Seminars 2009

Román Orús

In this talk I will give an introduction to the simulation of quantum many-body systems using the so-called tensor networks. After a brief historical review, I will introduce the basics on tensor network representations of quantum states, and will explain some recent developments. In particular, in the last part of my talk I will focus on recent results obtained in the simulation of 2-dimensional quantum lattice systems of infinite size.

Tassilo Keilmann

Mari Carmen Banuls

F. Brandao and M. Plenio, Nature Physics 4, 873 (2008).

Matteo Rizzi

Last year a news shocked the solid state community, 22 years later the seminal discovery of High-Tc superconducting cuprates by Bednorz and Müller. Namely, some compounds based on Iron were shown by Japanese and Chinese groups to be superconducting at fairly high temperatures. Since the mechanism of High-Tc superconductivity is one of the most elusive questions in modern physics, the appearence of such a new class of materials (up to now almost all HTS were cuprates) could hopefully help to unveal the 'beast'. Analogies, such as the almost 2D nature of the conducting planes, and substancial differences, such as having parent Mott insulating or metallic states, between the two families could indeed help to get a clearer view on the problem. I will go through a very general and not too specific review of such a huge topic, the aim of the talk being to stimulate interest in this new 'revitalized' playground with a big prize for the winner.

Karl Vollbrecht

Introduction on the connection between biophysics and quantum information based mainly on F. Caruso et.al.http://xxx.lanl.gov/abs/0901.4454 .

Alastair Kay

I will discuss the techniques that planet hunters use to try and find planets that are outside our solar system, and describe what we can learn about them. Finally, I will briefly discuss what measurements one might like to make in the future that could indicate whether there is life on any of these planets.

Miguel Aguado

The cosmological constant is a term in the equations of General Relativity that poses some of the deepest problems in current theoretical physics, including why it is ridiculously small *and* why it is nonzero. I will talk about points gathered from the following reviews: S. Weinberg, Rev. Mod. Phys. 61 (1989) 1; S. M. Carroll, arXiv:astro-ph/0004075; R. Bousso, arXiv:0708.4231.

Marco Roncaglia

After presenting some intuitive concept on soliton solutions of nonlinear equations, it will follow a discussion about the observation of soliton-like states in BEC's.

Mikel Sanz

We remind some physical properties of the carbon atom as well as the allotropic structures that this atom forms. We explain the differences between C and Si and Ge in the development of life. The second part is split into two parts: the first one is about the theoretical aspects of nanotubes and the relation between the cross section and the diameter with the physical properties shown by the nanotunes (conductivity, optical activity, elastic properties, etc). The second part is about the experiments and generation methods of nanotubes including the future perspectives for the industrial generation of nanotubes.

Anika Pflanzer

Tunneling is one of the hallmark effects of quantum mechanics and has been widely studied with ultracold atoms in double well traps. The central barrier in these traps is usually static and realized via light fields. But what happens if the barrier is flexible and material? This is the question to be addressed in this talk. One dimensional ultracold Bose-Bose mixtures under harmonic confinement are used to realize this setup. The localization of one of the species can be varied, changing its mass (effectively changing the strength of its trapping potential). Therefore the heavier species together with the harmonic confinement can be considered as an effective potential for the lighter species, representing a flexible, material double well. In the limit of complete localization of the heavier species, the results known for the static double well are recovered. Deviations from the infinite mass requirement allow the heavier bosons to move -this effects the ground state as well as the tunneling dynamics of the lighter bosons. The flexibility of the barrier induces an effective additional intra-species attraction between the lighter bosons. For small deviations an effective Bose-Hubbard Hamiltonian, describing the tunneling dynamics of the lighter species, is derived.

Maria Eckholt

I'll make a simple presentation of the article Nearest-Neighbor Detection of Atoms in a 1D Optical Lattice by Fluorescence Imaging, published recently by the group of Meschede in Bonn.

Birger Horstmann

Daniel Burgarth, Koji Maruyama; ArXiv: 0903.0612

Identifying the nature of interactions in a quantum system is essential in understanding any physical phenomena. Acquiring information on the Hamiltonian can be a tough challenge in many-body systems because it generally requires access to all parts of the system. We show that if the coupling topology is known, the Hamiltonian identification is indeed possible indirectly even though only a small gateway to the system is used. Surprisingly, even a degenerate Hamiltonian can be estimated by applying an extra field to the gateway.

