Program

Abstracts

M. Brune^1*, R. Celistrino Teixeira², C. Hermann-Avigliano³, T.L. Nguyen¹,
T. Cantat-Moltrecht¹, S. Gleyzes¹, J.M. Raimond¹, and S. Haroche<sup>1

1</sup> Laboratoire Kastler Brossel, ENS, UPMC-Paris 6, CNRS, Collège de France, 11 Place M. Berthelot, 75005, Paris, France
² Universidade Federal de São Carlos, Brazil
³ Joint Quantum Institute, Gaithersburg, United States
^* Email: michel.brune@lkb.ens.fr; http://www.cqed.org/

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Probing Dipole coupling between Cold Rydberg atoms, toward simulations of quantum transport
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Due to their huge polarizability, Rydberg atoms present strong, dipoledipole interactions. Studying many-body correlations in a dense cold cloud of Rydberg atoms may be of interest as a quantum simulator of model manybody hamiltonians of importance for understanding solid-states systems. We study a cold 87Rb atomic sample magnetically trapped on a superconducting atom chip. Due to the large polarizability of Rydberg atoms, an essential experimental issue is the control of stray electric fields in the vicinity of the atom chip surface. This problem was solved by coating the chip surface with a thick layer of Rubidium [1]. We observed coherence times in the ms range for the 60s to 61s microwave two-photon transition, opening interesting perspectives for the study of dipole interactions at higher density of Rydberg atoms. We then use microwave spectroscopy for measuring the interaction energy distribution of a single Rydberg atom with its neighbors [2]. The observed energy distribution carries information on spatial correlations between Rydberg atoms prepared in the dipole blockade regime. We study the energy distribution as a function of the detuning of the excitation laser. For blue detuning, we observe that we preferentially excite atoms in the neighborhood of previously excited atoms so that interaction energy shift compensates for laser detuning. We also observe the expansion of the Rydberg atom cloud due to repulsive dipole interaction and show that the cloud rapidly goes out of the ”frozen gas approximation” usually used in previous work. We will finally discuss further development in the direction of quantum simulations of transport in a 1D chain of Rydberg atoms.

[1] C. Hermann-Avigliano et al., Phys. Rev. A 90 040502(R) (2014).
[2] R. Teixeira et al., arXiv:1502.04179.

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S. Bernon, M. Bellouvet, P. Bouyer, C. Busquet, P. Lalanne, J. Zhang

LP2N, IOA, Rue François Mitterrand, F-33400, Talence, France

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Sub wavelength atoms traps and lattices
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The emerging field of on-chip integration of nanophotonic devices and cold atoms offers extremely-strong and pure light-matter interaction schemes, which may have profound impact on quantum information science, quantum simulation or atom interferometry. In this context, a long-standing obstacle is to achieve strong

interaction between single atoms and single photons, while at the same time trap atoms in vacuum at large separation distances from dielectric surfaces [1]. We will present our progress to push forward the creation of optical trapping field in regimes unattainable by conventional far-field experimental methods. Our goal is to lift the key technical issues that will allow to manipulate ultra-cold atoms in an electro-magnetic environment tailored by the near field of nano-structured surfaces. Controlling atoms in such complex environement is a challenging tasks that holds great promise to realize a unique hybrid quantum systems [2] . While ultra-cold atomic gazeous systems trapped in optical lattice potentials are nowadays model systems to simulate the quantum properties of solid state systems, our appoach will push forward these quantum simulators in regimes yet unattainable [3].

[1] X. Zang, J. Yang, R. Faggiani, C. Gill, P. G. Petrov, J. P. Hugonin, K. Vynck, S. Bernon, P. Bouyer, V. Boyer, P. Lalanne, Interaction between atoms and slow light: a waveguide-design study, Phys. Rev. Appl. 5, 024003 (2016).
[2] A. Goban, C.-L. Hung, S.-P. Yu, J. D. Hood, J. A. Muniz, J. H. Lee, M. J. Martin, A. C. McClung, K. S. Choi, D. E. Chang, O. Painter, and H. J. Kimble, Atom–light interactions in photonic crystals, Nat. Commun. 5, 3808 (2014).
[3] J. S. Douglas, H. Habibian, C.-L. Hung, A. V. Gorshkov, H. J. Kimble, and D. E. Chang, Quantum many-body models with cold atoms coupled to photonic crystals, Nat. Photonics 9, 326 (2015).

