Abstracts

Charles S. Adams	Light propagation in systems with strong dipole-dipole interactions
	The effect of strong dipole-dipole interactions on the propagation of light is studied in dense atomic ensembles with nanometer thickness [1,2] and in Rydberg systems [3]. In Rydberg ensembles, interactions can give rise to large optical non-linearities at the single photon level [4] and non-equilibrium phase transitions [5]. We will also discuss preliminary observations of exciton hopping in an emergent Rydberg lattice providing a powerful platform to study resonant energy transfer in systems with disorder. [1] J Keaveney et al., Phys. Rev. Lett. 108, 173601 (2012). [2] J Keaveney et al., , Phys. Rev. Lett. 109, 233001 (2012). [3] JD Prichard et al., Phys. Rev. Lett. 105, 193603 (2010). [4] D Maxwell et al., Phys. Rev. Lett. 110, 103001 (2013). [5] C Carr et al., arXiv:1302.6621
Eric Akkermans	Mesoscopic physics of photons in disordered, fractal and correlated systems
	The talk will first review basic notions of coherent transport at the mesocopic scale for both photonic and electronic systems. Then, we shall consider more specific problems (e.g., spontaneous emission, transport, cooperative effects, localization) related to underlying spectral properties of photons in either disordered or fractal mesoscopic systems.
Jonathan Bird	Mesoscopic Systems with Continuum Coupling
	The problem in which a mesoscopic device is coupled to an extended continuum of states provides a model system for investigating the transition from the classical to the quantum realm. In this presentation I address this problem experimentally, by demonstrating how small mesoscopic structures (quantum point contacts) may be used to embed discrete quantum states within a continuum. A key result is the demonstration that, at least on a sufficiently small scale, the continuum may support highly nonlocal interactions between separated mesoscopic devices. Notably, we demonstrate how this continuum coupling can support an interaction between a pair of discrete levels that is actually stronger than that which one might expect in the case where their wavefunctions overlap directly. The possibility of applying this continuum-based coupling to active electronic and photonic devices will also be considered. J. P. Bird and Y. Ochiai, Science 303, 1621 (2004). Y. Yoon, L. Mourokh, T. Morimoto, N. Aoki, Y. Ochiai, J. L. Reno, and J. P. Bird, Phys. Rev. Lett. 99, 136805 (2007). Y. Yoon, M.-G. Kang, T. Morimoto, L. Mourokh, N. Aoki, J. L. Reno, J. P. Bird, and Y. Ochiai, Phys. Rev. B 79, 121304(R) (2009). Y. Yoon, M.-G. Kang, T. Morimoto, M. Kida, N. Aoki, J. L. Reno, Y. Ochiai, L. Mourokh, J. Fransson, and J. P. Bird,. Rev. X 2, 021003 (2012).
Tobias Brandes	Excited states quantum phase transitions in Dicke-type models
	I will discuss some recent progress, mostly on non-analyticities in densities of states for superradiance phase transition (Dicke) models.
Luca Celardo	Subradiance localization in the open 3D Anderson-Dicke model
	Anderson localization is a paradigmatic coherence effect in disordered systems, often analyzed in the absence of dissipation. Here we consider the case of coherent dissipation, occuring for open system with coupling to a common decay channel. This dissipation induces cooperative Dicke super- and subradiance and an effective long range coupling, expected to destroy Anderson localization. We are thus in presence of two competing effects, i.e localization driven by disorder and delocalization driven by dissipative opening. Here we show that in an open 3D Anderson model, subradiance enables the system to preserve signatures of a localization transition. We demonstrate the existence of a subradiant localized regime, emerging from the interplay of opening and disorder, in which subradiant states are hybrid and localized, while superradiant states are extended. We also provide analytical predictions for this regime, confirmed by numerical simulations.
Philippe W. Courteille	Light scattering from ordered and disordered atomic clouds
	The scattering of light at an ensemble of scatterers is an intrinsically collective process. Matter wave superradiance and collective atomic recoil lasing are famous examples of how the irradiation of light into an ultracold gas can cause a collective instability resulting in a phase transition to a non-equilibrium state. In these cases, the instability is caused by correlations between scattering events either sustained by long-lived matter wave coherences or mediated by long-lived modes of a high finesse ring cavity. However, collective scattering even occurs with single photons. Recent experiments demonstrated collective effects via radiation pressure measurements and pinned down the respective roles of cooperativity, disorder and finite volume effects in the scattering process. In cases, where the atomic cloud is ordered within an optical lattice, cooperative scattering can give rise to photonic band structures, reminiscent to what is observed in photonic crystals.
