CoScaLi 2016

Workshop on Collective Scattering of Light

Ubatuba – SP, Brazil

May 9-12, 2016

CoScaLi 2016

The role of coherences between atomic dipoles hasgained an ever increasing importance with the advent ofultracold atoms and large trapped ion systems: Classicaland quantum correlations between particles can becreated and manipulated, with important consequencesfor the study of fundamental properties of matter and forquantum information.

This workshop addresses the collective properties of lightscattering in these many-body systems, at the frontierbetween the microscopic and macroscopic realms. Thecoupling between light and an ensemble of classical orquantum scatterers, and the induced correlations, will beintensively discussed.

The main topics are the following:

Cooperative scattering

Anderson localization of light

Dicke super and subradiance

Mesoscopic physics with cold atoms

Multiple scattering of light

Ab initio models for light scattering

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Program

Sergey Skipetrov – Localization of light in a cloud of coldatoms in magnetic field . . . . . . . . . . . . . . . . . 1

Igor M. Sokolov – Light trapping and localization in acold and dense atomic ensemble located in magneto-static field . . . . . . . . . . . . . . . . . . . . . . . . . 2

Carlos E. Maximo – Spatial and temporal localization oflight in two dimensions . . . . . . . . . . . . . . . . . 3

Gordon Robb – Optomechanical Self-Structuring in ColdAtomic Gases . . . . . . . . . . . . . . . . . . . . . . . 4

Francois Damanet – Collective spontaneous emission withquantized atomic motion . . . . . . . . . . . . . . . . . 5

Joachim von Zanthier – Dicke superradiance and HanburyBrown and Twiss intensity interference: two sides ofthe same coin . . . . . . . . . . . . . . . . . . . . . . . 6

Arturo Lezama – Fluctuations in light after traversing anatomic sample . . . . . . . . . . . . . . . . . . . . . . 8

Felipe Pinheiro – Probing scattering resonances of Vogel’sspirals with the Green’s matrix spectral method . . . . 9

Paulo Hisao Moryia – Coherent backscattering of inelasticphotons from single atoms and their mirror images . . 10

Marios C. Tsatsos – Beyond mean-field investigations ofBose-Einstein condensates: principles and applicationsin ultracold atomic gases . . . . . . . . . . . . . . . . . 11

Daniel Felinto – Resonant interaction of a broadband singlephoton with a dense atomic ensemble . . . . . . . . . . 12

Robin Kaiser – Dicke Sub- and Superradiance . . . . . . . 13

Tiago Jose Arruda – Tuning the electromagnetic wavetransport in two-dimensional disordered media contain-ing gyrotropic core-shell cylinders . . . . . . . . . . . . 14

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Monika Ritsch – Attractive Optical Forces from BlackbodyRadiation . . . . . . . . . . . . . . . . . . . . . . . . . 15

Nicolas Cherroret – From multiple scattering to van derWaals interactions and vice versa . . . . . . . . . . . . 16

Paulo Americo Maia Neto – Probing the Casimir forcewith optical tweezers . . . . . . . . . . . . . . . . . . . 17

Sebastian Slama – Cooperative coupling of ultracold atomsand surface plasmons . . . . . . . . . . . . . . . . . . 18

Michelle Araujo – Superradiance in a large cloud of coldatoms in the linear optics-regime . . . . . . . . . . . . 19

Pablo Saldanha – Single-photon superradiance in a quan-tum memory . . . . . . . . . . . . . . . . . . . . . . . 20

Nicola Piovella – Nonlinear effects in the cooperative scat-tering by cold atoms . . . . . . . . . . . . . . . . . . . 21

Thierry de Silans – Redistribution of light frequency bymultiple scattering in a resonant atomic vapor . . . . . 22

Patrıcia Castilho – Cooperative two-photon absortion in aBEC of sodium atoms . . . . . . . . . . . . . . . . . . 23

Sandra Vianna – Cooperative frequency shift in three-photonresonant four-wave mixing . . . . . . . . . . . . . . . . 24

Philippe Courteille – Gravimetry with in vivo monitoringof matter waves . . . . . . . . . . . . . . . . . . . . . 25

Marina Samoylova – Quantum Enhanced Interferometry . 26Helmut Ritsch – Spontaneous crystallization of light and

ultracold atoms in free space . . . . . . . . . . . . . . . 27Francois Impens – Recoherence from non-locality . . . . . 29Analabha Roy – Simulation of Quantum Spin Dynamics

by Phase Space Sampling of BBGKY Trajectories . . . 30Johannes Schachenmayer – Collective atomic emission

and its interplay with motional effects in dense cloudsof Sr atoms . . . . . . . . . . . . . . . . . . . . . . . . 31

Robert Bettles – Cooperativity in lattice monolayers ofdriven interacting dipoles . . . . . . . . . . . . . . . . 32

Natalia R. de Melo – Intrinsic Optical Bistability in aRydberg ensemble . . . . . . . . . . . . . . . . . . . . . 33

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Localization of light in a cloud of cold atoms in magneticfield Mon, 09/05

8:45-9:25

Sergey SkipetrovUniv. Joseph Fourier, France

We have recently shown that multiple scattering of light in a randomthree-dimensional ensemble of cold atoms cannot lead to the appear-ance of localized states, contrary to scattering of electrons on impu-rities in a solid-state sample [1]. This is due to the opening of anew energy transport channel by the dipole-dipole interactions be-tween nearby atoms [2], and was later confirmed by an independentstudy [3]. We find, however, that localized states can be induced byan external magnetic field that partially suppresses the dipole-dipoleinteractions [4]. Several questions then arise: is the resulting local-ization transition experimentally observable? Should it be consideredAnderson transition (as many other localization-delocalization transi-tions taking place in disordered systems)? And in which universalityclass should it be classified? After giving a quick review of our workon the topic of light localization, we will report on the recent progressthat we made in attempt to answer these questions.