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Thomas Elsaesser

Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Berlin, 12489, Germany
Email:elsasser@mbi-berlin.de

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Nonlinear charge transport and phonon amplification in solids induced by ultrashort electric field transients
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Charge transport in strong electric fields is highly relevant for both basic and device physics. Electric fields in nanometer devices are frequently in a range where high-field phenomena become important. Beyond the well-known drift-diffusion transport of electrons in metals and semiconductors, ballistic charge motion and nonlinear quantum-coherent transport can be induced with high external electric fields. Terahertz (THz) transients with electric field amplitudes of several tens to hundreds of kilovolts/cm represent an important tool for studying such phenomena on the relevant femto- to picosecond time scales. Sophisticated THz detection methods allow for resolving in amplitude and phase both the driving field and the field radiated by the moving charges, a direct probe of their dynamics. New methods such as multidimensional THz spectroscopy [1] provide highly specific insight and readily separate different contributions to the overall nonlinear behavior.
This talk gives an introduction in the generation of strong THz transients and spectroscopic methods [1,2], followed by a discussion of high-field transport in GaAs [3,4], the THz bulk photovoltaic effect in LiNbO3 [5], and acoustic phonon amplification in semiconductor superlattices. In bulk GaAs, ballistic transport of electrons is induced by THz fields with amplitudes of up to 300 kV/cm and results in partial Bloch oscillations through a major part of the first Brillouin zone. The transition from the ballistic to the drift transport regime is elucidated by studying the interaction of THz-driven electrons with optically excited electron-hole plasma.
The interaction of intense THz pulses with LiNbO3 in the nonperturbative regime generates an interband shift current with frequency components at the THz fundamental and higher harmonic frequencies. This novel type of bulk photovoltaic effect involves interband tunneling of electrons from the valence to the conduction band as the predominant generation mechanism of mobile charges. In the THz field, these charges undergo a driven motion along the crystal’s c-axis.
Energy exchange between electrons and the crystal lattice is mediated via the different types of electron-phonon coupling. This allows for amplifying acoustic phonons which are bosonic quasi-particles, by interaction with an electric current. Very recent results on the amplification of coherent superlattice phonons by interaction with an intra-miniband current in a GaAs/AlGaAs superlattice demonstrate a high gain originating from interactions via the acoustic deformation potential.

[1] T. Elsaesser, K. Reimann, and M. Woerner, J. Chem. Phys. 142, 212301 (2015).
[2] C. Somma et al., Opt. Lett. 40, 3404 (2015).
[3] W. Kuehn et al., Phys. Rev. Lett. 104, 146602 (2010).
[4] P. Bowlan et al., Phys. Rev. Lett. 107, 256602 (2011).
[5] C. Somma et al., Phys. Rev. Lett. 112, 146602 (2014).

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Regine Frank

Serin Physics Laboratory, Department of Physics and Astronomy, Rutgers University, Piscataway, USA

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Non-Equilibrium DMFT in the Frequency-Domain
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Dynamical mean field theory (DMFT) is one very efficient numerical method to solve many-body lattice problems in the physics of strongly correlated electronics by mapping them to local many-body problems such as e.g. the Single Impurity Anderson problem or the like [1]. Since its first setup in the late 1980s early 1990s DMFT has been considered in conjunction with a variety of local solvers in the time- and frequency-domain [2] using relatively easy to implement solvers like the semi-analytical iterative perturbation theory (IPT) [3] or sophosticated cluster- and plaquette solvers in space. DMFT has been applied presumambly to basic model Hamiltonians like the Falikov-Kimball model but also to e.g. the Hubbard model for single and multiple bands.

In this talk we give a brief introduction to dynamical mean field theory (DMFT) and an overview about several solver methods including so called frequency solvers using Floquet theory. The generalization of the setup to the non-equilibrium by use of Keldysh methods is presented and applied to various electrical field driven physical realizations.
The temporal dependency of the oscillatory electrical field is accounted for by the Floquet matrix where a very efficient iterative perturbative solver establishes the local self-consistency solution [4]. Results for plasmonic and excitonic polariton structures in the non-equilibrium are presented [5], as well as solutions for high-field excited graphene (honey-comb lattices). The extension of this solver to finite temperatures (thermal linewidth) of the underlying lattice is presented.