Maarten DeKieviet	Exploring geometric phases in a quantum mechanical experiment.
	TBA
Rosario Fazio	Quantum limits to optimal control
	Optimal control theory is a very promising candidate for a drastic improvement of the performance of quantum information processing. By exploring its ultimate limit of performance it is possible to show that it coincides with the maximum speed allowed by quantum evolution: The quantum speed limit. I will discuss some paradigmatic cases in BECs and in interacting spin systems and show recent related experiments.
Stefan Gehler	Experimental realization of resonant assisted tunneling in open systems
	In quantum mechanical billards with a mixed phase space direct tunneling from regular islands to the chaotic sea is well known [1],[2]. For quantum mechanical maps it was shown, that the tunneling rates are determined by another effect the so-called resonance-assisted tunneling [3], which is an indirect process whire another stable island. To verify this theory a Cosin-shaped microwave resonator with absorbers was designed, where the absorbers are located in such a way that the chaotic sea is replaced by the continuum. The experimental results are in good agreement with the theoretical predictions. [1] A. Bäcker et al., Phys. Rev. Lett. 100, 104101 (2008) [2] A. Bäcker et al., Phys. REv. Lett. 100, 174103 (2008) [3] S. Löck et al., Phys. Rev. Lett. 104, 114101 (2010)
Eva-Maria Graefe	Mean-field dynamics for Bose-Einstein condensates with non-Hermitian interactions
	Recently a generalised mean-field approximation for non-Hermitian many particle systems has been introduced for a Bose-Hubbard dimer with loss and gain. Here we apply this approximation to a Bose-Hubbard dimer with a complex particle interaction term, modelling particle losses due to interaction in a two mode Bose-Einstein condensate. We derive the mean-field equations of motion, and analyse the resulting stationary states and the dynamics. The resulting dynamics is contrasted to those arising from a Gross-Pitaevskii type equation with complex nonlinearities.
Thomas Guhr	Distribution of Off-Diagonal Scattering Matrix Elements
	Chaotic Scattering is ubiquitous in classical and quantum wave systems, comprising phenomena from the micro to the macroscale. In nuclear reaction theory, the Heidelberg approach was developed which models stochastic and chaotic behavior on the level of the Hamiltonian describing the scattering region. Due to its universality, it found fruitful applications to all systems where chaotic scattering occurs. Already in the 1980's, energy correlation functions of the scattering matrix were calculated exactly. However, for a long time, the distribution of the scattering matrix elements, which is of considerable theoretical and experimental interest, could not be calculated analytically within the Heidelberg approach. Only a few years ago, the problem was solved for the diagonal scattering matrix elements. In this talk, I show how we succeeded in closing the last remaining gap by calculating, exactly and completely, the distribution of the off-diagonal scattering matrix elements. In a collaboration with the Darmstadt group we compare our results with data from microwave experiments.
Susana Huelga	Exciton transport in light harvesting complexes
	TBA
Felix Izrailev	Open versus closed PT-symmetric models: Gain/loss induced localization
	We study the relevance of energy spectrum and eigenstates in a closed 1D tight-binding model with gain/loss, to transport properties of the same model with perfect leads. We show how the both models, either closed or open, can be rigorously analyzed in the unique approach. Main attention is paid to the properties of energy spectrum and structure of eigenstates (in closed model), and to the transmission coefficient and structure of scattering states (in open model). Our approach allows one to reveal the mechanism responsible for the onset of localization which is due to either loss or gain, or due to the PT-symmetric combination of gain/loss entries. Analytical results are complemented by the data obtained numerically. The influence of weak disorder is discussed as well.
Lev Kaplan	Superradiance Transition in Transport with Symmetric and Assymetric Coupling to the Leads
	Using an energy-independent non-Hermitian Hamiltonian approach to open systems, we fully describe transport through a sequence of potential barriers as external barriers are varied. We consider the general case of symemtric or asymmetric external barriers and variable coupling strength to the environment. We demonstrate that transport properties are very sensitive to the degree of opening of the system and determine the parameters for maximum transmission at any given degree of asymmetry. Analyzing the complex eigenvalues of the non-Hermitian Hamiltonian model, a single or double transition to a superradiant regime is shown to occur for symmetric and asymmetric coupling, respectively. A drastic change in the structure of resonances is demonstrated at these transitions. Finally, We extend our analysis to the presence of disorder and to higher dimensions.