[1] S.E. Skipetrov and I.M. Sokolov, Phys. Rev. Lett. 112, 023905(2014)[2] Th.M. Nieuwenhuizen et al., Phys. Lett. A 184, 360 (1994)[3] L. Bellando, A. Gero, E. Akkermans, and R. Kaiser, Phys. Rev. A90, 063822 (2014)[4] S.E. Skipetrov and I.M. Sokolov, Phys. Rev. Lett. 114, 053902(2014)

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Light trapping and localization in a cold and denseatomic ensemble located in magnetostatic fieldMon, 09/05

9:25-10:05

Igor M. SokolovSt Petersburg Univ., Russia

We study theoretically the influence of static magnetic field on lighttrapping and localization in a cold and dense atomic ensemble pre-pared in an atomic trap. The analysis is based on the consistentquantum-posed theoretical approach developed previously in [1]. Inthe framework of this approach we solve the nonstationary Schrodingerequation for the wave function of the joint system consisting of N mo-tionless atoms and a weak electromagnetic field. Restriction of thetotal number of states taken into account allows us to find approxi-mately the wave function of the system and consequently to analyzeboth the properties of atomic system and the light. Our study showsthat static magnetic field essentially modifies resonant dipole-dipoleinteratomic interaction. This effect influences strongly on light trap-ping. We found out appearance of long lived polyatomic collectivestates which lifetimes essentially exceed those take place in the ab-sence of magnetic field. Calculation of inverse participation rationshows that these states are localized. Scaling analysis of collectiveeigenstates distribution revealed that, in many aspects, this distribu-tion exhibits the behavior expected for the Anderson transition drivenby disorder. On the basis of our approach we analyze also the possi-bility to observe manifestation of these long-lived localized states inexperiment. We study the dynamics of fluorescence of atomic cloudsinitiated by pulse radiation. In the presence of magnetostatic field ra-diation trapping time increases considerably. We analyze as well theinfluence of magnetic field on the steady-state transmission of plainlayer of motionless atomic scatterers.

[1] I. M. Sokolov, D. V. Kupriyanov, and M. D. Havey, J. Exp. Theor.Phys. 112, 246 (2011).

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Spatial and temporal localization of light in twodimensions Mon, 09/05

10:05-10:30

Carlos E. MaximoIFSC-USP, Brazil

Multiple scattering of waves has been the subject of intense debates inthe context of disorder-induced Anderson localization. Nevertheless,despite several decades of research, the mere existence of Andersonlocalization of light in the prominent case of tree dimensions (3D)remains debated [1-2]. On the other hand, two dimensional (2D) ex-periments have reported Anderson localization of light for scatteringof scalar waves. However, the vector nature of light may be crucial incase of point scatterers [1]. For this reason we have focused our effortson a 2D vector model of scattering [3]. Our theoretical model predictsa reduced dimensionality by geometrical constraints which brings outto the existence two regimes of scattering: one corresponding to ascalar model of light and the other one corresponding to a vectorialmodel of light. Hence we are able to do a direct comparison betweenthe presence or the absence of polarization degrees of freedom and thepresence or the absence of near field terms interactions, making it anideal tool to investigate the role of polarization in light localizationphenomena. Performing a scaling analysis we observe in both caseslong lived atomic modes of the scattering, yet only the scalar caseexhibits Anderson localized modes. Investigating the reasons for theabsence of localization in cold atoms ensambles, it appears that boththe coupling of polarization and the presence of near field terms areable to prevent long lifetimes and Anderson localization. We finallyshow that, albeit modes with extremely long lifetimes are present onlyin the localization regime, surprisingly these lifetimes and their local-ization length are uncorrelated.

[1] S.E. Skipetrov and I.M. Sokolov, Phys. Rev. Lett. 112, 023905(2014)[2] T. Sperling, et al., New J. Phys. 18, 013039, (2016).[3] C. E. Maximo, N. Piovella, Ph. W. Courteille, R. Kaiser, and R.Bachelard, PRA 92, 062702 (2015).

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Optomechanical Self-Structuring in Cold Atomic GasesMon, 09/059:10-9:50

Gordon RobbUniv. of Strathclyde, United Kingdom

The interaction between light and cold atomic gases can give rise tononlinear , self-structuring instabilities which involve spontaneous for-mation of optical and atomic density patterns. Features of these in-stabilities in both thermal and degenerate gases will be described.

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Collective spontaneous emission with quantized atomicmotion Mon, 09/05

11:40-12:05

Francois Damanet 1, Daniel Braun 2 and John Martin1

1 Institut de Physique Nucleaire, Atomique et de Spectroscopie, Universite de Liege,4000 Liege, Belgium.

2 Institut fur theoretische Physik, Universitat Tubingen, 72076 Tubingen, Germany.

We derive and solve a markovian master equation for the internal dy-namics of an ensemble of indistinguishable two-level atoms includingall effects related to the quantization of their motion [1]. Our equationprovides a unifying picture of the consequences of recoil and indistin-guishability of atoms beyond the Lamb-Dicke regime on both theirdissipative and conservative internal dynamics, and is relevant for ex-periments with ultracold atoms. We give general expressions for thedecay rates and the dipole-dipole shifts for any motional states, andwe find analytical formulas for a number of relevant states (Gaussianstates, Fock states and thermal states). We show that dipole-dipoleinteractions and cooperative spontaneous emission can be modulatedthrough the motional state of atoms. As an application, we study theimpact of the quantized atomic motion on Dicke super- and subradi-ance [2]. In particular, we compute the radiated energy rate up to 30atoms and provide analytical predictions for large number of atomsbased on a mean-field approach [3,4].

