Dissipative Dicke Model with Collective Atomic Decay: Bistability, Noise-Driven Activation and Non-Thermal First Order Superradiance Transition

The Dicke model describes the coherent interaction of a laser-driven ensemble of two level atoms with a quantized light field. It is realized within cavity QED experiments, which in addition to the coherent Dicke dynamics feature dissipation due to e.g. atomic spontaneous emission and cavity photon loss. Spontaneous emission supports the uncorrelated decay of individual atomic excitations as well as the enhanced, collective decay of an excitation that is shared by $N$ atoms and whose strength is determined by the cavity geometry. We derive a many-body master equation for the dissipative Dicke model including both spontaneous emission channels and analyze its dynamics on the basis of Heisenberg-Langevin and stochastic Bloch equations. We find that the collective loss channel leads to a region of bistability between the empty and the superradiant state. Transitions between these states are driven by non-thermal, markovian noise. The interplay between dissipative and coherent elements leads to a genuine non-equilibrium dynamics in the bistable regime, which is expressed via a non-conservative force and a multiplicative noise kernel appearing in the stochastic Bloch equations. We present a semiclassical approach, based on stochastic nonlinear optical Bloch equations, which for the infinite-range Dicke Model become exact in the large-$N$-limit. The absence of an effective free energy functional, however, necessitates to include fluctuation corrections with $\mathcal{O}(1/N)$ for finite $N<\infty$ to locate the non-thermal first-order phase transition between the superradiant and the empty cavity.Comment: as published in Physical Review

Many-Body Quantum Optics with Decaying Atomic Spin States: (γ\gamma, κ\kappa) Dicke model

We provide a theory for quantum-optical realizations of the open Dicke model with internal, atomic spin states subject to spontaneous emission with rate $\gamma$. This introduces a second decay channel for excitations to irreversibly dissipate into the environment, in addition to the photon loss with rate $\kappa$, which is composed of individual atomic decay processes and a collective atomic decay mechanism. The strength of the latter is determined by the cavity geometry. We compute the mean-field non-equilibrium steady states for spin and photon observables in the long-time limit, $t\rightarrow \infty$. Although $\gamma$ does not conserve the total angular momentum of the spin array, we argue that our solution is exact in the thermodynamic limit, for the number of atoms $N\rightarrow \infty$. In light of recent and upcoming experiments realizing superradiant phase transitions using internal atomic states with pinned atoms in optical lattices, our work lays the foundation for the pursuit of a new class of open quantum magnets coupled to quantum light.Comment: 17 pages, 6 figures; added appendix for the derivation of a collective atomic decay mechanism in a Lindblad formalism; version as published in Physical Review

Theory of a weak-link superconductor-ferromagnet Josephson structure

We propose a model for the theoretical description of a weak-link Josephson junction, in which the weak link is spin-polarized due to proximity to a ferromagnetic metal [S-(F|S)-S, where S is a superconductor and F is a ferromagnetic metal]. Employing Usadel transport theory appropriate for diffusive systems, we show that the weak link is described within the framework of Andreev circuit theory by an effective self-energy resulting from the implementation of spin-dependent boundary conditions. This leads to a considerable simplification of the model, and allows for an efficient numerical treatment. As an application of our model, we show numerical calculations of important physical observables such as the local density of states, proximity-induced minigaps, spin-magnetization, and the phase and temperature dependence of Josephson currents of the S-(F|S)-S system. We discuss multivalued current-phase relationships at low temperatures as well as their crossover to sinusoidal form at high temperatures. Additionally, we numerically treat (S-F-S) systems that exhibit a magnetic domain wall in the F region and calculate the temperature-dependence of the critical currents

Magnetic Phase Transitions in Driven-Dissipative Atomic Ensembles Interacting with Quantum Light