Tsampikos Kottos	Taming wave propagation via Parity-Time symmetry: Examples from integrated optics and electronics
	We will introduce the notion of Parity-Time (PT) symmetry and present its implications in classical wave propagation. Using integrated optics and electronics as a playfields we show how one can construct new circuitry designs that allow for asymmetric transport due to interplay of the novel properties of PT-symmetry and non-linearity or gyrotropic elements.
Maksim Miski-Oglu	Graphene, photonic crystals and playing Dirac billiards with microwaves
	After a brief introduction into the salient features of the two-dimensional material Graphene and recapitulating the analogy between two-dimensional non-relativistic (Schrödinger) and relativistic (Dirac) quantum billiards and microwave billiards, the experimental modeling of certain spectral properties of Graphene and their theoretical interpretation in normal and superconducting billiards will be demonstrated. Main topics to be discussed are e.g. the band structure, the local density of states at the Dirac point, its relation to the scattering matrix and its use for determining the experimental length spectrum of periodic orbits in the relativistic regime around the Dirac point and in the non-relativistic one away from it, and the effect of edge states on the behavior of the mean density of states as function of quasi-momentum. Finally it is shown how the logarithmic divergence of states at the so-called Van Hove singularities can be interpreted as a Lifshitz topological phase transition. *Supported by the Deutsche Forschungsgemeinschaft (DFG) within the Collaborative Research Center SFB634.
Markus Oberthaler	Bifurcation – in the classical and quantum regime
	We report on our recent experimental results obtained in the context of bosonic internal Josephson junctions i.e. quantum dimer, which allow for the realization of a classical bifurcation scenario. The classical [1] as well as the quantum dynamics is experimentally investigated in the situation of a single external mode and the results are in good agreement with the theoretical expectations. Furthermore we will present our latest results on a driven quantum dimer. Analysing mean and variances of the particle number difference between the two states indicate the existence of mixed phase space. A further extension to more complex dynamics has been undertaken by adding more external degrees of freedom. With this system the structures emerging if the system is quenched to the quantum critical point has been investigated. The system has also the potential for a systematic study of emergent structures in a quench through the quantum phase transition [2]. We will present our recent results on scaling of the excitations generated by quenching the system close to the critical point. [1] Zibold, Tilman, Eike Nicklas, Christian Gross, und Markus Oberthaler. “Classical Bifurcation at the Transition from Rabi to Josephson Dynamics“. Physical Review Letters 105, 204101 (2010). [2] Jacopo Sabbatini, Wojciech H. Zurek, and Matthew J. Davis. “Phase Separation and Pattern Formation in a Binary Bose-Einstein Condensate”, Physical Review Letters 107, 230402 (2011)
Uri Peskin	Reservoir control of charge transport in molecular networks
	The question whether molecular systems can support coherent (phase-conserving) quantum transport in spite of their dissipative environment is raised again, following recent experiments on electron-energy transfer in bio-molecules. In this work we formulate conditions in which the coupling to a dissipative environment can promote fast coherent transport in molecules, and can be utilized to control it. This is demonstrated for models of charge transport in a multi-chromophoric molecular network in a solvent environment, and for charge transport through molecular junctions, in which coherent intra-molecular dynamics is coupled to macroscopic Fermions reservoirs.
Achim Richter	Chaotic scattering in open microwave billiards with and without time-reversal violation
	Chaotic quantum scattering occurs when the Schrödinger waves are scattered by a system with chaotic classical dynamics. In this talk it is discussed how open flat microwave resonators, which have long been known to be paradigms for quantum billiards, can be used for modeling chaotic quantum scattering in case of time-reversal invariance and its violation [1]. By performing reflection and transmission measurements in a resonator attached to two antennas the distribution of the scattering matrix and their fluctuations are determined in the regime of weakly overlapping resonances for both GUE and GOE systems, respectively. The results [2,3] are compared to predictions from the theory of chaotic scattering largely developed in the framework of nuclear reactions. [4]. Our work constitutes one of the most stringent tests of this statistical theory. [1] B. Dietz, T. Friedrich, H. L. Harney, M. Miski-Oglu, A. Richter, F. Schäfer, J. Verbaarschot, and H. A. Weidenmüller, Phys. Rev. Lett. 103, 064101 (2009). [2] B. Dietz, T. Friedrich, H. L. Harney, M. Miski-Oglu, A. Richter, F. Schäfer, and H. A. Weidenmüller, Phys. Rev. E 81, 036205 (2010). [3] B. Dietz, H. L. Harney, A. Richter, F. Schäfer, and H. A. Weidenmüller, Phys. Lett. B 685, 263 (2010). [4] G. E. Mitchell, A. Richter, and H. A. Weidenmüller, Rev. Mod. Phys. 82, 2845 (2010). *Supported by the Deutsche Forschungsgemeinschaft (DFG) within the Collaborative Research Center SFB634.