[1] F. Damanet, D. Braun, and J. Martin, Phys. Rev. A 93, 022124(2016).[2] R. H. Dicke, Phys. Rev. 93, 99 (1954).[3] M. Gross and S. Haroche, Phys. Rep. 93, 301 (1982).[4] H.-P. Breuer and F. Petruccione, The Theory of Open QuantumSystems (Oxford University Press, 2006).

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Dicke superradiance and Hanbury Brown and Twissintensity interference: two sides of the same coinMon, 09/05

12:05-12:30

Joachim von ZanthierUniversity Erlangen-Nuremberg, Germany

Superradiance is one of the outstanding problems in quantum opticssince Dicke introduced the concept of spontaneous emission of coher-ent radiation by an ensemble of two-level atoms in highly entangledDicke states [1]. The startling gist is that even though the atomshave no dipole moment they radiate with increased intensity in par-ticular directions. While a substantial amount of literature has beenpublished on this topic [2-6], the physical origin of the phenomenonremained largely obscure. A deeper understanding has emerged re-cently in terms of quantum interferences among multiple path waysproduced by systems in correlated states [7]. Based on this interpre-tation we investigate a new aspect of superradiance, namely that itcan be observed also with independent initially uncorrelated sources.In our approach the production of correlation among the sources andsubsequent superradiant emission relies on the successive measurementof photons at particular positions, such that the detection process isunable to identify the individual photon sources. In this case, theinitially fully excited system cascades down the ladder of symmetricDicke states each time a photon is recorded [8,9]. Detecting m photonsscattered from N > m atoms amounts to measuring the m-th orderphoton correlation function. Measuring this function allows (a) toproduce any desired symmetric Dicke state from initially uncorrelatedsources and (b) to observe the corresponding superradiant behavior ofthe resultant Dicke state. As it turns out the same approach is also ap-plicable for initially uncorrelated classical sources [8].The setup usedto display the superradiant emission characteristics of initially uncor-related sources is similar to the one employed in the Hanbury Brownand Twiss experiment. The detailed analysis shows that the two ef-fects derive indeed from the same cause, namely from multi-photoninterferences appearing in the m-th order photon correlation function.In this way it is shown that Hanbury Brown and Twiss intensity in-terference and Dicke superradiance are two sides of the same coin.

[1] R. H. Dicke, Phys. Rev. 93, 99 (1954).

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[2] N. E. Rehler and J. H. Eberly, Phys. Rev. A 3, 1735 (1971).[3] R. Friedberg et al., Phys. Rep. 7, 101 (1973).[4] M. Gross and S. Haroche, Phys. Rep. 93, 301 (1982).[5] G. Agarwal, Quantum Optics (Springer, Berlin, 1974).[6] M.O. Scully et al., Phys. Rev. Lett. 96, 010501 (2006).[7] R. Wiegner et al., Phys. Rev. A 84, 023805 (2011).[8] S. Oppel et al., Phys. Rev. Lett. 113, 263606 (2014).[9] R. Wiegner et al., Phys. Rev. A 92, 033832 (2015).

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Fluctuations in light after traversing an atomic sampleMon, 09/0519:30-20:10

Arturo LezamaUniv. de la Republica, Uruguay

The interaction of light with a homogeneous sample of atoms hasbeen used as a fundamental test bench for nonlinear and quantumoptics. While the simplest models only consider the coupling of lightwith a two-level atomic system, several important effects are due tothe internal atomic structure, namely the Zeeman degeneracy of theatomic energy levels. We review the role of the Zeeman degeneracyin nonlinear effects such as electromagnetically induced transparencyand absorption and consider the effect of the Zeeman degeneracy onthe fluctuations of the light field traversing the atomic sample. Wediscuss the fundamental difference between two-level and multi-levelatomic systems and describe recent experiments in which atom-lightinteraction results in quadrature and polarization squeezing as well astwo-mode entanglement.

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Probing scattering resonances of Vogel’s spirals with theGreen’s matrix spectral method Mon, 09/05

20:10-20:35

Felipe PinheiroUFRJ, Brazil

Using the rigorous Green’s function spectral method [1,2], we sys-tematically investigate the scattering resonances of different types ofVogel [3] spiral arrays of point-like scatterers. By computing the dis-tributions of eigenvalues of the Green’s matrix and the correspondingeigenvectors we obtain important physical information on the spatialnature of the optical modes, their lifetimes and spatial patterns, atsmall computational cost and for large-scale systems [4]. We empha-size the unique properties of aperiodic Vogel spirals with respect torandom scattering media, which have been investigated so far. Finally,we show that this method can be extended to the study of 3D Vogelaperiodic metamaterials and aperiodic photonic structures that mayexhibit a richer spectrum of localized resonances of direct relevance tothe engineering of novel optical light sources and sensing devices.

[1] F.A.Pinheiro, M.Rusek, A.Orlowski, and B.A. van Tiggelen, Phys.Rev. E 69, 026605 (2004).[2] A. Goetschy and S. E. Skipetrov, Phys. Rev. E 84, 011150 (2011).[3] J. Trevino, S. F. Liew, H. Noh, H. Cao, and L. D. Negro, Opt.Express 20, 3015 (2012).[4] A. Christofi, F. A. Pinheiro and L. Dal Negro, to be published inOpt. Lett. (2016).