Present-day experiments have started to couple traditional simulations of ultracold atom experiments with quantum light-fields in cavities. This has provided a wealth of op- portunities to enlarge the number of interaction potentials: cavity mediated long-range interactions compete with kinetic energies, longitudinal fields, short-ranged collisional or magnetic spin-spin interactions. The intracavity many-body lattice models often have to be maintained far from equilibrium through the presence of external driving lasers that help to boost and engineer the various interaction potentials. The steady influx of energy is compensated by a steady stream of energy out of the atom-cavity system for example by photon losses or atomic spontaneous emission. Several experiments have demonstrated that such environments can give rise to new competing quantum phases. But this long-standing ambition to push for models with tailorable interaction potentials can bring with it also considerable challenges in their theoretical description, since sponta- neous symmetry breaking transitions in many body lattice systems coupled to dynamical light-fields with single-photon character occur in the presence of drive and dissipation for the photonic force carriers. This clearly calls for model systems where the above men- tioned interplay of interactions, drive, dissipation and cooperative many-body behaviour can be theoretically studied to provide simple, experimentally verifiable predictions. The Dicke model is, through its simplicity (an exactly solved ferromagnet with infinite range atom-atom interactions mediated by a single cavity mode), an exceptionally well- suited candidate. As the generic model for atom-light interactions, it has been experimen- tally realized in a variety of modern quantum optical systems, highlighting its relevance for present-day research. The Dicke model is also highly versatile itself. It has been extended into the dissipative realm, was promoted to account for multiple optical light modes and was used to describe multiple, coupled single-mode cavity structures. It was adapted to treat spin-selective coupling to a cavity to describe superradiance phase tran- sitions in multi-level atomic systems. Moreover, it was realised in electronic circuits where the dipole coupling of real atoms to single mode fields is replaced by a capacitive cou- pling of artificial atoms to a resonator mode. This illustrates that the Dicke model and its extended variants are ’future-proof’ and continue to be of relevance for fundamental light-matter interactions and for driven-dissipative phase transitions. In this thesis, we investigate magnetic phase transitions in driven-dissipative atomic en- sembles interacting with quantum light. We present three research projects on variants of cooperative radiation of an ensemble of laser driven two-level atoms in a single mode optical cavity, as described by the Dicke model. Throughout the chapters 2,3 and 4 that contain the main body of research of this thesis, we investigate phase transitions between non-equilibrium stationary states in engineered quantum-optical systems each of which extends the conventional Dicke model physics. As a starting point, we map the quantum equations of motion onto a set of semiclassical nonlinear stochastic equations and analyse their stationary states and instabilities with master equations for atomic spin and photon mean-field amplitudes. These are used to obtain experimentally relevant parameters such as critical atom-light couplings for phase transitions, phase diagrams and properties of stationary non-equilibrium states in addition to cavity output spectra that identify the imprint of magnetic correlations in the light-field. In chapter 2, we help resolve a discrepancy between earlier experimental investigations of the critical atom-light coupling strength for the superradiance transition in the Dicke model: higher external pumping strengths than theoretically predicted were needed to observe a coherent, superradiant state of the light field in an optical cavity. By including incoherent spontaneous emission of atomic excitations, we extend the dissipative Dicke model to a two loss channel variant containing both photon leakage and atomic decay that reproduces the experimentally observed critical atom-light coupling. Recent experiments have started to interface quantum many body lattice models with coherent cavity fields, thereby allowing to realize new quantum phases through competing atom-cavity and atom-atom interactions. In chapter 3, we consider a simplified model for such a set-up where a single quantized mode of the light-field interacts with an ensemble of Rydberg-dressed atoms inside a high finesse optical cavity. This model provides a base case for further studies of quantum magnets in optical cavities. At the heart of this model is a competition of short- (dipolar atom-atom) and long-range (atom-light) interactions at the Hamiltonian level in the presence of both spontaneous emission and photon leak- age through the cavity mirrors. We show that different magnetic phases can coexist with coherent atomic radiation and provide clear experimental signatures to identify the mag- netic structure and intra-cavity dynamics. We suggest an experimental level-scheme for a quantum optical implementation of our model. In chapter 4 we consider a generic, collective decay for many-body excitations in the paradigmatic Dicke model. This extension drastically enriches the dynamics as it induces a bicritical point and a bistable regime dominated by true non-equilibrium fluctuations that induce a dissipative first-order phase transition that can only be resolved by including finite fluctuation corrections with the help of stochastic Langevin equations. We investigate the hysteretic response to time-dependent ramps of the atom-light coupling. Discontinuous first-order phase transitions where metastable states coexist in a hysteresis domain have been investigated in recent dissipative quantum-optical experiments. We review noise- activation far from thermal equilibrium in chapter 5

