Quantum Connections in Sweden-1 Workshop

8 Jun 2016, 08:00 → 14 Jun 2016, 18:00 Europe/Stockholm

AlbaNova universitetscentrum

Anders Karlhede, Antti Niemi, Frank Wilczek, Hans Hansson, Katie Freese, Lars Bergström

A week of workshops at the frontiers of quantum physics.

How observable phenomena in quantum systems bring in new ideas connecting emergent symmetry, topology, and entanglement.

Venue

Albanova University Centre, Stockholm University

Invited Speakers (tentative)

Egor Babaev Jordan Cotler Hans Hansson Andreas Hemmerich Cristiane Morais Smith Chetan Nayak Antti Niemi

Al Shapere Boris Svistunov Ari Turner Frank Wilczek Biao Wu

Application

If you want to participate in the workshop, please contact Antti.Niemi@physics.uu.se

Guidelines to speakers speakerGuidlines05.pdf

pictures Albanova_2012a.jpg

Poster Quantum-Connections-small.pdf

Program Master_Program.pdf

Friday, 10 June

1 Two-site equation of motion technique - a Hubbard model study

One of the main problems of understanding strongly correlated systems is that there is no universally accepted sufficiently simple reference system available for their description. The main workhorse of condensed matter physics, namely independent fermions, is not a good place to start. A better starting point is operators that are closer to creating the actual excitations in the interacting system. These can be identified by studying the equations of motion (EOM) for some chosen class of operators. In such a scheme there are a number of issues that must be dealt with: 1) Which class of operators should be used in the construction? The conventional approach is to use intuition from the physics to select the operators. 2) The next step is to identify or generate the operator dynamics. Ideally this dynamics is a reasonable approximation to reality. In practice this implies that the EOM have to be closed in a way that keeps the theory physical, for example keeping spectral weights positive. Also this step is not unique. 3) Once 1) and 2) are settled the EOM have to be solved and the averages that are necessary to construct the Green’s functions must be computable from within the theory itself. In our work, we suggest to use all operators that affect up to a pair of sites in the construction, making the scheme as unbiased as possible. This also has the advantage that the necessary averages can be calculated within the theory. We illustrate our scheme by an application to the Hubbard model. EOM technique introduction: Avella & Mancini, http://arxiv.org/abs/1112.1103 Mott physics introduction: Phillips, http://arxiv.org/abs/1001.5270

more information Quantum_Connections_Nilsson.pdf

2 Topological Superconductors and Majorana Fermions

Topological superconductors (TSCs) are a newly discovered class of superconductors with features uniquely advantageous for quantum computing. They have recently raised an immense amount of attention due to the possibility of them hosting Majorana fermion quasiparticles at surfaces, vortices, and other defects. Approximately one can say that a Majorana fermion quasiparticle is half an electron, or more accurately, in a material with Majorana fermions the wave function of an electron has split up into two separate parts. This non-local property of two Majorana fermions can be used for exceptionally fault-tolerant quantum computing, where disorder and decoherence, currently severely hampering quantum computation, is automatically rendered unimportant. Our current work focuses on the presently most promising TSCs in superconducting hybrid structures of well-known spin-orbit coupled materials, as well as on discovering new and experimentally feasible TSCs in especially graphene- like materials. We have recently developed a computationally highly efficient framework to study superconductivity self- consistently in realistic lattice models. This allows us to access important microscopic details in general hybrid structures at an essentially unprecedented level. In this talk I will show some of our recent results on vortices, single magnetic impurities, and magnetic impurity wire networks. General references to the field: J. Alicea, Rep. Prog. Phys. 75, 076501 (2012); Y. Ando and L. Fu, Annu. Rev. Condens. Matter Phys. 6, 361 (2015). Recent preprint: Bjornson and Black-Schaffer, arXiv:1605.00696.