[1] A. Georges, G. Kotliar, W. Krauth, and M. J. Rozenberg Rev. Mod. Phys. 68, 13 (1996).
[2] P. Schmidt, H. Monien, http://arxiv.org/abs/cond-mat/0202046 (2002).
[3] A. Lubatsch, J. Kroha, Annalen der Physik, Vol. 18, 12, 863-867 (2009).
[4] R. Frank, New Journal of Physics, Vol. 15, 123030 (2013); R. Frank, Applied Physics B 113, 41- 47 (2013).
[5] A. Lubatsch, R. Frank, arxiv.org/abs/1504.00949 (2015).

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Thomas Gasenzer

Kirchhoff-Institut für Physik, Ruprecht-Karls Universität Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany
Email: t.gasenzer@uni-heidelberg.de

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Prethermalisation, universal dynamics and non-thermal fixed points
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Non-equilibrated many-body systems show much richer characteristics than those in equilibrium. There is the possibility for universal dynamics, showing up with the same properties in very different systems irrespective of their concrete building blocks [1]. Prominent examples are the phenomenon of prethermalisation and the development of Generalised Gibbs Ensembles [2]. Superfluid turbulence in an ultracold atomic gas has the potential to show the same universal aspects as phenomena believed to have occurred after the inflationary period of the early universe [3]. This leads to the concept of non-thermal fixed points which lead beyond standard equilibrium universality. Phenomena in bosonic matter wave systems will be discussed which are characterized by universal scaling behavior in space and time [4]. This exhibits a close relation to quantum turbulence, the dynamics of topological defects, as well as magnetic and charge ordering phenomena [5,6]. Our results open a path to explore a new class of universal far-from-equilibrium dynamics which are accessible in ultracold gas experiments and are important beyond the realm of these systems.

[1] B. Nowak, S. Erne, M. Karl, J. Schole, D. Sexty, and T. Gasenzer, arXiv:1302.1448 [cond-mat.quant-gas]
to appear in Strongly Interacting Quantum Systems out of Equilibrium, edited by T. Giamarchi, et al. (Oxford University Press, 2016).
[2] T. Langen, S. Erne, R. Geiger, B. Rauer, T. Schweigler, M. Kuhnert, W. Rohringer, I. E. Mazets, T. Gasenzer, and J. Schmiedmayer, Science 348, 215 (2015).
[3] J. Berges, A. Rothkopf, and J. Schmidt, Phys. Rev. Lett. 101, 041603 (2008).
[4] E. Nicklas, M. Karl, M. Höfer, A. Johnson, W. Muessel, H. Strobel, J. Tomkovic, T. Gasenzer, M. K. Oberthaler, Phys. Rev. Lett. 115, 245301 (2015).
[5] B. Nowak, D. Sexty, and T. Gasenzer, Phys. Rev. B 84, 020506(R) (2011).
[6] M. Karl, B. Nowak, and T. Gasenzer, Sci. Rep. 3, 2394 (2013). .

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Antoine Georges

Collège de France, Paris
CPHT-École Polytechnique, France and DQMP-University of Geneva, Switzerland
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Theory of non-linear phononics for coherent light-control of solids
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The use of light to control the structural and electronic properties of solids is an area of great current interest. I will present a microscopic theory [Phys.Rev.B 89, 220301R (2014)] for ultrafast control of solids with high-intensity Tera-Hertz frequency optical pulses. When resonant with selected infrared-active vibrations, these pulses transiently modify the crystal structure and lead to new collective electronic properties. The theory predicts the dynamical path taken by the crystal lattice using first-principles calculations of the energy surface and classical equations of motion, as well as symmetry considerations. Two classes of dynamics are identified. In the perturbative regime, displacements along the normal mode coordinate of symmetry-preserving Raman active modes can be achieved by cubic anharmonicities. This validates the mechanism proposed by Först et al. [Nature Physics 7, 854 (2011)] and explains the light-induced insulator-to-metal transition of manganites reported experimentally by Rini et al. [Nature 449, 72 (2007)]. We also predict a non-perturbative regime in which ultra-fast instabilities that break crystal symmetry can be induced. Time permitting, I will also briefly describe applications of this theory to the transient crystal structure of YBa2Cu3O6.5 as measured in time-resolved X-ray diffraction [R.Mankowski et al., Nature 515, 71 (2014)].