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Coherent backscattering of inelastic photons from singleatoms and their mirror imagesMon, 09/05

20:35-21:00

Paulo Hisao MoryiaIFSC-USP, Brazil

Coherent Back Scattering (CBS) is a coherence effect in the propaga-tion of waves through disordered media involving two or more scatter-ers. This effect is understood as a constructive interference betweenphotons scattered by two time-reversed paths, leading to an enhancedscattering into the backward direction [1,2]. Unfortunately, severaleffects can lead to a loss of coherence and consequently to a reduc-tion in the CBS contrast, such as the presence internal structure inthe scatterers, saturation and media density. By using a simple trick,the coherence effects on the back scattered light can be recovered inoptically thin samples: the introduction of a dielectric mirror. In thisscheme, there will be constructive interference between light scatteredby the original atom and its image, eventually resulting in a CBSprocess [3]. Here we referred this effect as mirror-assisted CoherentBack-Scattering (mCBS) which has been observed for classical scat-terers [4]. In this talk, I will present the observation of the mCBSfrom a sample of laser cooled Strontium atoms and a dielectric mir-ror. Due to the robustness of the mCBS to dephasing induced bystrong saturation, I will present the contribution of inelastic photonsto the interference process.

[1] Y. Kuga and A. Ishimaru, J. Opt. Soc. Am. A 1, 831-835 (1984).[2] G. Labeyrie, F. de Tomasi, J.-C. Bernard, C.A. Mller, C. Miniatura,and R. Kaiser, Phys. Rev. Lett. 83, 5266 (1999).[3] J.-J. Greffet, Waves in Random Media, 3, S65, (1991).[4] G. Labeyrie, C.A. Muller, D.S. Wiersma, C. Miniatura, and R.Kaiser, J. Opt. B: Quantum Semiclass. Opt 2, 672 (2000).

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Beyond mean-field investigations of Bose-Einsteincondensates: principles and applications in ultracold

atomic gases Mon, 09/0521:00-21:25

Marios C. TsatsosIFSC-USP, Brazil

Bose-Einstein condensation, theoretically known to appear at ultralowtemperatures since the 1920s, was achieved in the laboratory only20 years ago, in dilute bosonic gases at temperatures close to abso-lute zero. The wide range of applications and controlabillity of thesystem parameters has made them unprecedented tools for exploringnovel quantum phases and behavior. The most common theoreticalmethod employed to tackle these complex systems, typically consistingof tens of thousands of interacting particles is the celebrated mean-field Gross-Pitaevskii model. Even though it has been proven success-ful in describing various types of nonlinear excitations it does not takeinto consideration fragmentation and correlations that can develop intime. I will briefly present a systematic theory, the MultiConfigu-rational Time-Dependet Hartree for Bosons (MCTDHB) [1] that hasbeen developed in order to solve the many-body Schroedinger equationbeyond the mean-field approach and can be in principle exact, and thelatest numerical implementation (solver) that is freely distributed inthe web [2]. I will then discuss some particular applications in Bosegases possessing angular momentum and interacting vortices and dis-cuss the novel concept of phantom vortices. The latter are topologicaldefects in rotating gases that evade detection in the density of the gasbut give their signature in the correlation function. Last I will presentrecent theoretical and experimental investigations in nearly-1D gaseswhere periodic modulation of the scattering length might give rise togranulated states and beyond-mean-field phenomena.

[1] Alexej I. Streltsov, Ofir E. Alon, and Lorenz S. Cederbaum, Phys.Rev. Lett. 99, 030402 (2007); Ofir E. Alon, Alexej I. Streltsov, andLorenz S. Cederbaum, Phys. Rev. A 77, 033613 (2008).[2] A.U.J. Lode, M.C. Tsatsos and E. Fasshauer, The Multiconfigu-rational Time-Dependent Hartree for Indistinguishable particles soft-ware (2016), http://ultracold.org.

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Resonant interaction of a broadband single photon with adense atomic ensembleTue, 10/05

8:30-9:10

Daniel FelintoUniv. Federal de Pernambuco, Brazil

The interaction of an ensemble of atoms with common vacuum modesmay lead to an enhanced emission into these modes. This phenomenon,known as superradiance, highlights the coherent nature of spontaneousemission, resulting in macroscopic entangled states in mundane situ-ations. The complexity of the typical observations of superradiance,however, masks its quantum nature, allowing alternative classical in-terpretations. Here we stress how this picture changed with the imple-mentation ten years ago of a new process for single-photon generationfrom atomic ensembles. We present then the last piece of evidence forthe superradiant nature of such process, reporting the observation ofan accelerated emission of the photon with a rate that may be tunedby controllably changing the number of atoms in the ensemble. Suchinvestigation paves the way to a new, bottom-up approach to the studyof superradiance.

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Dicke Sub- and Superradiance Tue, 10/059:10-9:50

Robin KaiserInstitut du Non Lineaire de Nice, France

The quest for Anderson localization of light is at the center of manyexperimental and theoretical activities. Cold atoms have emerged asinteresting quantum system to study coherent transport properties oflight. Initial experiments have established that dilute samples withlarge optical thickness allow studying weak localization of light. Thegoal of our research is to study coherent transport of photons in coldatomic samples. One important aspect is the quest of Anderson lo-calization of light with cold atoms and its relation to Dicke super- orsubradiance. In this talk I present our latest results on Dicke sub- andsuperradiance.

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Tuning the electromagnetic wave transport intwo-dimensional disordered media containing gyrotropic

core-shell cylindersTue, 10/059:50-10:15

Tiago Jose ArrudaUSP, Brazil

We study the scattering of normally irradiated gyroelectric and gyro-magnetic core-shell cylinders for p- and s-polarizations, respectively.Using the Lorenz-Mie theory, we derive multiple scattering quantitiesin the diffusion regime and weak disorder approximation of the radia-tive transfer equation. In particular, we calculate the energy trans-port velocity and the transport mean free path for two-dimensionaldisordered medium. Our system consists of parallel microcylinderscomposed of a dielectric core of SiO2 and a semiconductor shell ofInSb with realistic material parameters, irradiated by terahertz planewaves. Setting the aspect ratio of coated cylinders, we show thatvariations in the axial external magnetic field or temperature leadto alternating regimes of low and high diffusion in a terahertz band,effectively tuning the wave transport across the disordered medium.This result could be of interest to design magnetically and thermallytunable metamaterials.

