Theory Seminar

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About the Theory Seminar

New Rules (since May 2012), Old rules


Date Seminars 2014     (Go to Seminars 2013, Seminars 2012, Seminars 2011, and earlier therein)

15.01.2014 High Energy Physics in the Light of Quantum Simulators

Eliška Greplová

The goal of this seminar is to provide an overview of some of the latest work on the simulation of high energy physics using ultracold atoms. The basics of continuous and lattice field theory together with the gauge principle will be reviewed, and the engineering of the corresponding Hamiltonians using optical lattices will be discussed. The presentation will be mainly based on the following references:

[1] Phys. Rev. A 88, 023617 (2013)
[2] Ann. Phys. (Berlin) 525, No. 10–11, 777–796 (2013)

22.01.2014 Beyond Adiabatic Elimination

Vanessa Paulisch

Adiabatic elimination is a commonly used method for approximating the dynamics of multi-level systems. This procedure is, however, somewhat ambiguous and it is not clear how to improve on it systematically. Parts of my bachelor thesis, which deal with these problems, will be presented in this seminar. In addition, illustrative examples, applications and an alternative to the adiabatic elimination procedure for Raman transitions [1] will be discussed. The most important results of my thesis can be found on arXiv:1209.6568.

[1] R. Han et al. (2013) Raman transitions without adiabatic elimination: A simple and accurate treatment. J. Mod. Opt. 60, 255

29.01.2014 An elementary quantum network of single atoms in optical cavities

Guest Speaker: Carolin Hahn (MPQ, Quantum Dynamics Group)

Quantum networks are at the heart of quantum communication and distributed quantum computing. Single atoms trapped in optical resonators are ideally suited as universal quantum network nodes capable of sending, receiving, storing, and releasing photonic quantum information. The reversible exchange of quantum information between such single-atom cavity nodes is achieved by the coherent exchange of single photons. In my talk I will discuss the first experimental realization of an elementary quantum network consisting of two atom-cavity nodes that are located in remote, independent laboratories an MPQ in Garching [1]. We demonstrate the faithful transfer of arbitrary quantum states and the creation of entanglement between the two atoms. We characterize the fidelity and lifetime of the maximally entangled Bell states and manipulate the nonlocal state via unitary operations applied locally at one of the nodes. This cavity-based approach to quantum networking offers a clear perspective for scalability.

[1] S. Ritter et al., “An elementary quantum network of single atoms in optical cavities”, Nature 484, 195-200 (2012)

05.02.2014 The numerical renormalization group

Stefan Kühn

I will give a very basic introduction to the ideas of the renormalization group. After some background has been established, I would like to present the Kadanoff block spin method for the Ising model as a simple example for the application of the renormalization group. Finally I will discuss the numerical renormalization group approach to the single-impurity Anderson model.

12.02.2014 Fiber-based Fabry-Perot microcavities: A powerful tool for experiments with quantum- and nanosystems

Guest Speaker: David Hunger (MPQ, Laser Spectroscopy Group)

State of the art optical microcavities store light within volumes of wavelength scale for millions of optical cycles. Such spatio-temporal confinement of light can dramatically enhance light-matter interactions. To provide this capability on an accessible platform, we have developed microscopic Fabry-Perot cavities based on laser-machined optical fibers. The design achieves small mode volumes and large quality factors, combined with full tunability and open access to the cavity field. I will present our current efforts to build efficient single photon sources and to perform ultrasensitive microscopy with such cavities.

19.02.2014 Open quantum many-body systems: when interactions meet dissipations

Guest Speaker: Zi Cai (LMU Munich)

Understanding quantum systems embedded into environment is of particular theoretical and practical importance. The situation becomes particularly interesting and complex when the system itself is already an interacting many-body system, which provides novel perspectives to quantum many-body physics. In this talk, I will present two examples to show how the conspiracy of dissipation and interaction can significantly change the behaviors of the quantum systems and give rise to novel phenomena, including the algebraic decoherence behavior and a dissipation-induced localization. Our numerical methods can be applied to various open quantum many-body systems ranging from the Rydberg atoms and trapped ions to quantum computational systems based on solid-state devices (e.g. rf-SQUIDs, Solid-state qubits in diamonds nanostructures and quantum wells).

26.02.2014 Quantum information processing with closed timelike curves

Ivan Glasser

There is now a significant body of results on quantum interactions with closed timelike curves (CTCs) in the quantum information literature. In this talk I will present and compare two theories that make an attempt at treating time travel quantum-mechanically : Deutsch's theory of CTCs and the theory of CTCs through post-selected teleportation. Using a CTC we will kill our grandfathers and point out how the two theories have a different way of solving the causality paradox. Then we will show how CTCs enable us to distinguish non-orthogonal quantum states and to clone an arbitrary quantum state. Finally we will discuss the ambiguities that appear in non-linear extensions of quantum mechanics when dealing with proper and improper mixtures and show how these ambiguities may affect the interpretation of our previous results.

12.03.2014 Continuous Matrix Product States

Benedikt Herwerth Continuous matrix product states, introduced by F. Verstraete and J. I. Cirac in 2010, are an interesting generalization of the successful matrix product ansatz to continuous theories. As such, they provide a new, variational approach to study quantum field theories. In this seminar, we will discuss the definition of continuous matrix product states, their relation to discrete matrix product states, applications and we will consider possible future directions.

The talk will mainly focus on the studies
[1] F. Verstraete and J. I. Cirac, Phys. Rev. Lett. 104, 190405,
[2] Jutho Haegeman et al., Phys. Rev. Lett. 105, 251601,
[3] Jutho Haegeman et al., Phys. Rev. B 88, 085118.

19.03.2014 Area law and Tensor Networks

Andras Molnar

Area laws emerging from studies of the black hole entropy are a surprising feature of Nature. It tells us that information contained in a region is located on the boundary. This philosophy motivated the study of area laws for spin systems. I will overview these area laws and discuss their relation to the approximations by MPS and PEPS.

02.04.2014 Precision physics at low energy: The proton radius puzzle

Guest speaker: Randolf Pohl (MPQ, Laser Spectroscopy Group)

The recent discovery of the Higgs particle at CERN is a tremendous success of human ingenuity. Disappointingly, however, no sign of physics beyond the Standard Model has been seen so far. Precision measurements at low energy can give a complementary access to BSM physics. Here, two experiments involving muons, have displayed a large discrepancy to the Standard Model: The muon (g-2) measurement and the measurement of the proton rms charge radius from laser spectroscopy of muonic hydrogen. After a brief introduction to some precision tests of the Standard Model I will focus on the "proton radius puzzle".

09.04.2014 Inflation

Speaker: Lucas Clemente

I will give a very basic introduction to the theory of cosmic inflation, why it solves the horizon- & flatness problems and how quantum fluctuations during inflation explain the present-day structure of the universe. I will also talk about recent observations of the microwave background (WMAP, Planck, BICEP) and discuss how they confirm inflation. Please find my slides (once finished) at