Dmitri I. Yudin

The main object of our interest is a Bose gas with small admixture of Fermi gas at temperature below the temperature of Bose-Einstein condensation. In such mixture two fermionic atoms can attract each other by exchanging condensate-noncondensate and noncondensate-noncondensate particles. In lowest order in interaction between particles we derive the effective potential for fermion-fermion interaction due to the presence of Bose medium. The effective attraction reveals a posibility for a BCS-like superfluid transition in the Fermi subsystem. In a nonideal Bose gas there are two types of particles, condensate and noncondensate ones. One type of the indirect interaction, inherent in Bose gas, is associated with the exchange of condensate-noncondensate particles. This results in the short-range Yukawa-like interaction potential. The second type, associated with the exchange of noncondensate-noncondensate particles, exists also in the normal noncondensed state. Compared with the first one, the contribuion from the second type is weaker but has a long-range character, slowly decaying at infinity. The temperature of superfluid transition is calculated, and the properties of effective potential for fermion-fermion interaction are investigated.

The experimental realization of Bose-Einstein condensation in atomic vapors has allowed to observe a great variety of macroscopic quantum effects. In particular, there arises a considerable interest to the study of the Josephson effect in the Bose-condensed gases as one of intriguing possibilities to explore the macroscopic quantum effects related directly to the broken symmetry in the quantum systems. The dynamics of the Josephson effect is governed by the difference between the phases of the condensates, playing a role of macroscopic quantum variable. The theoretical treatment of the Josephson effect includes both the internal effect for atoms of a gas in the different hyperfine states and the case of the Bose- condensates spatially separated with a potential barrier which acts as a tunneling junction. The latter case due to its direct analogy with superconductors seems to be attractive. The dissipative dynamics of a Josephson junction in the Bose-gases (Bose-Bose mixture as well) is considered within the framework of the model of a tunneling Hamiltonian. The effective action which describes the dynamics of the phase difference across the junction is derived using functional integration method.

II-Quantum coherence from vortex patterns in phase transitions.

Manuel Donaire

In the first part of this talk I will explain the basis of a microscopical diagrammatic approach to the diople emission problem. I will deduce formulae for the emission rates of atoms placed in both virtual and real cavities embedded in generic media. The real cavity approach is potentially useful for c-QED.

In the second part, to the extent that time limitations allow, I will explain how the vorticity parttern generated during the phase transition of a condensate can be related to quantum coherence.

Leonardo Mazza

In this talk the main theoretical and experimental results of the recent research field on intersubband cavity polaritons in semiconductor quantum wells will be discussed. After a general introduction of the polariton concept for planar microcavities, it will be showed how the peculiarities of this system allow the reach of a new quantum regime of the light-matter interaction, namely the ultrastrong coupling, which could hopefully allow, in a not far future, the detection of quantum vacuum radiation outcoming from the microcavity.

Main references: (1) Dini, Koehler, Tredicucci et al; PRL, 90, 116401 (2003); (2) Carusotto, Bastard, Ciuti; PRB, 72, 115303 (2005); (3) Guenter, Anappara, Hees et al; Nature, 458, 178 (2009).

Andrew J. Daley

Christine Muschik

Main reference: S. Olmschenk, D. N. Matsukevich, P. Maunz, D. Hayes, L.-M. Duan, C. Monroe; Science Vol. 323. no. 5913, pp. 486 - 489 (2009)

Abstract of the article: Quantum teleportation is the faithful transfer of quantum states between systems, relying on the prior establishment of entanglement and using only classical communication during the transmission. We report teleportation of quantum information between atomic quantum memories separated by about 1 meter. A quantum bit stored in a single trapped ytterbium ion (Yb+) is teleported to a second Yb+ atom with an average fidelity of 90% over a replete set of states. The teleportation protocol is based on the heralded entanglement of the atoms through interference and detection of photons emitted from each atom and guided through optical fibers. This scheme may be used for scalable quantum computation and quantum communication.