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Attractive Optical Forces from Blackbody Radiation Tue, 10/0510:15-10:40

Monika RitschMedical University of Innsbruck, Austria

Blackbody radiation around hot objects induces ac Stark shifts of theenergy levels of nearby atoms and molecules. These shifts are roughlyproportional to the fourth power of the temperature and induce a forcedecaying with the third power of the distance from the object. Weexplicitly calculate the resulting attractive blackbody optical dipoleforce for ground state hydrogen atoms. Surprisingly, this force cansurpass the repulsive radiation pressure and actually pull the atomsagainst the radiation energy flow towards the surface with a forcestronger than gravity. We exemplify the dominance of the “blackbodyforce” over gravity for hydrogen in a cloud of hot dust particles. Thisoverlooked force appears relevant in various astrophysical scenarios,in particular, since analogous results hold for a wide class of otherbroadband radiation sources.

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From multiple scattering to van der Waals interactionsand vice versaTue, 10/05

11:10-11:50

Nicolas CherroretLaboratoire Kastler Brossel, France

Neutral polarizable atoms brought close to each other can exhibit avan der Waals (or “induced dipole-dipole”) interaction: a fluctuationof the electric field polarizes one atom, which emits a radiation thatpolarizes its neighbor. The field the first atom receives back from thesecond then yields a binding energy between the two particles. Thisfundamental phenomenon may have important consequences at themacroscopic scale.

In the talk, I will discuss two manifestations of van der Waals in-teractions in macroscopic, random ensembles of point scatterers. Inthe first one, I will focus on the interaction aspect by considering theCasimir-Polder force between an isolated atom and a random collec-tion of other atoms. The Casimir force here results from radiationscattering between the probe atom and all the constituents of themacroscopic random medium. In the second scenario, I will focus onthe transport aspect and discuss the effect of van der Waals interac-tions on the propagation of an electromagnetic wave through a diluteatomic cloud. Starting from a rigorous, energy-conserving scatteringapproach, I will show how these interactions affect fundamental trans-port properties such as the energy transport velocity or the transportmean free path.

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Probing the Casimir force with optical tweezers Tue, 10/0511:50-12:30

Paulo Americo Maia NetoUFRJ, Brazil

Optical tweezers (OT) are single-beam laser traps for neutral parti-cles, usually applied to dielectric microspheres immersed in a fluid.The stiffness is proportional to the trapping beam power, and hencecan be tuned to very small values, allowing one to measure femtonew-tonforces, once the device is carefully calibrated. In this work, weemploy OT to measure the Casimir (or retarded van der Waals) forcebetween polystyrene beads in a fluid, for distances between 50 and500 nanometers. The spherical beads have diameters ranging from3 to 7 micrometers. This configuration is well beyond the validityof the commonly employed Proximity Force (or Derjaguin) approxi-mation (PFA), thus revealing unprecedented features of the Casimirinteraction. For the comparison with experimental data, we computethe Casimir force using the scattering approach applied to the spheri-cal geometry. We also present experimental results for the total forcebetween a mercury microdroplet and a polystyrene bead immersed inethanol, with similar distances and diameters.

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Cooperative coupling of ultracold atoms and surfaceplasmonsTue, 10/05

19:30-20:10

Sebastian SlamaUniv. Tubingen, Germany

Cooperative coupling between optical emitters and light fields is oneof the outstanding goals in quantum technology. It is both funda-mentally interesting for the extraordinary radiation properties of theparticipating emitters and has many potential applications in photon-ics. Although this goal has been achieved using high-finesse opticalcavities, attention has turned to broadband, easy to build cavity-freeapproaches. Here we demonstrate cooperative coupling of ultracoldatoms with surface plasmons propagating on a plane gold surface.While the atoms are moving towards the surface they are excited byan external laser pulse. The interaction between the excited atomfluorescence and surface plasmons is probed by detecting the photonsemitted into the substrate when the plasmon excitations decay. Amaximum Purcell factor of ηP = 4.9 is reached at an optimum dis-tance of z = 250 nm from the surface. The coupling leads to theobservation of a Fano-like resonance in the spectrum.

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Superradiance in a large cloud of cold atoms in the linearoptics-regime Tue, 10/05

20:10-20:35

Michelle AraujoUniversite de Nice Sophia Antipolis, France

Dicke’s superradiance is a phenomenon in the context of cooperativescattering of light where the spontaneous emission is coherently en-hanced by a system of N particles. In the past many works wereperformed in inverted systems (where many atoms are excited by theincident light), but since 2006 a regime called “single photon superra-diance” (where there is only one excited atom) has been investigated,mainly in large samples. In cold atoms, this regime is also known asthe linear optics regime and it is achieved by a weak and far-detunedlaser which interacts with the cold atoms. After the steady state isreached, the incident laser is switched off and the emitted fluores-cence is collected to measure the superradiant decay rate. Theoreticalworks predict, as a main result, a fluorescent emission in the forwarddirection of the incident laser.

Here, we demonstrate superradiance out of the forward direction(off-axis superradiance) in a dilute cloud of cold Rb atoms, in thelinear optics regime. The physical system is described by a scalarcoupled-dipole model for N two-level atoms and large detuned light.In the experiment, the Rb atoms interact with a sequence of pulsesobtained from an AOM and an EOM in series. The superradiant decayrates are extracted from a fit of the measured fluorescence.

