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.

24.02.2016 A beginner's introduction to gravitational waves surfing

Speaker: Nicola Pancotti

02.03.2016 Lattice models and wave functions for the Fractional Quantum Hall effect

Speaker: Ivan Glasser

There is currently a lot of interest in finding models displaying the Fractional Quantum Hall effect in lattice systems. In this talk I will give an introduction to the Fractional Quantum Hall effect and the model wave functions that have contributed to much of our understanding of this effect. I will then show how these wave functions can be applied to lattice systems in the context of infinite dimensional matrix product states using the examples of the Laughlin and the Moore-Read states. I will discuss the properties of these lattice states and show that in most (but not all) cases the topological properties on the lattice are the same as in the continuum. Finally I will explain how to derive exact parent Hamiltonians for these states and how deformations of these parent Hamiltonians lead to local spin models realizing Fractional Quantum Hall physics.
References :
- H.-H. Tu, A. E. B. Nielsen, J. I. Cirac, G. Sierra, New J. Phys. 16, 033025 (2014)
- I. Glasser, J. I. Cirac, G. Sierra, and A. E. B. Nielsen, New J. Phys. 17, 082001 (2015)
- I. Glasser, J. I. Cirac, G. Sierra, and A. E. B. Nielsen, in preparation

09.03.2016 Long range order and symmetry breaking in PEPS

Speaker: Manuel Rispler

I will start by briefly motivating how projected entangled pair states (PEPS) can be used as a framework for the construction of many body models from a single tensor. I will then consider PEPS with Z2-symmetry and study the occurrence of long range order and its relation to the spectrum of the so called transfer operator. I will demonstrate how properties of the transfer operator naturally lead to spontaneous symmetry breaking in its fixed point space and how we can extract physical properties from the latter. I will conclude with discussing the implications of symmetry breaking for the boundary physics of the PEPS model and how it leads to local entanglement Hamiltonians also in symmetry broken phases.
Reference: MR, K. Duivenvoorden, N.Schuch, Phys. Rev. B 92 155133 (2015)

15.03.2016 Artificial intelligence, learning and quantum mechanics

Guest speaker: Vedran Dunjko (University of Innsbruck, Austria)

In the last few years there has been an increasing interest in the potential of quantum improvements in machine learning (ML) and artificial intelligence (AI). In this talk, I will present an overview of some of the recent results in this field, but also in the `dual’ discipline, which focuses on the utility of ML/AI techniques when applied to quantum mechanics problems. Following this, I will focus on the research of our group in Innsbruck, which addresses both sides of this coin, primarily through the study of the physics-oriented Projective Simulation model for AI. In the end, I will introduce some of our most recent results which concern provable quantum improvements in learning from quantum-accessible task environments.

23.03.2016 Casimir-Polder forces and their consequences: from matter-wave scattering in complex geometries to CP violation and quantum friction

Guest speaker: Stefan Buhmann (University of Freiburg, Germany)

Casimir-Polder forces between a single atom and a macroscopic body are effective electromagnetic interactions that may be attributed to the vacuum fluctuations of the electromagnetic field. I will introduce macroscopic quantum electrodynamics as a powerful tool to study this force for arbitrary geometries. As examples, I will consider a dielectric sphere and a grating, showing how the position-dependent Casimir-Polder energy influences the scattering of cold atomic matter waves. In the second part of my talk, I will discuss two more exotic types of Casimir-Polder interaction. Firstly, I will show how media violating the Lorentz reciprocity property give rise to force which are sensitive to violations of charge-parity symmetry. Secondly, I will discuss how atoms moving parallel to a smooth surface may experience a quantum friction force.

06.04.2016 TUM-MPQ Seminar: Universal Quantum Physics in Driven Open Many-Body Systems

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

Quantum optics and many-body physics increasingly merge together in ultracold atomic gases and certain classes of solid state systems. This gives rise to new non-equilibrium scenarios even in stationary state, where coherent and dissipative dynamics appear on an equal footing. Oftentimes however, in the transition from microscopic scales to macroscopic distances, much of the underlying quantum dynamics is washed out, and the long distance many-body physics is (semi-)classical.