Oriol Romero-Isart

Quantum Darwinism aims at disentangling the "Gordian knot" between the classical and quantum realm --that is, not just cutting it and describing two separate worlds, as Bohr did. This is also motivated with recent experiments, which confirm the validity of quantum laws on scales that begin to infringe on the "macroscopic". Therefore, the origin of "the classical" can no longer be dismissed. I will try to review the research along this direction following the recent progress article "Quantum Darwinism", W.H. Zurek, Nature Physics 5, 181 March 2009.

Christina Kraus

In this talk I will introduce a notion of generalized entanglement which goes beyond the conventional definition based on quantum subsystems. Here, entanglement is defined as a property of a quantum state relative to a distinguished set of observables. The relation to the standard view of entanglement, its application to systems of indistinguishable particles and its connection to quantum phase transitions will be discussed. See: quant-ph/0701124, 0403044, 0403043

Fernando Pastawsky

The Kuramoto model describes a large population of coupled limit-cycle oscillators whose natural frequencies are drawn from some prescribed distribution. If the coupling strength exceeds a certain threshold, the system exhibits a phase transition: some of the oscillators spontaneously synchronize, while others remain incoherent. The mathematical analysis of this bifurcation has proved both problematic and fascinating. We review 25 years of research on the Kuramoto model, highlighting the false turns as well as the successes, but mainly following the trail leading from Kuramoto’s work to Crawford’s recent contributions. It is a lovely winding road, with excursions through mathematical biology, statistical physics, kinetic theory, bifurcation theory, and plasma physics.

1. Steven H. Strogatz, “>From Kuramoto to Crawford: exploring the onset of synchronization in populations of coupled oscillators,” Physica D: Nonlinear Phenomena 143, no. 1-4 (September 1, 2000): 1-20.

Michael Möckel

Motivated by recent experiments in ultracold atomic gases that explore the nonequilibrium dynamics of interacting quantum many-body systems, we investigate the opposite limit of Landau’s Fermi liquid paradigm: We study a Hubbard model in more than one dimension with a sudden interaction quench, that is the interaction is switched on at time t = 0. Using the flow equation method, we are able to study the real time dynamics for weak interaction U in a systematic expansion and find three clearly separated time regimes: i) An initial buildup of correlations where the quasiparticles are formed. ii) An intermediate quasi-steady regime resembling a zero temperature Fermi liquid with a nonequilibrium quasiparticle distribution function. iii) The long time limit described by a quantum Boltzmann equation leading to thermalization of the momentum distribution function with a temperature T ∝ U.

M. Moeckel and S. Kehrein, Phys. Rev. Lett. 100, 175702 (2008) M. Moeckel and S. Kehrein, arXiv:0903.1561 (2009), accepted for publication in Annals of Physics, N.Y.

Lian-Ao Wu

Density functional theory (DFT) is shown to provide a novel conceptual and computational framework for entanglement in interacting many-body quantum systems. DFT can, in particular, shed light on the intriguing relationship between quantum phase transitions and entanglement. We use DFT concepts to express entanglement measures in terms of the first or second derivative of the ground state energy. We illustrate the versatility of the DFT approach via a variety of analytically solvable models. As a further application we discuss entanglement and quantum phase transitions in the case of mean field approximations for realistic models of many-body systems.

Norbert Schuch

Lucas Lamata

We will discuss the progress of the past decade in testing for a possible temporal variation of the fine structure constant. Advances in atomic sample preparation, laser spectroscopy and optical frequency measurements led to rapid reduction of measurement uncertainties. Eventually laboratory tests became the most sensitive tool to detect a possible variation of the fine structure constant at the present epoch. We explain the methods and technologies that helped make this possible.

Kolachevsky et al., arXiv:0904.1663

Valentin Murg

Fabio Mezzacapo

I will discuss recent experimental findings showing that the cell migration mechanism is a cooperative phenomenon due to traction forces arising at the front edge of a migrating cell sheet as well as many cell rows behind the leading one, and extending across enormous distances. Similarities and differences between the physics of a migrating cell sheet and that of granular materials are discussed on the basis of a simple theoretical model.

Roman Schmied

I present two sets of experiments where single particles (neutrons, Rb-, and Cs-atoms) bounce up and down in earth's gravitational field. In the atomic case, bouncing atoms can be made to interfere with themselves, resulting in an extraordinarily sensitive means for measuring the local gravitational acceleration.