Concentrations des artefacts et structures d'habitation. L'interprétation des accumulations des vestiqes archéologiques du Paléolithique final

International audienceAn den Fundkonzentrationen des Spätpaläolithikums (Federmessergruppen) Niederbieber (Fläche IV), Andernach-Martinsberg 3 und Berlin-Tegel IX zeigt eine Kombination verschiedener Methoden der räumlichen Analyse Hinweise auf eine frühere Trennung der jeweils größten Ansammlungen von Artefakten von ihrer Umgebung. Alle drei Konzentrationen sind relativ kompakt und weisen am Rand einen deutlichen Abfall der Fundmenge auf. Die Verteilung größerer Objekte und die Verbindungslinien von Artefaktzusammenpassungen sind auf die Ränder der Fundstreuungen bezogen. In einigen Fällen werden bestimmte Funde wie unverbrannte größere Knochenfragmente oder größere Steine nicht innerhalb der Fundstreuung der Steinartefakte gefunden. Wir interpretieren diese Indizien für eine ehemals wirksame Begrenzung der Fundkonzentrationen als Hinweise auf das ehemalige Vorhandensein von Behausungen.At the Final Palaeolithic (Federmessergruppen) sites of Niederbieber (Area IV), Andernach-Martinsberg 3 and Berlin-Tegel IX a combination of different methods of spatial analysis reveals indications of a former separation of the main accumulations of finds from the surrounding area. All three find scatters are very dense and show a marked sudden decrease at their edges. The distribution of larger objects and refitting lines between artefacts respect the edges of the find scatters. In some cases, certain finds, such as unburned larger bone fragments or larger stones, are not found within the artefact find scatter. We interpret these as indications of a formerly effective limitation of the find concentrations that show the former presence of dwelling structures.Pour les concentrations d’objets lithiques du Paléolithique final (Groupes à Federmesser) de Nieberbieber (zone IV), Andernach-Martinsberg 3 et Berlin-Tegel IX, la présence des habitations peut être démontrée par la combinaison de différents méthodes d'analyse spatiale. Chacune des ces trois concentrations est relativement compacte tandis que la quantité des objets lithiques diminue rapidement en périphérie. La distribution des objets de taille supérieure et des lignes de raccordement entre les remontages se rapporte à la périphérie de l’étendue lithique. Dans certains cas, quelques objets, comme des fragments d'os plus gros non brûlés ou des pierres plus grosses, ne sont pas trouvées dans les zones riches en artefacts. Nous interprétons ces indications concernant les limites des concentrations autrefois en vigueur comme indiquant la présence des logements

Dissipative Dicke model with collective atomic decay: Bistability, noise-driven activation, and the nonthermal first-order superradiance transition

