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.

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 MooreRead 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.

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 Z2symmetry 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.

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 physicsoriented 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 quantumaccessible task environments.

23.03.2016  CasimirPolder forces and their consequences: from matterwave scattering in complex geometries to CP violation and quantum friction
Guest speaker: Stefan Buhmann (University of Freiburg, Germany) CasimirPolder 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 positiondependent CasimirPolder energy influences the scattering of cold atomic matter waves. In the second part of my talk, I will discuss two more exotic types of CasimirPolder interaction. Firstly, I will show how media violating the Lorentz reciprocity property give rise to force which are sensitive to violations of chargeparity symmetry. Secondly, I will discuss how atoms moving parallel to a smooth surface may experience a quantum friction force.

06.04.2016  TUMMPQ Seminar: Universal Quantum Physics in Driven Open ManyBody Systems
Guest speaker: Sebastian Diehl (University of Cologne, Germany) Quantum optics and manybody physics increasingly merge together in ultracold atomic gases and certain classes of solid state systems. This gives rise to new nonequilibrium 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 manybody 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 nonequilibrium 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 nonequilibrium 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  TUMMPQ 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 spinmodel with long range interactions, both showing dynamical criticality in their prethermal steadystates. 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 Fmoves and the pentagon equation as well as Rmoves 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 KleinGordon 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 lowenergy 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:

01.06.2016  From Geometry To Topology
Speaker: Julian Roos I will give a pedagogical nonformal introduction to the notion of topology. The idea is to outline an intuitive pathway from geometrical to topological concepts by increasing the number of allowed transformations within the congruence classes of Euclidean geometry. I will then pick one or the other basic system from the class of topological phases of matter and discuss why the word topological is used to describe it.

15.06.2016  Digital Quantum Simulations
Speaker: Daniel González Cuadra Simulating quantum systems is a hard problem, meaning that the resources that a classical computer requires to study them grows exponentially with the size of the system. Quantum simulations, on the other hand, exploit the quantum nature of highly controllable physical systems, such as trapped ions or ultracold atoms, to perform efficient simulations of a broad range of quantum phenomena. In this seminar I will review the topic of quantum simulation, focussing on the exponential improvement in efficiency with respect to its classical counterpart. I will consider in particular the case of digital quantum simulations, that can be regarded as a class of quantum computation algorithms. Then I will explain how a quantum simulator can be built using a trapped ion setup to study, for example, quantum spin systems. I will finish by showing a recent experiment where a lattice gauge theory was simulated with a fourqubit digital quantum simulator.

22.06.2016  Topological systems on a lattice
Speaker: Henrik Dreyer We will discuss the meaning of fusion rules and fusion trees for spin systems on a lattice and derive them for the Kitaev Toric Code. An efficient PEPS construction of its ground state wavefunction will be highlighted. This construction also works for the generalised case  the Quantum Double Models, which we subsequently discuss, in particular in terms of an example that hosts nonabelian anyons.

29.06.2016  Symmetries in classical and quantum physics  from groups to Hopf C*algebras
Speaker: Andreas Bauer Symmetry is one of the leading concepts in physics. For classical, deterministic systems, a symmetry is given by a group and its action on the configuration space via permutations. Using a graphical language, I will motivate that an attempt to generalise the notion of symmetry to a (quantum) probabilistic theory naturally leads to Hopf C*algebras and their representations. There are many examples of such Hopf C*algebras that go beyond the case of groups and their representations, like the ones obtained from the socalled quantum double construction. In the end, I will show how the latter can be used to define excitations in the quantum double models.

06.07.2016  Quantum algorithms for Hamiltonian simulation  an overview of 20 years of progress
Speaker: Yimin Ge The simulation of the dynamics of quantum systems, as originally envisioned by Feynman, is one of the main motivations for quantum computers. The first explicit quantum algorithm for this problem, due to Lloyd in 1996, simulates the evolution of a local Hamiltonian using a simple Trotter expansion. Although elementary, its complexity is unsatisfactory in several ways. Since then, a wide spectrum of improvements have been proposed, including exponentially better error dependencies, linear time scalings and generalisations to sparse Hamiltonians. The methods range from graph colourings to quantum walks and most recently quantum signal processing. In this talk, I will give an overview of this "zoo" of algorithms, including recent developments towards essentially optimal runtime complexities.

20.07.2016  Coherently driven 2component Bose gases
Guest speaker: Alessio Recati (TUM Munich, Germany) In the present talk I will give an overview on the physics of 2component Bose gases in presence of a spin exchange Rabi term. The Rabi term reduces the gauge symmetry of the system and a gap opens for relative phase excitations. The latter has many consequences. In particular it leads to the formation of vortex dimers coupled through sineGordon solitons. Results on the dynamics of the vortex dimer are presented. The system exhibits also a second order (quantum) phase transition between a neutral (paramagnetic) and a polarised (ferromagnetic) state, provided the interspecies interaction is large enough. I will discuss some phenomena related to the phase transition and to the two phases and the interplay between Rabi coupling and densitydensity interaction: (i) at a meanfield level for a continuous model  which well described atoms in a shallow trap. (ii) for a onedimensional single band Hubbard model  which describes atoms confined in an onedimensional optical lattice.

28.07.2016  Steady states and dynamics in manybody drivendissipative systems
Guest speaker: Mohammad Faghfoor Maghrebi (University of Maryland, USA) Manybody systems with both coherent dynamics and dissipation constitute a rich class of models which are nevertheless much less explored than their dissipationless counterparts. The advent of numerous experimental platforms that simulate such dynamics poses an immediate challenge to systematically understand and classify these models. In particular, nontrivial manybody states emerge as steady states under nonequilibrium dynamics. In this talk, I use a systematic approach to study nonequilibrium phases and phase transitions and the dynamics in such models. I show that an effective thermal behavior generically emerges as a result of dissipation. Finally I use this equivalence to make quantitative predictions about the steadystate and dynamical properties of these systems. 