References: quantized states of neutrons in earth's gravity field: T.J. Bowles, News and Views , Nature 415, 267 (2002)

atoms bouncing in earth's gravity field: K.J. Hughes, J.H.T. Burke, C.A. Sackett, PRL 102, 150403 (2009) K.Y. Chung, S. Chiow, S. Herrmann, S. Chu, H. Müller, arXiv:0905.1929 (2009)

Heike Schwager

The seminar will be on the Zeno and the Anti-Zeno effect. I will start 2500 years back with Zeno's paradox, explain the Quantum Zeno and Anti-Zeno effect and discuss "milestone" experiments such as http://link.aps.org/doi/10.1103/PhysRevA.41.2295 and http://link.aps.org/doi/10.1103/PhysRevLett.87.040402.

Norbert Schuch

One of the central problems in quantum mechanics is to determine the ground state properties of a system of electrons interacting via the Coulomb potential. Since its introduction by Hohenberg, Kohn, and Sham, Density Functional Theory (DFT) has become the most widely used and successful method for simulating systems of interacting electrons, making their original work one of the most cited in physics. In this work, we show that the field of computational complexity imposes fundamental limitations on DFT, as an efficient description of the associated universal functional would allow to solve any problem in the class QMA (the quantum version of NP) and thus particularly any problem in NP in polynomial time. This follows from the fact that finding the ground state energy of the Hubbard model in an external magnetic field is a hard problem even for a quantum computer, while given the universal functional it can be computed efficiently using DFT. This provides a clear illustration how the field of quantum computing is useful even if quantum computers would never be built.

arXiv: 0712.0483

Géza Giedke

Lloyd et al., arXiv:0906.2762 and arXiv:0906.2758
We show that the minimum output entropy for all single-mode Gaussian channels is additive and is attained for Gaussian inputs. This allows the derivation of the channel capacity for a number of Gaussian channels, including that of the channel with linear loss, thermal noise, and linear amplification.

Karl Vollbrecht

I will discuss about the following paper.
Authors: J. Kiukas, A. Ruschhaupt, R. F. Werner. (Submitted on 10 Jul 2009)
Abstract: We consider the time delay of massive, non-relativistic, one-dimensional particles due to a tunneling potential. In this setting the well-known Hartman effect asserts that often the sub-ensemble of particles going through the tunnel seems to cross the tunnel region instantaneously. An obstacle to the utilization of this effect for getting faster signals is the exponential damping by the tunnel, so there seems to be a trade-off between speedup and intensity. In this paper we prove that this trade-off is never in favor of faster signals: the probability for a signal to reach its destination before some deadline is always reduced by the tunnel, for arbitrary incoming states, arbitrary positive and compactly supported tunnel potentials, and arbitrary detectors. More specifically, we show this for several different ways to define ``the same incoming state and the same detector when comparing the settings with and without tunnel potential. The arrival time measurements are expressed in the time-covariant approach, but we also allow the detection to be a localization measurement at a later time.

Oliver Buerschaper

I'll try and explain what topological insulators are, how many of them there are and why you should care about them at all.

Ines de Vega

Photosynthetic complexes are tuned by nature to capture solar light efficiently, and then transmit the excitation energy to reaction centers, where this energy is used to realize chemical reactions. This transfer mechanism has been considered for a long time as a classical process. Nevertheless, recent experiments realized with in-vitro photosynthetic complexes have shown that quantum coherences play indeed an important role in the process. This has driven the attention of several theoretical quantum physicists, that have tried to give intuitive pictures that explain the importance of coherence in the transport process, as well as the crucial role of the dephasing noise. In this talk, I will review some of the most relevant experimental and theoretical proposals that have appeared in the last couple of years.

Eric Kessler

The relation between theory and experiment in Quantum Mechanics is a delicate question. Since the famous disputes on the completeness of QM between Einstein and Bohr in the 1920's it became the standard procedure to divide the system and the measurement device /ad hoc/ into quantum and classical parts.
I will show that including the measurement device into the quantum description on acquires a powerful tool for the observation of pure quantum behavior. In that spirit I will briefly discuss the von Neumann model of quantum measurements and introduce generalized expectation values the so called /weak values/. On this foundation I will then present experiments, glancing at the weird quantum world, that measure negative kinetic energy of tunneling particles, unbound spins and super light speed.