This talk is based on work with Alaska Subedi and Andrea Cavalleri [Phys.Rev.B 89, 220301R (2014)], supported by a grant (ERC-319286 QMAC) from the European Research Council.

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Emanuel Gull

University of Michigan, United States

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Diagrammatic Monte Carlo for real-time propagation
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We present an overview of recent progress in the simulation of strongly correlated non-equilibrium problems using diagrammatic Monte Carlo techniques, including methods for obtaining Green’s functions, methods for reaching long times and low temperatures, and a recent ‘inchworm’ algorithm that allows to overcome the exponential scaling at long times. We will then examine the dynamics of a correlated quantum dot in the mixed valence regime. We perform numerically exact calculations of the current after a quantum quench from equilibrium by rapidly applying a bias voltage in a wide range of initial temperatures. The current exhibits short equilibration times and saturates upon the decrease of temperature at all times, indicating Kondo behavior both in the transient regime and in steady state. The time-dependent current saturation temperature matches the Kondo temperature at small times or small voltages; a substantially increased value is observed outside of linear response. These signatures are directly observable by experiments in the time-domain.

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Walter Hofstetter

Goethe-Universität Frankfurt am Main, Germany

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Competing Quantum Phases in Bosonic Lattice Systems with Rydberg Dressing
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Recent experiments have shown that (quasi-)crystalline phases of Rydberg-dressed quantum many body systems in optical lattices (OL) are within reach. While conventional neutral atomic OL gases lack strong long-range interactions, they arise naturally in Rydberg systems, due to the large polarisability of Rydberg atoms. In combination with the bosonic character of the systems considered in our work, a wide range of quantum phases have been predicted. Among them are a devil’s staircase of lattice-incommensurate density waves, as well as the more exotic supersolid lattice order. High experimental tunability opens up a wide range of parameters to be studied. Based on our previous analysis of the ”frozen” gas case, we have studied the ground state phase diagram at finite hopping amplitudes and in the vicinity of resonant Rydberg driving. Since different types of lattice-incommensurate order are to be expected, we have applied a real-space extension of bosonic dynamical mean-field theory (RB-DMFT). This method allows a non-perturbative treatment of local quantum correlations and also takes lowest-order nearest-neighbour correlations into account. It therefore improves upon basic mean-field theories such as the Gutzwiller approximation (GA), yielding a rich phase diagram which illustrates the competition between interaction and condensation.

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Daniel T. Kulp

American Physical Society, 1 Research Road, Ridge, NY USA

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Publishing in Scientific Journals: Physical Review and You
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The Physical Review family of journals has a long and rich history. Since its establishment in 1893 and under the guidance of the American Physical Society since 1913, it has grown and expanded into a broad, international family of journals covering all fields of physics. We will briefly discuss the history and growth of the journals before jumping into the current state of publishing, including the peer-review process and the roles played by authors, referees, and editors; and the business and pressure of publishing, including the expenses and the effect of open access.

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Andreas Lubatsch

Electrical Engineering, Precision Engineering and Information Technology
Georg-Simon-Ohm University of Applied Sciences Nürnberg, Germany
Physikalisches Institut, Rheinische Friedrich-Wilhelms University of Bonn, Germany

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Weighted essentially non-oszillatory solver for transport of light in disordered complex random media
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The Anderson transition was originally proposed for electrons [1], however it has been soon theoretical demontrated for several different kinds of waves in disordered media. Among different approaches the diagrammatic methods have been very successfully applied to describe localization in terms of interference, as so-called Cooperon diagrams added to the regular form of the Bethe-Salpeter equation [2].
Anderson localization of light became extremely interesting with the application of short pulse high amplitude lasers, where the medium response is expected to also exhibit non-linear effects, i.e. the medium's response will depend on light intensity. In other words, the material is no longer only spatially disordered but its complexity results even more in intensity- and even time-dependent microscopic non-linear feedback [3]. It may act on incoherent as well as coherent contributions which makes it difficult to compute and numerically expensive since large scale correlations, responsible for the Anderson transition, will be affected in principal.
We give in this talk an introduction to several numerical methods, e.g. Galerkin solvers and the so-called Weighted Essentially Non-Oszillatory (WENO) solvers, which yield valid and stable numerical solution for multiple scattering in disordered random complex media on the basis of diagrammatic transport of light [4,5]. We present novel results on the basis of multiscale WENO and explain our choice of solver at hand: As the name non-oscillatory already suggests, this method is especially designed to suppress and eliminate numerical artefacts so often associated with obtained solutions of diffusion type equations by Galerkin methods, where in particular the first order spatial derivative may be the source of trouble. In present numerical results we identify the actual amount of incoherently and coherently scattered light through finite sized slabs comprised of poly-disperse resonant complex nanoparticles.

