810.05619. M. Asa, C. Autieri et al. Anomalous Hall effect at the antiferromagnetic/non magnetic interface, in manuscript

I will discuss ways in which the quantum world is different from the classical world, and how these differences might be used to design quantum computers which are more powerful than classical computers, at least for certain problems. I shall focus on one approach to quantum computing, called the Quantum Adiabatic Algorithm, which is used to solve optimization problems, which are of great importance in science, engineering and industry. An optimization problem is one in which one has to minimize (or maximize) an energy function in which there is competition between different terms. I will discuss results both from computer modelling of the Quantum Adiabatic Algorithm and real experiments on a device which has of order 1,000 superconducting quantum bits. The difficulty of getting a "quantum speedup" from this device due to the great sensitivity of the ground state to changes in parameters (chaos) will be discussed.