Miguel Aguado

Holography is a broad (and sometimes fuzzy) set of ideas connecting physics in the bulk of a system with a related system defined in its boundary. Entropic area laws are cited as an example of holographic concepts in Condensed Matter. Here I will talk about holography as seen by a high energy theorist (actually a string theorist): it has been proposed as the principle underlying any possible theory that unifies gravity and the other fundamental interactions. The cast of this seminar: a Black Hole (gloomy), a Light Sheet (null), and the couple formed by an Anti-de Sitter (negative) and a Conformal Theory (superficial).

References:

R. Bousso, Rev. Mod. Phys. 74, 825 (2002).

Birger Horstmann

A short introduction to the ideas of optical olography and its applications in imaging and interferometry. It's a very basic review.

Mari Carmen Banuls

I will review some recent experimental and theoretical works that have focused on the problem of whether and how non-equilibrium dynamics of isolated many-body quantum systems leads to thermal equilibrium. Based on Nature 452, 854 (2008).

Tassilo Keilmann

Maarten Van den Nest

Topological methods are widely used in many fields in physics and are the topic of an increasing number of papers, also in our own research areas. In this talk I will give a very basic introduction to some of the elementary concepts in (Algebraic) Topology. The idea of the seminar will be to introduce some fancy-sounding notions in this very beautiful branch of mathematics on an intuitive level.

The basis of the talk will be a review article by N. D. Mermin (Rev. Mod. Phys. 51, 591 (1979)).

Richard Jozsa - University of Bristol, @ 15.15 in the Herbert Walther Auditorium

Quantum computations that can be classically efficiently simulated offer no direct computational benefits over classical computation, yet their study is of great interest for illuminating possible origins of quantum computational power. We will review some of the best known examples of simulatable computations and describe a simple general formalism for generating such classes. We will show that the efficient simulation of Clifford circuits (Gottesman-Knill theorem) and of matchgate circuits (Valiant's theorem), appear as two special cases.

Further examination of the techniques used in the simulations can reveal a more precise characterisation of the computational power of the circuits. Correspondingly we may briefly describe how matchgate circuits can be related to so-called log-space bounded quantum computation.

Martin Kiffner

In one-dimensional systems, the quantum statistics of particles is not independent of their interaction. A prominent example are bosons interacting via strong repulsive forces that can enter a Tonks-Girardeau (TG) gas regime, where they behave with respect to many observ- ables as if they were fermions. A TG gas can be described as the strong interaction limit of the Lieb-Liniger model, and the recent progress in cooling and trapping of atoms allowed the experimental observation of a TG gas. Recently, an experiment with cold molecules showed that not only elastic interactions, but even two-particle losses alone are able to create a TG gas where two molecules never occupy the same position, thereby avoiding dissipation of particles. This counterintuitive result can be regarded as a manifestation of the quantum Zeno effect.
In my talk I will present a scheme for the generation of a TG gas of photons with purely dis- sipative interaction. Based on a master equation approach for the description of stationary light in atomic four-level media we show that, under suitable conditions, two particle decays are the dominant photon loss mechanism. These dissipative two-photon losses increase the interaction strength between photons by at least one order of magnitude as compared to dis- persive two-photon processes. Our scheme allows for measurements of various characteristic correlations of the TG gas via standard quantum optical techniques, including quantities that distinguish it from free fermions.

Matteo Rizzi

The seminar is intended to be a tutorial on this topic for the many new people in the group and an opportunity for the others to understand what is within experimental feasibility (and then is sensible to be proposed theoretically for implementing quantum simulations and/or quantum info-computation).

Michael Möckel

Since its development in the 1950ies by Landau, Fermi Liquid theory became a paradigmatic description of correlated many-particle systems which provides a generic framework to study as diverse physical systems as electron gases in metals, neutron stars, liquid 3He and, recently, correlated fermions in optical lattices. In my talk (which I will keep very basic) I will present fundamental derivations and results which might be helpful to understand the behavior of recent and future experiments. All group members are kindly invited!