Ralf Röhlsberger	Quantum Optics with X-rays: Cooperative Emission from Nuclei in Cavities
	The nuclear resonances of Mössbauer isotopes constitute almost ideal two-level systems to study cooperative effects in the interaction of x-rays with matter. Embedding an ensemble of resonant Mössbauer nuclei into a planar cavity facilitates the excitation of superradiant oscillatory eigenmodes via pulses of synchrotron radiation. These modes exhibit a large collective Lamb shift that can be controlled via the excitation conditions and the cavity geometry [1]. Cooperative emission in connection with the radiative coupling of the nuclei in the cavity leads to phenomena like electromagnetically induced transparency [2] and spontaneously generated coherences [3], opening interesting perspectives to explore concepts of quantum control, quantum state engineering and quantum many-body physics in the x-ray regime. This contribution gives a review on cooperative effects in nuclear resonant scattering of synchrotron radiation with special emphasis on cavity mediated effects. [1] R. Röhlsberger, K. Schlage, B. Sahoo, S. Couet, and R. Rüffer, Collective Lamb Shift in Single-Photon Superradiance, Science 328, 1248 (2010). [2] R. Röhlsberger, H.-C.Wille, K. Schlage, and B. Sahoo, Electromagnetically Induced Transparency with Resonant Nuclei in a Cavity, Nature 482, 199 (2012). [3] K. P. Heeg, H.-C. Wille, K. Schlage, T. Guryeva, D. Schumacher, I. Uschmann, K. S. Schulze, B. Marx, T. Kämpfer, G. G. Paulus, R. Röhlsberger, and J. Evers, in preparation.
Stefan Rotter	Relating Coherent Transport to Modes in Random and PT-symmetric media
	In my talk I will present both theoretical and experimental work on the connection between coherent transport and the internal modes in complex scattering systems. In the first part, I will focus on microwave scattering through strongly disordered media. In the deeply localized limit we find that transmission is dominated by a single transmission channel which is formed by a unique localized mode or spectrally overlapping necklace modes. This situation leads to conspicuous signatures in the statistical properties of transmission which I will discuss in detail. In the second part of my talk, I will focus on PT–symmetric scattering systems with gain and loss. In particular, I will show how the PT–symmetry breaking points of an unbounded scattering system relate to those of the underlying bounded systems. Based on the insights drawn from this relation, we are able to show that, under very general conditions, the PT–transitions in the scattering matrix are entirely insensitive to a variable coupling strength between the bounded region and the unbounded asymptotic region. I will end my talk with making specific predictions for how this result can be tested in the experiment.
Ingrid Rotter	Does Fermi's golden rule hold in open quantum systems?
	We study generic features of open quantum systems embedded into a continuum of scattering wavefunctions and compare them with results discussed in optics. The environment exists always and cannot be deleted. A dynamical phase transition appears at high level density in a many-particle system and also in a two-level system which is studied mainly in optics. The coupling $W$ between the states of the system via the environment is complex. Re$(W)$ causes level repulsion while Im$(W)$ causes width bifurcation. In the case of two neighboring states with equal widths, two singular (exceptional) points exist. In the parameter range between these two points, $W$ is imaginary and the widths bifurcate as a function of a certain parameter, without any enhancement of $W$. The larger $W$ the stronger width bifurcation (up to a natural limit). Similar results are obtained numerically for $N>2$ states and for neighboring states with different widths. Time reversal symmetry breaking and its violation in the neighborhood of exceptional points is discussed. Using a unitary representation of the $S$ matrix, the cross section is calculated for a two-level system and for different $W$, including at the exceptional point (double pole of the $S$ matrix). The results obtained for the transition of level repulsion at small (real) $W$ to width bifurcation at large (imaginary) $W$ show qualitatively the same features that are observed experimentally in the transition from Autler-Townes splitting to electromagnetically induced transparency in optics. Fermi's golden rule holds only below the dynamical phase transition. It is violated beyond the dynamical phase transition.