Here we present two instances which do not follow this generic pattern. The first addresses the critical dynamics in a driven open system with a dark state realizable in microcavity arrays. We find it governed by a new non-equilibrium universality class, characterized by the absence of decoherence and thermalization. The second focuses on an ensemble of driven Rydberg atoms, in which the interplay of constrained coherent quantum dynamics and dissipation leads to a new variant of a non-equilibrium absorbing state transition beyond the classical paradigm of directed percolation.

20.04.2016 (Quantum) Boltzmann machines

Speaker: András Molnár

In the talk I will introduce Boltzmann machines. Boltzmann machines are a special model for machine learning. I will try to explain how they can be used to solve certain typical tasks such as classification or dimension reduction. Finally I will discuss a possible quantum generalization recently proposed by Amin et al.

27.04.2016 TUM-MPQ Seminar: Probing dynamical phase transitions with the statistics of excitations

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

Dynamical phase transitions can occur in isolated quantum systems that are brought out of equilibrium by either a sudden or a gradual change of their parameters. Theoretical examples rage from the behaviour of the O(N) model in the large N limit as well as spin-model with long range interactions, both showing dynamical criticality in their prethermal steady-states. In this talk I will start by discussing the characterization of such dynamical phase transitions based on the statistics of produced excitations. I will focus both on the role of fluctuations as well as on the difference between sudden and gradual changes of the parameters. Finally, I will discuss a second type type of dynamical criticality discussed in the literature, related to the emergence of zeroes in the Loschmidt amplitude, and show that this phenomenon is much less generic and robust than standard dynamical criticality.

11.05.2016 An Introduction to Topological Quantum Field Theory

Speaker: Anna Hackenbroich

Intrinsically topologically ordered systems such as fractional quantum Hall samples possess elementary bulk excitations with anyonic statistics. For this reason they are very interesting both in their own right and as possible platforms for topological quantum memories or topological quantum computation. In the limit of very low energies, topologically ordered systems are described by a topological quantum field theory (TQFT). In this talk, we give an elementary introduction to a particular version of TQFT that describes the fusion and braiding of anyons. In particular, we define anyons as representations of the braid group and discuss the concept of fusion and fusion rules. We then introduce fusion and splitting spaces and discuss F-moves and the pentagon equation as well as R-moves and the hexagon equation. These definitions allow us to derive a physical characterisation of the quantum dimension. We conclude by discussing some examples.

20.05.2016 To what extent can information propagation decouple from energy propagation?

Guest speaker: Robert Jonsson (University of Waterloo, Canada)

The way in which quantities such as energy, classical and quantum information, and entanglement propagate through extended systems is of great interest from quantum field theory to condensed matter systems. Here, I will explore the extent to which the propagation of information requires a flow of energy. I will show that even in the basic case of a massless Klein-Gordon field in the vacuum, the propagation of information can decouple from the propagation of energy to a large extent and, in special circumstances, it can even decouple completely. The new phenomenon arises from the quantization of classical wave phenomena, associated with the strong Huygens principle. These result in signals which propagate partially slower than the speed of light even in massless fields. Potential applications range from cosmology to quantum information.

25.05.2016 The functional renormalization group and the Hubbard model

Speaker: Benedikt Herwerth

The functional renormalization group provides exact flow equations for generating functionals of quantum field theories under an energy scale. Truncations of these flow equations lead to approximations schemes, which can be used to gain physical insight about the low-energy physics of a model. In this talk, the flow equations for fermionic fields are introduced and applications to the Hubbard model are discussed.

The talk will mainly be based on the following references:
[1] P. Kopietz et al., "Introduction to the functional renormalization Group", Springer, 2010.
[2] W. Metzner et al., "Functional renormalization group approach to correlated fermion systems", Reviews of Modern Physics 84.1 (2012).
[3] C. Honerkamp, "Functional Renormalization Group for Interacting Many-Fermion Systems on Two-Dimensional Lattices", Springer Series in Solid-State Sciences, Volume 171 (2012).

01.06.2016 Tba

Speaker: Julian Roos


15.06.2016 Tba

Speaker: Daniel González Cuadra