Theory Seminar
From MPQTheory

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 manybody 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 manybody state can often be inferred from its natural orbitals (eigenvectors of the reduced onebody 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 naturalorbital analysis can help unravelling complex manybody processes and intriguing aspects of local correlations. Secondly, it will be shown how twobody correlation measurements can in turn be utilized for reconstructing the naturalorbital densities for a certain class of manybody states. Finally, I will discuss in the context of binary bosonic mixtures how the interplay between interspecies correlations and excitation transfer can be made transparent by applying the naturalorbital analysis generalized to a whole species.

28.01.2016  TUMMPQ 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 twodimensional 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 nonunitary conformal field theories, are pathological. We will also show the direct characterization of the ReadRezayi quasihole excitations from their MPS description.

03.02.2016  Fixedparameter Algorithms
Speaker: Stefan Kühn Many practically relevant computational problems are NPcomplete 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. Fixedparameter 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  TUMMPQ Seminar: Tba
Guest speaker: Sebastian Diehl (University of Cologne, Germany) Tba

27.04.2016  TUMMPQ Seminar: Tba
Guest speaker: Alessandro Silva (SISSA, Trieste, Italy) Tba 