[1] P. W. Anderson, Phys. Rev. 109, 1492 (1958).
[2] D. Vollhardt and P. Wölfle, Phys. Rev. B 22, 4666 (1980).
[3] G Maret, T Sperling, W Buhrer, A Lubatsch, R. Frank & C.M. Aegerter, Nature Photonics 7 (12), P. 934-935 (2013).
[4] R Frank, A Lubatsch, J Kroha, Physical Review B 73 (24), 24510 (2006).
[5] R Frank, A Lubatsch, Physical Review A 84 (1), 013814 (2011).

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Gilles Montambaux

Laboratoire de Physique des Solides
CNRS, Université Paris-Sud, Université Paris-Saclay, F-91405 Orsay, France
Email: gilles.montambaux@u-psud.fr

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Artificial graphenes : Dirac matter beyond condensed matter
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The so many fascinating properties of graphene, like the massless propagation of the electrons, are the subject of an intense research activity. On the other hand there is a growing interest for the study of “artificial graphenes”, that are totally different and new systems which bear exciting similarities with graphene, among them: lattices of ultracold atoms, microwave or photonic lattices or “molecular graphene”. The advantage of these artificial structures is that they serve as new playgrounds for measuring and testing physical phenomena which may not be reachable in graphene. In particular the possibility of controlling the position of the pair of Dirac points (or Dirac cones) existing in the electronic spectrum of graphene.
These cones, which describe the band structure in the vicinity of the two connected energy bands, are characterized by a topological “charge”, that is essentially a Berry phase. The cones can be moved in reciprocal space by appropriate modification of external parameters (pressure, twist, sliding, stress, etc.). They can be manipulated, created or suppressed under the condition that the total topological charge be conserved. The merging between two Dirac cones is thus a topological transition that may be described by two distinct universality classes, according to whether the two cones have opposite or like topological charges [1,2].
In this presentation, I will discuss several aspects of the scenarios of merging or emergence of Dirac points as well as the experimental investigations of these scenarios in various condensed and not condensed matter.

[1] Merging of Dirac points in a two-dimensional crystal, G. Montambaux, F. Piéchon, J.-N. Fuchs, M.O. Goerbig, Phys. Rev. B 80 (2009) 153412 ; A universal Hamiltonian for the motion and the merging of Dirac cones in a 2D crystal, ibid, Eur. Phys. J. B 72 (2009) 509
[2] Manipulation of Dirac points in graphene-like crystals, R. de Gail, J.-N. Fuchs, M. O. Goerbig, F. Piéchon, G. Montambaux, Physica B Condensed Matter 407 (2012) 1948