The Dicke model describes the coherent interaction of a laser-driven ensemble of two-level atoms with a quantized light field. It is realized within cavity QED experiments, which in addition to the coherent Dicke dynamics feature dissipation due to, e.g., atomic spontaneous emission and cavity photon loss. Spontaneous emission supports the uncorrelated decay of individual atomic excitations as well as the enhanced collective decay of an excitation that is shared by N atoms and whose strength is determined by the cavity geometry. We derive a many-body master equation for the dissipative Dicke model including both spontaneous emission channels and analyze its dynamics on the basis of Heisenberg-Langevin and stochastic Bloch equations. We find that the collective loss channel leads to a region of bistability between the empty and the superradiant state. Transitions between these states are driven by nonthermal Markovian noise. The interplay between dissipative and coherent elements leads to a genuine nonequilibrium dynamics in the bistable regime, which is expressed via a nonconservative force and a multiplicative noise kernel appearing in the stochastic Bloch equations. We present a semiclassical approach, based on stochastic nonlinear optical Bloch equations, which for the infinite-range Dicke model become exact in the large- N-limit. The absence of an effective free-energy functional, however, necessitates the inclusion of fluctuation corrections with O(1/N) for finite N < ∞ to locate the nonthermal first-order phase transition between the superradiant and the empty cavity

Huttes ou ciel ? Structures d'habitation latentes au Paléolithique final de l'Europe centrale

International audienceFür die Fundkonzentrationen der Federmessergruppen von Niederbieber I & IV, Andernach-Martinsberg 3 und Berlin-Tegel IX lassen sich jeweils Kombinationen von Indizien für das Vorhandensein von Behausungen aufzeigen. Alle vier Konzentrationen sind relativ kompakt und weisen am Rand einen deutlichen Abfall der Fundmenge auf: Teils ist die Verteilung größerer Objekte und der Verbindungslinien von Artefaktzusammenpassungen auf die Ränder bezogen, teils sind die Artefaktkonzentrationen von der Verteilung größerer Knochen ausgespart. Die Fundkonzentrationen unterscheiden sich im Hinblick auf den Grad der Verwischung kleinräumiger Artefakthäufungen, die Arbeitsbereiche anzeigen. Diese Verwischung könnte möglicherweise als ein Hinweis für die Aufenthaltsdauer herangezogen werden. Die von uns zur Diskussion gestellten latenten Behausungsbefunde nehmen eine geringere Fläche ein als die Trapezzelte des Magdalénien und der frühen Rückenspitzengruppen. Sie haben aber z.T. einige Merkmale mit diesen gemein: den trapezförmigen Grundriss und die Aufteilung in fundreiche und fundarme Hälften sowie einen deutlich begrenzten fundreichen Bereich um die Feuerstelle.At the Final Palaeolithic (Federmessergruppen) sites of Niederbieber (Area I & IV), Andernach-Martinsberg 3 and Berlin-Tegel IX, a combination of different methods of spatial analysis reveals indica tions for the presence of dwelling structures. All four find scatters are very dense and show a marked, sudden decrease at their edges. The distribution of larger objects and refitting lines between artefacts respect the edges of the find scatters. In some cases bones are not found within the artefact find scatter. The find concentrations differ in their degree of blurring of small scale artefact accumulations, which indicate the locations of working areas. The dwelling features discussed in this paper are smaller than trapezoid dwellings of the Magdalenian and the early Federmessergruppen. We could nevertheless identify some features common to both: the trapezoidal floor plan, partitioning into rich and poor sectors and a clearly defined area with a high density of finds around the hearth.Pour les concentrations d’objets lithiques de Federmessergruppen dans les sites de Nieberbieber I et IV, Andernach-Martinsberg 3 et Berlin-Tegel IX, la présence d’habitats humains peut être démontrée par la combinaison de différents indices. Chacune des ces quatre concentrations est relativement compacte tandis que la quantité des objets lithiques diminue rapidement en périphérie: en partie la distribution des objets de taille supérieure et des lignes de raccordement entre les remontages se rapportent à la périphérie de l’étendue lithique, en partie la concentration d’objets lithiques se trouve sans rapport avec la répartition des ossements. Les concentrations d’objets lithiques se distinguent concernant le degré de dispersion des petites accumulations de pièces retouchées qui indiquent des zones d’activités spécialisées. Cette dispersion des zones d’activités pourrait probablement constituer un indice pour la durée de l’occupation du site. L’étude concerne des preuves latentes d’une existence d’habitats humains qui occupent une surface inférieure à celle des structures trapézoïdales du Magdalénien et des groupes précoces aux pointes à dos courbe avec lesquelles elles partagent néanmoins quelques caractéristiques: le plan trapézoïdal, la division en parties riches et pauvres en objets lithiques ainsi qu’une zone riche et clairement délimitée autour du foye

