Theory Seminar

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About the Theory Seminar

New Rules (since May 2012), Old rules


Date Seminars 2016   (Go to Seminars 2015, Seminars 2014, Seminars 2013, and earlier therein)

13.01.2016 The History and Future of Time (Measurement)

Vanessa Paulisch

In this seminar, I will talk about why and how time has been, is and (probably) will be measured. Especially during the last centuries, since the need for exact time keeping for navigation became clear, the goal of more and more precise clocks was pursued. Not only for navigation, but also for geodetic applications and an exact definition of the SI unit second.

20.01.2016 Phase Transitions in Topological Tensor Network States

Mohsin Iqbal

Tensor networks can be used as a tool to study topological phases of matter. These phases come with exotic properties such anyonic excitations and topological degeneracy which are manifested as the virtual (hidden) symmetry in the tensor network description of these phases. An important object to study with in tensor network framework is transfer operator. And it is possible characterize different topological phases by the symmetry properties of the transfer operator fixed points. Symmetry breaking in the transfer operator fixed points corresponds to anyon condensation (where anyons identify themselves with the vacuum). In this talk, I will try illustrate these ideas using the test case of Z_4 invariant tensors. Different possible phases and phases transitions that can be encoded in tensor networks with Z_4 virtual symmetry will be described.

28.01.2016 Natural orbitals of ultracold many-body systems: from experimental reconstruction to correlation analysis in coordinate and energy space

Guest speaker: Sven Krönke (University of Hamburg, Germany)

Deep insights into the structure of a many-body state can often be inferred from its natural orbitals (eigenvectors of the reduced one-body density operator) and their populations. In this talk, various aspects of this theoretical concept will be discussed in the context of ultracold bosonic atoms: First, I will consider the decay of dark solitons due to dynamical quantum depletion as an example for how a natural-orbital analysis can help unravelling complex many-body processes and intriguing aspects of local correlations. Secondly, it will be shown how two-body correlation measurements can in turn be utilized for reconstructing the natural-orbital densities for a certain class of many-body states. Finally, I will discuss in the context of binary bosonic mixtures how the interplay between inter-species correlations and excitation transfer can be made transparent by applying the natural-orbital analysis generalized to a whole species.

28.01.2016 TUM-MPQ Seminar: Matrix Product States and the Fractional Quantum Hall Effect

Guest speaker: Nicolas Regnault (ENS Paris, France)

The fractional quantum Hall effect is the most celebrate example of a two-dimensional phase that exhibits intrinsic topological order. We will show that many fractional quantum Hall states have an exact infinite matrix product states (MPS) representation. We will discuss how a controlled truncation can be performed on this representation and we will give a natural interpretation from the entanglement spectrum perspective. Through the MPS, We will give evidences why certain model states related to non-unitary conformal field theories, are pathological. We will also show the direct characterization of the Read-Rezayi quasihole excitations from their MPS description.

03.02.2016 Fixed-parameter Algorithms

Speaker: Stefan Kühn

Many practically relevant computational problems are NP-complete and thus it is commonly believed that there are no efficient (meaning polynomial time) algorithms for them. In practice one therefore often has to resort to approximations or heuristics which might not yield an exact/optimal solution. Fixed-parameter algorithms by contrast allow to compute an exact solution. While the runtime is still exponential, the key idea is to limit the combinatorial explosion to a parameter additionally given as input to the problem. Hence, if the parameter is small, this might still result in an acceptable complexity for practical purposes.

In this week's seminar I am going to give a very basic introduction to fixedparameter algorithms and parametrized complexity theory. I will illustrate the method with examples and show some techniques commonly used in this field.

15.02.2016 Understanding jumps and spikes in continuous quantum trajectories

Guest speaker: Antoine Tilloy (ENS Paris, France)

When a quantum system is subjected to a continuous measurement, its evolution becomes stochastic and in a proper limit, it can be described by a continuous equation with Gaussian noise. On the other hand, it is known since Bohr that a quantum system subjected to successive von Neumann measurements undergoes rare quantum jumps. The objective of this talk is to show how this simple jumpy behavior can be obtained as a Limit of the finer continuous picture and to unravel a genuinely new phenomenon: quantum spikes. After a short reminder of continuous measurement theory, I will explain why continuous quantum trajectories show seemingly discontinuous jumps when the measurement strength is increased and give an idea of the general proof. I will then say a few words about Quantum spikes, sharp scale invariant fluctuations which decorate the jumps even in the infinitely strong measurement limit and which are simply lost in the von Neumann approximation.

06.04.2016 TUM-MPQ Seminar: Tba

Guest speaker: Sebastian Diehl (University of Cologne, Germany)


27.04.2016 TUM-MPQ Seminar: Tba

Guest speaker: Alessandro Silva (SISSA, Trieste, Italy)