We show numerically and experimentally how it is possible to haveoff-axis superradiance. Far from resonance, the superradiant decayrate increases with the on-resonance optical thickness. The validity ofthe measurements is checked from the investigation of the superradiantdecay rates for different intensities of the probe beam.

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Single-photon superradiance in a quantum memoryTue, 10/0520:35-21:15

Pablo SaldanhaUFMG, Brazil

We present a theoretical and experimental study of the mechanism ofextraction of information stored in a quantum memory. The mem-ory contains a single excitation of a collective atomic state, which ismapped into a single photon during the reading process. A theoryis developed for the wavepacket of the extracted photon, leading toa simple analytical expression depending on the key parameters ofthe problem, like detuning and the intensity of the read field and thenumber of atoms in the atomic ensemble. This theory is compared toa large set of experimental situations and a satisfactory quantitativeagreement is obtained. We were then able to make a detailed study ofthe single-photon superradiance in the system, since the superradiantbehavior is associated to a single fitting parameter of the experimentalcurves. In particular, we showed that the effect of superradiance inthe single-photon emission increases lineary with the number of atomsof the quantum memory, as expected from the Dicke theory.

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Nonlinear effects in the cooperative scattering by coldatoms Wed, 11/05

8:30-9:10

Nicola PiovellaUniv. degli Studi di Milano, Italy

We studied the impact of inelastic scattering from strongly driven,saturated atomic dipole transitions, calculating the power spectrumof the scattered light diagram. Modification of the satellite rays in thewell-known Mollow spectrum, both in frequency shift and line widthhave been investigated. Understanding this dephasing mechanism, in-duced by quantum mechanical frequency fluctuations of the scatteredphotons, is important for the transition from weak to strong localiza-tion, which is expected to occur at increased atomic densities, whenatoms exchange multiple photons, and even a single photon is able tosaturate the atomic transition.

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Redistribution of light frequency by multiple scattering ina resonant atomic vaporWed, 11/05

9:10-9:50

Thierry de SilansUFPB, Brazil

In this presentation we analyze the frequency redistribution of lightdiffused by a resonant vapor. We have developed a Monte-Carlo sim-ulation that allows us to follow the evolution of the emission spectrumas a function of the number of scattering events suffered by the light.For an incident light at the line center Complete Frequency Redis-tribution occurs after a few scattering events and the light transportexhibits anomalous diffusion behavior (Levy Flights). For excitationat some Doppler width from the line center, correlation between in-cident and emitted frequency at each scattering events implies in thesubsistence of emission far from resonance and a normal diffusion be-havior. We also investigate the how the ratio between Doppler andhomogeneous width of the vapor influences the frequency redistribu-tion and the photon step size distribution.

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Cooperative two-photon absortion in a BEC of sodiumatoms Wed, 11/05

9:50-10:15

E. Pedrozo-Penafiel ; P. C. M. Castilho ; E. D.Mercado-Gutierrez ; G. Roati ; K. M. Farias ; V. S. Bagnato

IFSC-USP, Brazil

Cooperative atom-light interaction in cold atomic samples has beenthe subject of intense investigation in the last few years. In such sys-tems, the Doppler broadening of the spectral lines is negligible and theatom-atom interaction can dominate over the other energy scales. Thistwo ingredients can lead to the cooperative two-photon absorption by apair of atoms or molecules. The first experimental observation of sucheffect in a cold atomic system was done in a MOT of sodium atoms.In this experiment, the cooperative two-photon excitation peak wasobserved at halfway between the D1 and D2 transitions and showed aquadratic scaling with respect to the excitation laser intensity, whichis the expected signature for the effect. The asymmetric profile ofthe two-photon excitation line provided an additional evidence of theinteraction potential between the two atoms. Here, we propose a newexperiment to investigate this effect in a Bose-Einstein condensate ofsodium atoms. The increased density of these samples and the pos-sibility to tune the atomic interaction through Feshbach resonanceswill allow to investigate the dependence of the excitation line shapeand intensity on the interaction strength. The spatial coherence of theBEC can also be studied through the measurement of the two-photonexcitation probability as a function of the displacement of two slightlydetuned excitation laser beams. Finally, the ability of this system togenerate correlated photons at different frequencies will be character-ized by temporal correlation measurements of the atomic fluorescencesignal.

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Cooperative frequency shift in three-photon resonantfour-wave mixingWed, 11/05

10:15-10:40

Sandra ViannaUFPE, Brazil

Modifications of resonant absorption and emission spectra lines bydense atomic samples are of fundamental importance in the investi-gations of a variety of quantum and nonlinear phenomena. In fact,intriguing properties can be obtained when many atoms collectivelyinteract via a common electromagnetic field, such as Dicke superra-diance and cooperative frequency Lamb shift [1]. In this work, wereport on the investigation of the nonlinear response of a rubidiumvapor, in which case two- and three-photon resonant transitions aredriven by a four-wave mixing process. The experiment is performedwith nanosecond lasers, using two beams in a non-collinear configura-tion, i. e., θ 6= 0, and high atomic density. In particular, the coherenceinduced on the three-photon resonant transition from 5s to 6p states,excited via intermediate Rydberg states, unveiled interesting featuresrelated to the interaction of the atoms with the internally generatedradiation field [2]. First, the odd-photon destructive interference be-tween the incident and generated fields, observed in a collinear con-figuration (θ = 0) [3], is strongly inhibited for θ 6= 0. Second, andmost importantly, the three-photon transition presents a cooperativefrequency shift, which decreases as the angle θ between the two exci-tation beams increases and increases linearly with the atomic density.The results are consistent with the Maxwell-Bloch equation, when thegenerated radiation field is considered self-consistently. The measuredfrequency shift is strongly enhanced for small, but nonzero values ofθ, due to the factor (1 − cos θ)−1 , in agreement with the descriptionbased on a cooperative frequency shift.