Henning Schomerus	Topologically protected states in photonic systems
	One of the principal goals in the design of photonic crystals is the engineering of band gaps and defect states. Drawing on the concepts of band-structure topology, I describe the formation of exponentially localized, topologically protected midgap states [1]. When gain and loss are suitably arranged these states maintain their topological protection and then acquire a selectively tunable amplification rate. This finds applications in the beam dynamics along a photonic lattice and in the lasing of quasi-one-dimensional photonic crystals. I also describe the formation of highly degenerate Landau-like levels in strained photonic honeycomb lattices [2]. These states display a related selective amplification mechanism and could form the basis of a degenerate laser. [1] Topologically protected midgap states in complex photonic lattices H. Schomerus, arXiv:1301.0777 [physics.optics]. [2] Parity anomaly and Landau-level lasing in strained photonic honeycomb lattices H. Schomerus and N. Yunger Halpern, Phys. Rev. Lett. 110, 013903 (2013), arXiv:1208.2901 [cond-mat.mes-hall].
Marlan O. Scully	Quantum Thermodynamics: Increasing Quantum Heat Engine Efficiency via Quantum Coherence
	Laser and photocell quantum heat engines (QHEs) are powered by thermal light and governed by the laws of quantum thermodynamics. To appreciate the deep connection between quantum mechanics and thermodynamics we need only recall that in 1901 Planck introduced the quantum of action to calculate the entropy of thermal light, and in 1905 Einstein's studies of the entropy of thermal light led him to introduce the photon. We here show how to use quantum coherence induced by external coherent fields [1] or by quantum noise2[2] to improve the efficiency of a laser or photocell QHE. Surprisingly, this coherence can be induced by the same noisy (thermal) emission and absorption processes that drive the QHE. Furthermore, this noise-induced coherence can be robust against environmental decoherence. Application of these ideas to photosynthesis3[3] will also be discussed. [1] M.O. Scully, “Quantum Photocell: Using Quantum Coherence to Reduce Radiative Recombination and Increase Efficiency,” Physical Review Letters, 104, 207701 (2010). [2] M.O. Scully, K. R. Chapin, K, E. Dorfman, M. B. Kim, and A. Svidzinsky, “Quantum heat engine power can be increased by noise-induced coherence”, PNAS, 108, 15097 (2011). [3] G.S. Engel et al., “Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems”, Nature, 446, 782-786 (2007).
Hans-Jürgen Stöckmann	Resonance Widths in Open Microwave Cavities studied by Harmonic Inversion
	From the measurement of a reflection spectrum of an open microwave cavity the poles of the scattering matrix in the complex plane have been determined. The resonances have been extracted by means of the harmonic inversion method. By this it became possible to resolve the resonances in a regime where the line widths exceed the mean level spacing up to a factor of 10, a value inaccessible in experiments up to now. The obtained experimental distributions of line widths were found to be in perfect agreement with predictions from random matrix theory when wall absorption and fluctuations caused by couplings to additional channels are considered (U. Kuhl et al., Phys. Rev. Lett. 100, 254101 (2008)).
A. Douglas Stone	A Scattering approach to Lasers and Coherent Perfect Absorbers
	Despite being open systems described by a non-hermitian wave equation, lasers have traditionally been treated as closed cavities, with the coupling to the continuum included phenomenologically or perturbatively. This approach is ill suited to treating modern complex micro-lasers, and particularly very low-Q systems, such as random lasers. We have developed a unified approach to laser theory which treats the cavity + gain medium as a scattering system described by a non-unitary S-matrix. In this approach, which we call Steady-state Ab initio Laser Theory (SALT), the openness of the cavity is treated exactly, and the non-linear mode competition above threshold is included to infinite order. The lasing threshold is determined by the condition that a pole of the S-matrix reach the real axis, and the multi-mode steady-state is described by a non-linear S-matrix with multiple lasing poles. Solutions of the SALT equations are in excellent agreement with brute force time integration of the semiclassical Maxwell-Bloch laser equations, but are orders of magnitude faster. Random and chaotic lasers can be studied rigorously in this approach, and a number of new results have been obtained. Using input-output theory the approach can be extended to quantum fluctuations of laser properties, and a more general form of the Schawlow-Townes linewidth formula has been derived, which finds results beyond all previous linewidth theories. Applying the time-reversal operator to the laser equations at threshold uncovers a new effect, coherent perfect absorption, in which a lossy cavity described by the complex conjugate of the laser susceptibility, will absorb the incoming version of the threshold mode of the corresponding laser. This effect, which is a generalization of critical coupling to a cavity, has recently been demonstrated experimentally.

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