Thermalization in a 1D Rydberg gas: validity of the microcanonical ensemble hypothesis

We question the microcanonical hypothesis, often made to account for the thermalization of complex closed quantum systems, on the specific example of a chain of two-level atoms optically driven by a resonant laser beam and strongly interacting via Rydberg-Rydberg dipole-dipole interactions. Along its (necessarily unitary) evolution, this system is indeed expected to thermalize, i.e. observables, such as the number of excitations, stop oscillating and reach equilibrium-like expectation values. The latter are often calculated through assuming the system can be effectively described by a thermal-like microcanonical state. Here, we compare the distribution of excitations in the chain calculated either according to the microcanonical assumption or through direct exact numerical simulation. This allows us to show the limitations of the thermal equilibrium hypothesis and precise its applicability conditions.Comment: v2: Add comparison with Bettelli et al.'s Monte-Carlo simulation (App. A) + typo correctio

Cavity Quantum Electrodynamics with a Rydberg blocked atomic ensemble

We propose to implement the Jaynes-Cummings model by coupling a few-micrometer large atomic ensemble to a quantized cavity mode and classical laser fields. A two-photon transition resonantly couples the single-atom ground state |g> to a Rydberg state |e> via a non-resonant intermediate state |i>, but due to the interaction between Rydberg atoms only a single atom can be resonantly excited in the ensemble. This restricts the state space of the ensemble to the collective ground state |G> and the collectively excited state |E> with a single Rydberg excitation distributed evenly on all atoms. The collectively enhanced coupling of all atoms to the cavity field with coherent coupling strengths which are much larger than the decay rates in the system leads to the strong coupling regime of the resulting effective Jaynes-Cummings model. We use numerical simulations to show that the cavity transmission can be used to reveal detailed properties of the Jaynes-Cummings ladder of excited states, and that the atomic nonlinearity gives rise to highly non-trivial photon emission from the cavity. Finally, we suggest that the absence of interactions between remote Rydberg atoms may, due to a combinatorial effect, induce a cavity-assisted excitation blockade whose range is larger than the typical Rydberg dipole-dipole interaction length.Comment: 9 pages, 6 figure

Rydberg-induced optical nonlinearities from a cold atomic ensemble trapped inside a cavity

We experimentally characterize the optical nonlinear response of a cold atomic medium placed inside an optical cavity, and excited to Rydberg states. The excitation to S and D Rydberg levels is carried out via a two-photon transition in an EIT (electromagnetically induced transparency) configuration, with a weak (red) probe beam on the lower transition, and a strong (blue) coupling beam on the upper transition. The observed optical nonlinearities induced by S states for the probe beam can be explained using a semi-classical model with van der Waals' interactions. For the D states, it appears necessary to take into account a dynamical decay of Rydberg excitations into a long-lived dark state. We show that the measured nonlinearities can be explained by using a Rydberg bubble model with a dynamical decay.Comment: 8 pages, 6 figure

Photonic Controlled-Phase Gates Through Rydberg Blockade in Optical Cavities

We propose a novel scheme for high fidelity photonic controlled phase gates using Rydberg blockade in an ensemble of atoms in an optical cavity. The gate operation is obtained by first storing a photonic pulse in the ensemble and then scattering a second pulse from the cavity, resulting in a phase change depending on whether the first pulse contained a single photon. We show that the combination of Rydberg blockade and optical cavities effectively enhances the optical non-linearity created by the strong Rydberg interaction and thereby reduces the requirements for photonic quantum gates. The resulting gate can be implemented with cavities of moderate finesse which allows for highly efficient processing of quantum information encoded in photons. As a particular example of this, we show how the gate can be employed to increase the communication rate of quantum repeaters based on atomic ensembles.Comment: main manuscript 5 pages with 11 pages of supplementary informatio

The Golgi and the centrosome: building a functional partnership

The mammalian Golgi apparatus is characterized by a ribbon-like organization adjacent to the centrosome during interphase and extensive fragmentation and dispersal away from the centrosome during mitosis. It is not clear whether this dynamic association between the Golgi and centrosome is of functional significance. We discuss recent findings indicating that the Golgi–centrosome relationship may be important for directional protein transport and centrosome positioning, which are both required for cell polarization. We also summarize our current knowledge of the link between Golgi organization and cell cycle progression

Contrôle Quantique et Protection de la Cohérence par effet Zénon, Applications à l'Informatique Quantique

Raporteurs : MM. Philippe Grangier et Dima Shepelyansky.Examinateurs : MM. Lev Ioffe, Pierre Pillet et Jean-Michel Raimond.Quantum control is quite an important issue in modern Physics. In particular, in the context of Quantum Computation, controlling the evolution operator enables to implement any quantum gate and thus to perform any computational task on the information stored in the computer. In the first part of this dissertation, after a brief review of the field, a method, called the non-holonomic control, is put forward in order to control the evolution operator. This method is then applied to a realistic physical situation, involving cold atoms of Caesium. The interaction of the computer with its environment constitutes a threat for reliable quantum information storage and processing. However, the development of quantum error-correcting methods, inspired of their classical counterparts, suggests that this danger may be avoided. In the second part of this study, an information protection scheme is presented, based on the quantum Zeno effect, which is then applied to a Rubidium atom.Le contrôle quantique constitue un enjeu majeur de la Physique contemporaine. Après un bref tour d'horizon du domaine, nous présentons une méthode, appelée contrôle non holonôme, qui permet d'imposer à système quantique quelconque une évolution unitaire arbitrairement choisie. Dans le contexte de l'Informatique Quantique, cette technique peut être utilisée pour réaliser n'importe quelle porte quantique : à titre d'exemple, nous montrons comment appliquer une porte CNOT à un système de deux atomes de Césium froids en interaction.L'interaction de l'ordinateur avec son environnement risque de compromettre sa fiabilité. Le développement récent de la correction d'erreurs quantiques, inspirée des techniques classiques, suggère néanmoins que ce danger peut être évité. Après une présentation succincte du cadre général de la correction d'erreurs, nous proposons une méthode de protection de l'information fondée sur l'effet Zénon. Cette méthode est ensuite appliquée à un atome de Rubidium

Contrôle quantique et protection de la cohérence par effet zénon, applications à l'Informatique quantique

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Thermalization in a 1D Rydberg gas: validity of the microcanonical ensemble hypothesis

v2: Add comparison with Bettelli et al.'s Monte-Carlo simulation (App. A) + typo correctionWe question the microcanonical hypothesis, often made to account for the thermalization of complex closed quantum systems, on the specific example of a chain of two-level atoms optically driven by a resonant laser beam and strongly interacting via Rydberg-Rydberg dipole-dipole interactions. Along its (necessarily unitary) evolution, this system is indeed expected to thermalize, i.e. observables, such as the number of excitations, stop oscillating and reach equilibrium-like expectation values. The latter are often calculated through assuming the system can be effectively described by a thermal-like microcanonical state. Here, we compare the distribution of excitations in the chain calculated either according to the microcanonical assumption or through direct exact numerical simulation. This allows us to show the limitations of the thermal equilibrium hypothesis and precise its applicability conditions