Structures latentes des habitations du Paléolithique final : Niederbieber IV, Andernach-Martinsberg 3, Berlin-Tegel IX

International audienceAn den Fundkonzentrationen des Spätpaläolithikums (Federmessergruppen) Niederbieber (Fläche IV), Andernach-Martinsberg 3 und Berlin-Tegel IX zeigt eine Kombination verschiedener Methoden der räumlichen Analyse Hinweise auf eine frühere Trennung der jeweils größten Ansammlungen von Artefakten von ihrer Umgebung. Alle drei Konzentrationen sind relativ kompakt und weisen am Rand einen deutlichen Abfall der Fundmenge auf. Die Verteilung größerer Objekte und die Verbindungslinien von Artefaktzusammenpassungen sind auf die Ränder der Fundstreuungen bezogen. In einigen Fällen werden bestimmte Funde wie unverbrannte größere Knochenfragmente oder größere Steine nicht innerhalb der Fundstreuung der Steinartefakte gefunden. Wir interpretieren diese Indizien für eine ehemals wirksame Begrenzung der Fundkonzentrationen als Hinweise auf das ehemalige Vorhandensein von Behausungen.At the Final Palaeolithic (Federmessergruppen) sites of Niederbieber (Area IV), Andernach-Martinsberg 3 and Berlin-Tegel IX a combination of different methods of spatial analysis reveals indications for a former separation of the main accumulations of finds from the surrounding area. All three find scatters are very dense and show a marked sudden decrease at their edges. The distribution of larger objects and refitting lines between artefacts respect the edges of the find scatters. In some cases certain finds like unburned larger bone fragments or larger stones are not found within the artefact find scatter. We interpret these indications for a formerly effective limitation of the find concentrations as showing the former presence of dwelling structures.Pour les concentrations d’objets lithiques de Federmessergruppen dans les sites de Nieberbieber IV, Andernach-Martinsberg 3 et Berlin-Tegel IX, la présence d’habitats humains peut être démontrée par la combinaison de différents indices. Chacune des ces quatre concentrations est relativement compacte tandis que la quantité des objets lithiques diminue rapidement en périphérie. La distribution des objets de taille supérieure et des lignes de raccordement entre les remontages se rapporte à la périphérie de l’étendue lithique. En partie, la concentration d’objets lithiques se trouve sans rapport avec la répartition des ossements. Nous interprétons ces indices comme une ancienne barrière effective entre les répartitions qui indique l’existence des structures habitats humains

Structures latentes des habitations du Paléolithique final : Niederbieber IV, Andernach-Martinsberg 3, Berlin-Tegel IX