[1] R. Friedberg, S. R. Hartmann and J. T. Manassah, Phys. Rep.C 7, 101 (1973); M. O. Scully, et al., Phys. Rev, Lett., 96, 010501(2006); M. O. Scully, ibid 102, 143601 (2009).[2] N. R. de Melo, S. S. Vianna, Phys. Rev. A 92, 053830 (2015).[3] N. R. de Melo, S. S. Vianna, J. Opt. Soc. Am. B 31, 1735 (2014).

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Gravimetry with in vivo monitoring of matter waves Wed, 11/0511:10-11:50

Philippe CourteilleIFSC-USP, Brazil

Collective scattering of light can be employed for non-destructive mon-itoring of the dynamics of matter waves provided the light-matter in-teraction occurs in a ring cavity, where the scattered light can becoherently recycled in the counterpropagating cavity mode. We pro-pose to apply this idea to monitor in vivo the Bloch oscillations ofultracold atoms in an optical lattice under the action of a constantexternal force. In the proposed scheme, the atoms interact with a uni-directionally pumped optical ring cavity whose one arm is collinearwith the optical lattice. We find that the feedback provided by thecavity field on the atomic motion synchronizes Bloch oscillations via amode-locking mechanism, steering the atoms to the lowest Bloch band.It also stabilizes Bloch oscillations against noise, and even suppressesdephasing due to atom-atom interactions. Furthermore, it generatesperiodic bursts of light emitted into the counter-propagating cavitymode, providing a non-destructive monitor of the atomic dynamics.

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Quantum Enhanced InterferometryWed, 11/0511:50-12:15

Marina SamoylovaUniversity of Southampton, United Kingdom

Within the UK Quantum Technology Hub on sensing and metrologywe are currently involved in theoretical studies of gravity and magneticsensors based on nonlinear atom interferometry. More specifically, weare devising a protocol that may be implemented in a two-componenttrapped Bose-Einstein condensate and surpasses the super-Heisenbergsensitivity scaling. This particular research area helps further ourunderstanding of potential nonlinear interferometric applications andis relevant for the future development of portable and practical sensingdevices.

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Spontaneous crystallization of light and ultracold atomsin free space Wed, 11/05

19:30-20:10

Helmut Ritsch

Universitat Innsbruck, Austria

Coherent scattering of light from ultracold atoms involves an exchangeof energy and momentum introducing a wealth of non-linear dynami-cal phenomena. As a prominent example particles can spontaneouslyform stationary periodic configurations which simultaneously maxi-mize the light scattering and minimize the atomic potential energyin the emerging optical lattice. Such self-ordering effects resulting inperiodic lattices via bimodal symmetry breaking have been experimen-tally observed with cold gases and Bose-Einstein condensates (BECs)inside an optical resonator. Here we study a new regime of periodicpattern formation for an atomic BEC in free space, driven by far off-resonant counterpropagating and non-interfering lasers of orthogonalpolarization. We predict that in suitable geometries roton instabili-ties originating from non-linear free space atom light interactions canbe tailored to generate stationary crystalline states. They involve anoptical lattice showing an emergent spacing and phononic excitations,trapping the atoms at the intensity maxima.

The required translation invariant, mirror symmetric geometry canbe realized using two orthogonal polarization degrees of freedom orfrequency shifted counterpropagating beams. We estimate that thedynamics studied in this work should be accessible in already exist-ing experimental setups on large quasi-1D Bose-Einstein condensates.Actually, in comparison with standard crossed beam dipole traps, onesimply has to adapt and control the polarizations of the trapping lasersand choose suitable detunings. The ordering process should be eas-ily observable not only by measuring the atomic distributions butdirectly by looking at the reflected light from the condensate. Thisnon-destructive measurement allows for a real-time monitoring of thedynamics. Our results open up an intriguing new direction in quan-tum simulations with ultracold atoms in optical lattices, where thelatter are enriched by the presence of collective phononic excitationsresulting from the spontaneous crystallisation of light. In this spirit,the application of our approach to two-dimensions and the inclusions

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of retardation effects as well as quantum fluctuations constitute thenatural extension of this study.

In contrast to previous works, no spatial light modes are preselectedby any boundary conditions and the transition from homogeneous toperiodic order amounts to a crystallization of both light and ultracoldatoms breaking a continuous translational symmetry. In the crystal-lized state the BEC acquires a phase similar to a supersolid with anemergent intrinsic length scale whereas the light-field forms an opti-cal lattice allowing phononic excitations via collective back scattering.The studied system constitutes a novel configuration allowing the sim-ulation of synthetic solid state systems with ultracold atoms includinglong-range phonon dynamics.

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Recoherence from non-locality Wed, 11/0520:10-20:50

Francois ImpensUFRJ, Brazil

We show that the decoherence rate of an open quantum system can bedecomposed into a local contribution, related to the amplitude of thisphysical process responsible for the decoherence, and into a non-localcontribution which captures the relevance of the process with respectto which-path information on the system. Both terms arise naturallyin the framework of the influence functional, in the form of imagi-nary phase contributions. The non-local phase can be understood interms of coupling between the backward and forward histories in theinfluence action. While the local phase always yields a positive de-coherence rate, the non-local phase may provide a recoherence whenonly partial information about the system is obtained from the dis-turbed environment. We discuss these concepts in the framework ofatom interferometry in an optical lattice.