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Ulrich Schneider

Cavendish Laboratory, Cambridge, UK

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Non-Ergodic dynamics of interacting many body systems
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Following any kind of perturbation or quantum quench, generic interacting many-body systems will at least locally relax back to thermal equilibrium. This relaxation effectively deletes any initial quantum information about the states, since all local degrees of freedom become fully entangled with the rest of the system. Furthermore, it leads to effectively classical hydrodynamics and diffusive transport. However, several counter examples are known. The first consists of so-called integrable systems, such as 1D bosons with point-like interactions or 1D hard-core bosons on a lattice. In these systems, local thermalization is hindered by the existence of many conservation laws. This can lead to a markedly different behaviour, such as ballistic transport of interacting particles. Integrable systems are however ‘fine-tuned’ in the sense that the addition of small but generic terms to the Hamiltonian typically destroys integrability. The second, more robust class of non-ergodic systems, which has emerged during the last years, are so-called many-body localized systems (MBL), where an external disorder has induced the localization of interacting particles. These systems show an exact absence of mass transport even for finite energy densities. Both systems open the door to observe quantum effects over unprecedented timescales, since thermalization and the associated loss of accessible quantum information is absent.
We have experimentally investigated both the generic diffusive transport in 2D interacting lattice gases [1], as well as the fast ballistic expansion of 1D hard-core lattice bosons [2]. In the second case, we have additionally observed a dynamical quasi-condensation, that is the emergence of coherence with a phase order that differs from the ground-state order [3]. In a second set of experiments, we have studied the localization for interacting fermions in a one-dimensional quasirandom optical lattice and identified the MBL transition through the relaxation dynamics of an initially prepared charge density wave [4]. In addition we have studied the effects of coupling several localized systems, which highlights the crucial differences between MBL and Anderson localization of non-interacting atoms [5].

[1] U. Schneider, L. Hackermüller, J. P. Ronzheimer, S. Will, S. Braun, T. Best, I. Bloch, E. Demler, S. Mandt, D. Rasch, and A. Rosch, Nature Physics 8, 213-218 (2012)
[2] J. P. Ronzheimer, M. Schreiber, S. Braun, S. S. Hodgman, S. Langer, I. P. McCulloch, F. Heidrich- Meisner, I. Bloch, and U. Schneider, Expansion Dynamics of Interacting Bosons in Homogeneous Lattices in One and Two Dimensions, Phys. Rev. Lett. 110, 205301 (2013).
[3] L. Vidmar, J. P. Ronzheimer, M. Schreiber, S. Braun, S. S. Hodgman, S. Langer, F. Heidrich-Meisner, I. Bloch, and U. Schneider, Phys. Rev. Lett. 115, 175301 (2015)
[4] M. Schreiber, S. S. Hodgman, P. Bordia, H. P. Lüschen, M. H. Fischer, R. Vosk, E. Altman, U. Schneider, and I. Bloch, Science 349, 842 (2015)
[5] P. Bordia, H. P. Lüschen, S. S. Hodgman, M. Schreiber, I. Bloch, and U. Schneider, Arxiv: 1509.00478 (2015)

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Hadrien Kurkjian, Yvan Castin, and Alice Sinatra*

Laboratoire Kastler Brossel, Ecole Normale Supérieure, Paris, France
Email: alice.sinatra@lkb.ens.fr

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Coherence time of a paired Fermi gas
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The question of the coherence time of an isolated macroscopic quantum system, is both fundament and of practical interest for all the applications aiming to exploit the macroscopic coherence of the system, as in interferometry or quantum information. It is generally assumed that a condensate of paired fermions at equilibrium is characterized by a macroscopic wave function with a well-defined, immutable phase. In reality, all systems have a finite size and are prepared at non-zero temperature; the condensate has then a finite coherence time, even when the system is isolated in its evolution and the particle number N is fixed. The loss of phase memory is due to interactions of the condensate with the excited modes that constitute a dephasing environment. While coherence time measurements are presently being performed in cold Bose gases, experiments on Fermi gases, which up to now focused on traditional aspects of the N-body problem, are moving towards correlation and coherence measurements. This turn will open a new research field, including the strong coupling regime : that of fermionic quantum optics. However, a theory predicting the coherence time of a pair-condensed Fermi gas was missing. We present a microscopic theory bridging this theoretical gap in a general way, and we propose a method to measure the coherence time with ultra-cold atoms, which we predict to be tens of milliseconds for the canonical ensemble unitary Fermi gas [1,2].

[1] H. Kurkjian, Y. Castin, A. Sinatra, Phys. Rev. A 88, 063623 (2013).
[2] H. Kurkjian, Y. Castin, A. Sinatra, https://hal.archives-ouvertes.fr/hal-01118346

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Emina Soljanin

Rutgers University, Department of Electrical and Computer Engineering, USA
Alcatel-Lucent, Bell Labs, Murray Hill, USA

