Institut für Theoretische Physik I
Universität Erlangen-Nürnberg
Staudtstraße 7
91058 Erlangen

How to find us

Phone: +49-9131-85 28442
Fax: +49-9131-85 28444

Department Physik
Teilbibliothek Physik Web-Opac
UnivIS FAU Erlangen-Nürnberg

Physikalisches Kolloquium
Kolloquium der Theor. Physik
Gruppenseminar der Theorie 1

Kolloquium der Theoretischen Physik

Sommersemester 2014

ca. jeden zweiten Dienstag jeweils 16–18h c.t. im Hörsaal F
um zahlreiches Erscheinen wird gebeten

  • 15.04.2014
    Andreas Döring (Erlangen)
    Contexts, Quantum State Spaces, and Foundations of Quantum Theory [show abstract]

    Classical physics is a highly geometric theory, based on the concept of a state space. There is a close link between topological-geometric aspects and logical aspects of the theory, mathematically given by suitable (categorical) dualities. In contrast, quantum theory lacks such a nice geometric underpinning, and no straightforward duality results exist for noncommutative algebras of observables. In this talk, I will show how a contextual description of quantum systems leads to a new notion of quantum state spaces and hence to a more geometric picture of quantum theory. Partial duality results exist in this setting, which also leads to a powerful new form of logic for quantum systems. This work is part of the so-called topos approach to quantum theory by Isham, Butterfield, Döring, Landsman et al.

  • 29.04.2014
    Eli Barkai (Bar-Ilan, Israel)
    Weak ergodicity breaking: from Blinking quantum dots to mRNA di using in the cell [show abstract]

    In nature the noisy signal representing a physical observable is in many cases unpredictable, though the long time average of the signal converges in statistical sense to the ensemble average (ergodicity). On the nano-scale noise levels are large and many processes exhibit ergodicity breaking. Examples include blinking nano-crystals [1] and single molecules (e.g. mRNA) sub-di using in live cells [2]. Such processes are analyzed within the statistical framework of weak ergodicity breaking. Foundations of the theory are discussed based on random walk models. Since time averages of physical observables remain random in such systems, usual ergodic statistical mechanics cannot be applied.

    [1] F. D. Stefani, J. P. Hoogenboom, and E. Barkai Beyond Quantum Jumps: Blinking Nano-scale Light Emitters Physics Today 62 nu. 2, p. 34 (February 2009).
    [2] E. Barkai, Y. Garini and R. Metzler Strange Kinetics of Single Molecules in the Cell Physics Today 65(8), 29 (2012).

  • 27.05.2014
    Kilian Singer (Mainz)
    Moving Ions [show abstract]

    Novel ion trap geometries for deterministic high resolution ion implantation are presented which are obtained by highly efficient field calculation methods [1]. I will present our recent progress with a segmented ion trap with mK laser cooled ions which serves as a high resolution deterministic single ion source. It can operate with a huge range of sympathetically cooled ion species, isotopes or ionic molecules. We have deterministically extracted a predetermined number of ions on demand [2], moved the ions inside the trap[3] without exciting any motional quanta and reached extraction accuracies of less than 10nm. These results a first step in the realization of an atomic nano assembler, a novel device capable of placing an exactly defined number of atoms or molecules into solid state substrates with sub nano meter precision in depth and lateral position. The project is motivated by the quest for novel tailored solid state quantum materials generated by deterministic high resolution ion implantation. Ions can also be moved by heat realizing a heat engine scaled down to a single ion [4]. We propose driving the trapped ion in an Otto cycle, oscillating in a specially designed linear Paul trap and coupled to engineered laser reservoirs. We present detailed Monte Carlo simulations and the calculation of the efficiency of such single ion heat engine, exceeding the standard Carnot limit when employing a squeezed thermal reservoir [5]. Finally, I will discuss how ions can be moved in the radial direction performing the experimental investigation of the Kibble-Zurek mechanism, where control parameters are tailored such that a structural phase transition from a linear to a zigzag configuration of the crystal is crossed. Trapped ions serve here as a clean model system to investigate universal laws of defect formation when such transition is crossed fast and causally separated regions form. The amount of defects is predicted by the Kibble-Zurek mechanism. We have experimentally determined the universal scaling exponent for defect formation and confirm the scaling law for the inhomogeneous Kibble-Zurek effect accurately at the percent level [6].

    [1] K. Singer, U. G. Poschinger, M. Murphy, P. A. Ivanov, F. Ziesel, T. Calarco, F. Schmidt- Kaler, Rev. Mod. Phys. 82, 2609 (2010).
    [2] W. Schnitzler, N. M. Linke, R. Fickler, J. Meijer, F. Schmidt-Kaler, and K. Singer, Phys. Rev. Lett. 102, 070501 (2009).
    [3] A. Walther, F. Ziesel, T. Ruster, S. T. Dawkins, K. Ott, M. Hettrich, K. Singer, F. Schmidt-Kaler, U. G. Poschinger, Phys. Rev. Lett. 109, 080501 (2012).
    [4] O. Abah, J. Roßnagel, G. Jacob, S. Deffner, F. Schmidt-Kaler, K. Singer, E. Lutz, Physical Review Letters 109, 203006 (2012).
    [5] J. Roßnagel, O. Abah, F. Schmidt-Kaler, K. Singer, E. Lutz, Physical Review Letters 112, 030602 (2014).
    [6] S. Ulm, J. Roßnagel, G. Jacob, C. Degünther, S. T. Dawkins, U. G. Poschinger, R. Nigmatullin, A. Retzker, M. B. Plenio, F. Schmidt-Kaler, K. Singer, Nature Communications 4, 2290 (2013).

  • 24.06.2014
    Benjamin Bahr (Hamburg)
    On background-independent renormalization in state-sum models [show abstract]

    In this talk I will give an overview of the state sum models that have been proposed in recent years in an attempt to define a path integral for quantum gravity in a non-perturbative and background-independent way. The form of these models is motivated from discrete gravity, so emphasis will lie on the questions of how to define a continuum limit, and in which way one should expect diffeomorphism symmetry (which plays the role of a gauge symmetry in classical GR) to arise in these models. A proposal for a background-independent formulation of renormalization group flow is presented, which does not require a notion of length scale, and which can be applied to these models. Time permitting, examples and numerical approximation methods will be presented as well.

  • 08.07.2014
    Tobias Donner (ETH Zürich)
    Exploring cavity-mediated long-range interactions in a quantum gas [show abstract]

    Creating quantum gases with long-range atom-atom interactions is a vibrant area of current research, possibly leading to the observation of novel quantum phases and phase transitions. In our approach, we couple the external degree of freedom of a Bose-Einstein condensate to the vacuum mode of a high-finesse optical cavity using a non-resonant standing-wave transverse pump beam. This gives rise to a driven-dissipative system with cavity-mediated atom-atom interactions of global range, which are tunable in both magnitude and sign.

    Increasing the interaction strength leads to a softening of an excitation mode at finite momentum, preceding a superfluid-to-supersolid phase transition, which can be mapped onto the Dicke quantum phase transition. This mode softening, which is reminiscent of the roton-minimum in liquid Helium, is accompanied by diverging fluctuations of the atomic and photonic fields. The openness of the cavity offers natural access to the system and allows us to apply quantum optical methods to investigate the many-body system. We study density fluctuations of the quantum gas in real-time while the system undergoes the Dicke quantum phase transition. The observed critical exponent deviates significantly from a mean-field model. Our experiment can be seen as a basic building block for quantum simulations of key models in quantum-many body physics.