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Simulation of Quantum Spin Dynamics by Phase SpaceSampling of BBGKY TrajectoriesWed, 11/05

20:50-21:15

Analabha RoyNational Institute for Theoretical Physics, Stellenbosch, South Africa

Recent experimental developments in quantum simulators with latticespins in ion traps have necessitated the search for numerical techniquesthat adequately describe their dynamics. While methods based onmatrix product states have been successful with one-dimensional sys-tems up to intermediate time-scales, computational methods of equiv-alent accuracy for larger dimensions remain elusive. I will present anovel method that is suitable for simulating the dynamics of quan-tum spin models of any dimension. The method samples the manybody Wigner function and evaluates the evolution equations obtainedfrom the Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierar-chy. Higher orders in the hierarchy allow for systematic refinements.Quantum correlations can be treated through both, the Wigner func-tion sampling and the BBGKY evolution, bringing about highly ac-curate estimates of correlations and entanglement witnesses. Themethod is particularly suitable for nonintegrable systems, especiallythose with long-range interactions. I will demonstrate its efficacy bycomparing with exact results, as well as other numerical methods. Fi-nally, I conclude with outlooks into modelling the Lindblad dynamicsof the collective scattering of classical light by cold quantum gases,especially phenomena like superradiance, as well as experimental cor-rections in lattice spin simulators due to decoherence.

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Collective atomic emission and its interplay withmotional effects in dense clouds of Sr atoms Thu, 12/05

8:30-9:10

Johannes SchachenmayerJILA, United States of America

Understanding interactions between light and matter in a dense atomicmedium is a long-standing problem in physical science. In additionto their fundamental importance in optical physics, such interactionsplay a central role in enabling a range of new quantum technologiesincluding optical lattice atomic clocks and quantum networks. Here,we report how experimental results of collective light emission from acoherently driven ultracold gas of Strontium atoms at JILA (Boulder,USA) can be theoretically understood from a microscopic model ofradiating quantum dipoles. In particular, we report on how the motionof the atoms affects the interaction induced broadening and shifting ofthe emitted line-shape. These motional effects can be experimentallyidentified in micro-Kelvin temperature Strontium gases, by makinguse of two atomic transition, a strong and a weak one with naturallinewidths of 32 MHZ and 7.5 kHz, respectively.

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Cooperativity in lattice monolayers of driven interactingdipolesThu, 12/05

9:10-9:35

Robert BettlesDurham University, United Kingdom

We investigate the cooperative behaviour of regular monolayers ofdriven two-level dipoles, using classical electrodynamics simulations.The dipolar response results from the interference of many cooperativeeigenmodes, each frequency-shifted from the single resonant dipolecase, and with a modified lifetime, due to the interactions betweendipoles. We show how subradiant behaviour of the dominant eigen-mode in a two-dimensional triangular or square dipolar lattice canlead to significant enhancement in the optical extinction of a resonantdriving field [1]. Such an enhancement in the optical cross sectionis an enticing goal in light-matter interactions, due to its fundamen-tal role in quantum and non-linear optics. Also of interest is thekagome lattice, where the semiregular geometry permits simultaneousexcitation of two dominant modes, one strongly subradiant, leadingto an electromagnetically-induced-transparency-like interference in atwo-level system. The interfering modes are associated with ferroelec-tric and antiferroelectric ordering in alternate lattice rows with longrange interactions [2].

[1] Enhanced optical cross section via collective coupling of atomicdipoles in a 2D array, R.J. Bettles, S.A. Gardiner, and C.S. Adams,arXiv:1510.07855 (2015).[2] Cooperative ordering in lattices of interacting two-level dipoles,R.J. Bettles, S.A. Gardiner, and C.S. Adams, Phys. Rev. A 92,063822 (2015).

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Intrinsic Optical Bistability in a Rydberg ensemble Thu, 12/059:35-10:00

Natalia R. de MeloUFPE, Brazil

The optical bistability is a phenomenon which has been receiving con-siderable attention by providing a rich contribution to the understand-ing of the non-equilibrium systems. Intrinsic Optical bistability origi-nates when the bistability arises in a system without external cavity,where the dynamical equilibrium, necessary for these observations,is provided by a driving field excitation and the dissipation due theatom-atom interaction [1]. In this work, we investigate intrinsic op-tical bistability in a diluted ensemble of cesium atoms. For this, weuse a three photon excitation scheme, where three lasers co- prop-agating are focused in a 2mm vapor cell and, using an interferencefilter, we measure the transmission of a probe beam. We observephase transition between two phases, low and high, of Rydberg occu-pancy and an intrinsic optical bistability for a big range of Rydbergstates [2]. The behavior of the hysteresis window width is investi-gated for different parameters like atomic density, principal quantumnumber and the power of the excitation laser. The experimental re-sults reveal widening of the hysteresis window and a subsequent nar-rowing with the increase of the principal quantum number. Also,we observe a saturation of the hysteresis window whilst the power ofthe excitation laser is increased. The experimental results show goodagreement with the prediction from a simple theoretical model basedon semiclassical Maxwell-Bloch equations including the effects of self-broadening and frequency shift. Furthermore, we are able to find thenumeric value for the self-broadening coefficient involving Rydberglevels [3]. This work was supported by Durham University, EPSRC(Grant No.EP/M014398/1 and EP/M013103/1 ), CNPq and FACEPE(Brazilian Agencies).

[1] H.M. Gibbs, Optical Bistability:Controlling Light with Light (Aca-demic press Inc, 1985).[2] C. Carr, R. Ritter, C. G. Wade, C. S. Adams and K. J. Weatherill.Rev. Lett. 111, 113901 (2013).[3] N. R. de Melo and S.S. Vianna. J. Opt. Soc. Am. B. 31, 1735(2014).

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