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Multicast in Quantum Networks
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Communication networks, unlike their transportation or fluid counterparts, can duplicate, merge, or in general, process entities (bits) they carry. While a car at an intersection can proceed along only one highway, a bit available at a communication network router can be copied and sent out to multiple destinations simultaneously, and while two cars reaching a Y-junction have to proceed one behind the other, two bits at a router can be combined by binary addition and sent out as a single bit carrying the information on whether the two input bits are the same or different. This property of classical information has been exploited (albeit only recently) to achieve high data throughputs for given link capacities in multicast networks in which data generated by multiple sources have to be simultaneously delivered to multiple receivers. An interesting question to ask is whether anything can be gained by allowing processing of quantum information at nodes in quantum networks. Since quantum states are represented by physical entities, the problem of quantum multicast at first seems nothing more than the multicommodity flow problem of shipping a collection of different commodities through a common network so that the total flow going through each edge in the network does not exceed its capacity. On the second thought, one realizes that although quantum states cannot be cloned or broadcast, they can be, in certain limited ways, processed and compressed with no information loss. We show that quantum multicast has simlarities with both transportation multicommodity networks and classical communication networks. In particular, we show that lossless compression of special multicast quantum states is possible and significantly reduces the link capacity requirements of the multicast.

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Martin Weitz

Institut für Angewandte Physik, Universität Bonn, Wegelerstr. 8, 53115 Bonn, Germany
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Coherence of a Bose-Einstein Condensed Light Field
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Bose-Einstein condensation has been observed with cold atomic gases, quasiparticles in solid state systems as polaritons, and more recently also with photons in a dye-filled optical microcavity [1]. I will here describe measurements of our Bonn group determining both the first and the second order coherence of a photon Bose-Einstein condensate. The optical condensate is generated in a wavelength-sized optical cavity, where the small mirror spacing imprints a low-frequency cutoff with a spectrum of photon energies restricted to well above the thermal energy. Thermal equilibrium of the photon gas is achieved by repeated absorption re-emission cycles in dye molecules [2]. In this system the photo-excitable dye molecules act as a reservoir for the condensate particles, which allows to reach a regime with large "grand canonical" number fluctuations, of order of the total particle number [3]. Photon bunching occurs, as in a lamp-type (chaotic) source. To study the first order coherence of the condensate, we have examined the temporal interference signal of the photon Bose-Einstein condensate with a narrowband laser source acting as a phase reference [4]. We in situ observe phase jumps of the condensate associated with the large statistical number fluctuations. Our experimental data reveals a regime that is macroscopically phase coherent and displays a spontaneously broken symmetry, despite the large statistical number fluctuations. Grandcanonical statistics optical sources in the condensed phase hold prospects for both fundamental studies and optical imaging technology.

[1] See, e.g.: Novel superfluids, Vol. 1, K. H. Bennemann and J. B. Ketterson (eds.) (Oxford University Press, Oxford, 2013).
[2] J. Klaers, J. Schmitt, F. Vewinger, and M. Weitz, Nature 468, 545 (2010).
[3] J. Schmitt, T. Damm, D. Dung, F. Vewinger, J. Klaers, M. Weitz, Phys. Rev. Lett. 112, 030401 (2014).
[4] J. Schmitt et al., in preparation.

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Thomas Wellens

Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Germany
E-mail: Thomas.Wellens@physik.uni-freiburg.de
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Scattering laser light on cold atoms: Multiple scattering signals from single-atom responses
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The theory of multiple scattering in dilute media that consist of a disordered collection of discrete scatterers relies on the division of the total scattering process into single scattering events. In standard multiple scattering theory, these are assumed to be linear (scattered field proportional to incident field). For atomic scatterers with transition frequency close to the laser frequency, however, nonlinear multi-photon scattering processes are induced at high laser intensities. To account for the impact of these processes on the multiple scattering signal, we present an approach which combines tools of diagrammatic multiple scattering theory (ladder and crossed diagrams) with quantum-optical methods (optical Bloch equations) [1,2]. This approach allows us to evaluate how quantum-mechanical scattering processes influence, both, diffusive propagation of the average light intensity through a dilute cloud of cold atoms (with distances between the atoms much larger than the laser wavelength), as well as effects of coherent light propagation such as coherent backscattering.

[1] T. Wellens and B. Grémaud, Phys. Rev. Lett. 100, 033902 (2008).
[2] T. Wellens, T. Geiger, V. Shatokhin, and A. Buchleitner, Phys. Rev. A 82, 013832 (2010).

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