# Talks

coffee, tea, cookies at 16:15 in the main hall
Monday 16:30-17:30
Seminar room 1+2
- monthly seminars -
Wednesday 16:30 - 18:00
Seminar room 1+2
- weekly seminars -
Monday 11:00-12:00
Room 1D1
Wednesday 16:30 - 17:30
Seminar room 1D1
Thursday 14:00-15:00
Seminar room 3

## Talks in chronological order

22 Jan 2018
02:00 PM

### Clustering of spins on Ising and Heisenberg models on kagome and pyrochlore lattices

Tomonari Mizoguchi (Gakushuin University)

Fractionalization is one of the fundamental properties of topological phases. For example, in a spin ice, an emergent fractionalized degrees of freedom is called a magnetic monopole [1], which is a topological defect on top of a spin-ice manifold. In the present work, we investigate the many-body effects of such topological charges that emerge in classical spin liquids on corner-sharing lattices such as kagome and pyrochlore lattices. In the first part of my talk, I will show the thermodynamic properties of a J1-J2-J3 Ising model on a kagome lattice [2], where J2 and J3 give rise to the nearest-neighbor interaction between topological charges. We find that the interactions between topological charges give rise to a novel classical spin liquid state characterized by the formation of clusters, or “hexamers”, of same-sign topological charges. Then, in the second part, I will show the results for J1-J2-J3 Heisenberg models on kagome and pyrochlore lattices [3]. Using a large-N approximation and Monte Carlo simulations, we elucidate the static structure factors, and find that the obtained patterns are very similar to those in Ising models. This indicates that the short-range correlations similar to topological-charge clusters develop even in the Heisenberg model. [1] C. Castelnovo, R. Moessner, and S. L. Sondhi, Nature (London) 451, 42 (2008). [2] T. M., L. D. C. Jaubert, and M. Udagawa, PRL 119, 077207 (2017). [3] T. M., L. D. C. Jaubert, and M. Udagawa, in preparation.

Room 1D1 iCal Event
22 Jan 2018
04:30 PM

### Colloquium: The long way to steady state fusion plasmas - the superconducting stellarator device Wendelstein 7-X

Prof. Thomas Klinger (MPI fuer Plasmaphysik, Greifswald)

The stable generation of high temperature Hydrogen plasmas (ion and electron temperature in the range 10-20 keV) is the basis for the use of nuclear fusion to generate heat and thereby electric power. The most promising path is to use strong, toroidal, twisted magnetic fields to confine the electrically charged plasma particles in order to avoid heat losses to the cold, solid wall elements. Two magnetic confinement concepts have been proven to be most suitable: (a) the tokamak and (b) the stellarator. The stellarator creates the magnetic field by external coils only, the tokamak by combining the externally created field with the magnetic field generated by a strong current in the plasma. “Wendelstein 7-X” is the name of a large superconducting stellarator that goes into operation after 15 years of construction. With 30 $m^3$ plasma volume, 3 T magnetic field on axis, and 10 MW micro wave heating power, Hydrogen plasmas are generated that allow one to establish a scientific basis for the extrapolation to a future fusion power plant. It is a unique feature of Wendelstein 7-X to be able to operate high-power Hydrogen plasmas under steady-state conditions, more specifically for 1800 s (note that the world standard is now in the 10 s ballpark). This talk provides a review of the principles of nuclear fusion and discusses the key physics subjects of optimized stellarators. The sometimes adventurous undertaking to construct such a first-of-a-kind device is summarized as well as the most important findings during the first operation phase of Wendelstein 7-X. We finish with an outlook towards the fusion power station and address the most important remaining issues to be addressed in the framework of the world-wide fusion research endeavor.

Seminarroom 1+2+3 iCal Event
25 Jan 2018
02:00 PM

### From the early universe to ultracold quantum gases

Jürgen Berges (Heidelberg)

Seminarroom 4 iCal Event
01 Feb 2018
02:00 PM

### tba

Antonello Scardicchio (ICTP)

Room 1D1 iCal Event
07 Feb 2018
11:00 AM

### Thermodynamics of computation

Dominique Chu (University of Kent)

Computation is not limited to electronic computers, but many natural systems can process information as well. Studying such systems is interesting because it can lead to new unconventional computers, for example in synthetic biology. Understanding computation in natural systems, however, also reveals gaps in our understanding of what it means to compute. In this talk I will concentrate on computation in and by biological systems and describe under which conditions biochemical reactions can be understood as processing information and how this relates to thermodynamics.

Room 1D1 iCal Event
08 Feb 2018
02:00 PM

### tba

Angelo Russomanno (ICTP)

Room 1D1 iCal Event
26 Feb 2018
04:30 PM

### Colloquium: Chern numbers counted in a synthetic-dimension quantum Hall strip

Prof. Ian Spielman (University of Maryland, USA)

Ultra cold atoms are remarkable systems with a truly unprecedented level of experimental control and one application of this control is creating topological band structures. The most natural approach centers on creating suitable real-space lattice potentials that the atoms experience. Here we present our experimental work which uses the internal atomic states as an additional synthetic'' dimension. We engineered a two-dimensional magnetic lattice in an elongated strip geometry, with effective per-plaquette flux about 4/3 times the flux quanta. The long direction of this strip is formed from a 1D optical lattice while the short direction is built from the 5 mF states comprising the f=2 ground state hyperfine manifold of Rb-87. We imaged the localized edge and bulk states of atomic Bose-Einstein condensates in this strip, with single lattice-site resolution along the narrow direction. In this 5-site wide strip we are able to delineate between bulk behavior quantified by Chern numbers and edge behavior which is not.

Seminarroom 1+2+3 iCal Event
05 Mar 2018
04:30 PM

### Colloquium: The Quantum Way of Doing Computations

Prof. Dr. Rainer Blatt (Institute for Experimental Physics, University of Innsbruck and Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Austria)

Since the mid-nineties of the 20th century, it became apparent that one of the centuries’ most important technological inventions, computers in general and many of their applications could possibly be further enhanced by using operations based on quantum physics. This is timely since the classical roadmap for the development of computational devices, commonly known as Moore’s law, will cease to be applicable within the next decade. This is due to the ever-smaller sizes of the electronic components that will enter the realm of quantum physics. Computations, whether they happen in our heads or with any computational device, always rely on real physical devices and processes. Data input, data representation in a memory, data manipulation using algorithms and finally, data output require physical realizations with devices and practical procedures. Building a quantum computer then requires the implementation of quantum bits (qubits) as storage sites for quantum information, quantum registers and quantum gates for data handling and processing as well as the development of quantum algorithms. In this talk, the basic functional principle of a quantum computer will be reviewed. It will be shown how strings of trapped ions can be used to build a quantum information processor and how basic computations can be performed using quantum techniques. In particular, the quantum way of doing computations will be illustrated with analog and digital quantum simulations, which range from the simulation of quantum many-body spin systems over open quantum systems to the quantum simulation of a lattice gauge theory.

Seminarroom 1+2+3 iCal Event
26 Mar 2018
04:30 PM

### Colloquium tba

Prof. Dr. Martin Weitz (Universität Bonn)

Seminarroom 1+2+3 iCal Event
16 Apr 2018
04:30 PM

### Colloquium: tba

Jean-François Joanny (ESPCI Paris)

Seminarroom 1+2+3 iCal Event
23 Apr 2018
04:30 PM

### Colloquium tba

Seminarroom 1+2+3 iCal Event
30 Apr 2018
04:30 PM

### Colloquium: tba

Francoise Brochard (Curie Institute)

Seminarroom 1+2+3 iCal Event