International audienceAn den Fundkonzentrationen des Spätpaläolithikums (Federmessergruppen) Niederbieber (Fläche IV), Andernach-Martinsberg 3 und Berlin-Tegel IX zeigt eine Kombination verschiedener Methoden der räumlichen Analyse Hinweise auf eine frühere Trennung der jeweils größten Ansammlungen von Artefakten von ihrer Umgebung. Alle drei Konzentrationen sind relativ kompakt und weisen am Rand einen deutlichen Abfall der Fundmenge auf. Die Verteilung größerer Objekte und die Verbindungslinien von Artefaktzusammenpassungen sind auf die Ränder der Fundstreuungen bezogen. In einigen Fällen werden bestimmte Funde wie unverbrannte größere Knochenfragmente oder größere Steine nicht innerhalb der Fundstreuung der Steinartefakte gefunden. Wir interpretieren diese Indizien für eine ehemals wirksame Begrenzung der Fundkonzentrationen als Hinweise auf das ehemalige Vorhandensein von Behausungen.At the Final Palaeolithic (Federmessergruppen) sites of Niederbieber (Area IV), Andernach-Martinsberg 3 and Berlin-Tegel IX a combination of different methods of spatial analysis reveals indications for a former separation of the main accumulations of finds from the surrounding area. All three find scatters are very dense and show a marked sudden decrease at their edges. The distribution of larger objects and refitting lines between artefacts respect the edges of the find scatters. In some cases certain finds like unburned larger bone fragments or larger stones are not found within the artefact find scatter. We interpret these indications for a formerly effective limitation of the find concentrations as showing the former presence of dwelling structures.Pour les concentrations d’objets lithiques de Federmessergruppen dans les sites de Nieberbieber IV, Andernach-Martinsberg 3 et Berlin-Tegel IX, la présence d’habitats humains peut être démontrée par la combinaison de différents indices. Chacune des ces quatre concentrations est relativement compacte tandis que la quantité des objets lithiques diminue rapidement en périphérie. La distribution des objets de taille supérieure et des lignes de raccordement entre les remontages se rapporte à la périphérie de l’étendue lithique. En partie, la concentration d’objets lithiques se trouve sans rapport avec la répartition des ossements. Nous interprétons ces indices comme une ancienne barrière effective entre les répartitions qui indique l’existence des structures habitats humains

Concentrations des artefacts et structures d'habitation. L'interprétation des accumulations des vestiqes archéologiques du Paléolithique final

International audienceAn den Fundkonzentrationen des Spätpaläolithikums (Federmessergruppen) Niederbieber (Fläche IV), Andernach-Martinsberg 3 und Berlin-Tegel IX zeigt eine Kombination verschiedener Methoden der räumlichen Analyse Hinweise auf eine frühere Trennung der jeweils größten Ansammlungen von Artefakten von ihrer Umgebung. Alle drei Konzentrationen sind relativ kompakt und weisen am Rand einen deutlichen Abfall der Fundmenge auf. Die Verteilung größerer Objekte und die Verbindungslinien von Artefaktzusammenpassungen sind auf die Ränder der Fundstreuungen bezogen. In einigen Fällen werden bestimmte Funde wie unverbrannte größere Knochenfragmente oder größere Steine nicht innerhalb der Fundstreuung der Steinartefakte gefunden. Wir interpretieren diese Indizien für eine ehemals wirksame Begrenzung der Fundkonzentrationen als Hinweise auf das ehemalige Vorhandensein von Behausungen.At the Final Palaeolithic (Federmessergruppen) sites of Niederbieber (Area IV), Andernach-Martinsberg 3 and Berlin-Tegel IX a combination of different methods of spatial analysis reveals indications of a former separation of the main accumulations of finds from the surrounding area. All three find scatters are very dense and show a marked sudden decrease at their edges. The distribution of larger objects and refitting lines between artefacts respect the edges of the find scatters. In some cases, certain finds, such as unburned larger bone fragments or larger stones, are not found within the artefact find scatter. We interpret these as indications of a formerly effective limitation of the find concentrations that show the former presence of dwelling structures.Pour les concentrations d’objets lithiques du Paléolithique final (Groupes à Federmesser) de Nieberbieber (zone IV), Andernach-Martinsberg 3 et Berlin-Tegel IX, la présence des habitations peut être démontrée par la combinaison de différents méthodes d'analyse spatiale. Chacune des ces trois concentrations est relativement compacte tandis que la quantité des objets lithiques diminue rapidement en périphérie. La distribution des objets de taille supérieure et des lignes de raccordement entre les remontages se rapporte à la périphérie de l’étendue lithique. Dans certains cas, quelques objets, comme des fragments d'os plus gros non brûlés ou des pierres plus grosses, ne sont pas trouvées dans les zones riches en artefacts. Nous interprétons ces indications concernant les limites des concentrations autrefois en vigueur comme indiquant la présence des logements