Direct measurement of the van der Waals interaction between two Rydberg atoms

We report on the direct measurement of the van der Waals interaction between two isolated, single Rydberg atoms separated by a controlled distance of a few micrometers. By working in a regime where the single-atom Rabi frequency of the laser used for excitation to the Rydberg state is comparable to the interaction energy, we observe a \emph{partial} Rydberg blockade, whereby the time-dependent populations of the various two-atom states exhibit coherent oscillations with several frequencies. A quantitative comparison of the data with a simple model based on the optical Bloch equations allows us to extract the van der Waals energy, and to observe its characteristic $C_6/R^6$ dependence. The magnitude of the measured $C_6$ coefficient agrees well with an \emph{ab-initio} theoretical calculation, and we observe its dramatic increase with the principal quantum number $n$ of the Rydberg state. Our results not only allow to test an important physical law, but also demonstrate a degree of experimental control which opens new perspectives in quantum information processing and quantum simulation using long-range interactions between the atoms.Comment: 4 pages, 3 figures, published versio

Coherent dipole-dipole coupling between two single atoms at a F\"orster resonance

Resonant energy transfers, i.e. the non-radiative redistribution of an electronic excitation between two particles coupled by the dipole-dipole interaction, lie at the heart of a variety of chemical and biological phenomena, most notably photosynthesis. In 1948, F\"orster established the theoretical basis of fluorescence resonant energy transfer (FRET), paving the ground towards the widespread use of FRET as a "spectroscopic ruler" for the determination of nanometer-scale distances in biomolecules. The underlying mechanism is a coherent dipole-dipole coupling between particles, as already recognized in the early days of quantum mechanics, but this coherence was not directly observed so far. Here, we study, both spectroscopically and in the time domain, the coherent, dipolar-induced exchange of electronic excitations between two single Rydberg atoms separated by a controlled distance as large as 15 microns, and brought into resonance by applying a small electric field. The coherent oscillation of the system between two degenerate pair states occurs at a frequency that scales as the inverse third power of the distance, the hallmark of dipole-dipole interactions. Our results not only demonstrate, at the most fundamental level of two atoms, the basic mechanism underlying FRET, but also open exciting prospects for active tuning of strong, coherent interactions in quantum many-body systems.Comment: 4 pages, 3 figure

Contribution to the Active Generator Principle:the Gate-commutated Polyphased Matrix Converter

This work is part of the innovative "Active Generator" (AG) project. AG is a concept that suggests a new arrangement of the turbine-generator line of a high power utility (a few hundred of MW) in order to de-synchronize the rotation speed of the turbine-generator group from the fixed grid frequency (50 Hz or 60 Hz). This de-synchronization has essentially two advantages. First, the variable speed of the group enables the operation of the turbine at its best available efficiency in function of the delivered power. Second, the de-synchronization allows to eliminate the gearbox between the turbine and the generator without losing the important degree of freedom in the choice of optimal nominal rotation speed of the turbine. The latter advantage is particularly interesting for high power utilities, whose prime mover is a gas turbine, because for this power range the gearbox constitutes a heavy burden. The de-synchronization is realized with a static frequency converter which is a power electronics circuit composed of silicon power devices. The converter must ensure the same nominal frequency ratio than the gearbox it replaces, which can go above 50%. For such ratio the converter must be inserted between the stator windings of the generator and the grid. There are numerous different frequency converters. Some of them are available as industrial products and others are still in a development state. Not all of these different frequency converters are well adapted to high power applications. In the AG literature, a few recommendations suggest to use a low frequency commutation sequence, combined with a high number of input phases. The high number of input phases ensures a sufficient resolution of the converter's output voltage. Compared to others, this sequence is supposed to decrease the commutation losses of the converter, avoid the usual overdesign of the nominal power of the generator, and, finally, does not require the converter to include bulky intermediary DC storage components (capacitor or inductor). This sequence is a variant of the "Cosine Waveform Crossing" (CWC) method used for Naturally Commutated Cyclo-converters (NCC) and is named slowCWC. However, up till now, there is no converter that is able to run properly with this sequence. Thus a new converter is needed. This PhD work introduces a new converter that is able to fulfill the slowCWC sequence. It is derived from a slight modification of an existing topology (NCC) and is called "gate-commutated Polyphased Matrix Converter" (PPMC). It is a direct frequency converter with a high number of input phases, generally greater than twenty, and a matrix structure of the valves that allows to connect each of the three output phases to each of the generator (input) phases. The valves are bi-directional in voltage and current and are transistor-based to achieve the turn-off capability required by the commutation sequence. The PPMC requires to add protection circuits across each generator stator winding. These circuits protect the stator windings from overvoltages which appear during some forced commutations. In its first part this PhD work uses an analytical approach and the results are expressed in a per unit system that is also adequate to describe the electrical machines. In this first part, it is about the development of design rules for the components of the protection circuits. In addition the energy losses linked to these circuits are evaluated. Those losses strongly depend on the commutation type, which is itself influenced by the presence of the protection circuits. The expression of the duration of natural commutations in the per unit system is also developed in this first part and it constitutes a key parameter in the determination of the commutation type. These theoretical developments are illustrated with numerical simulations. In its second part this PhD work presents the realization of a small-scale experimental set-up with reduced power (1 kW) but a high input phase number (27). The aim of the experimental set-up is to implement and experiment in real-time the command and control algorithm of the PPMC as well as to verify the theoretical predictions developed in the first part. The results of those developments lead to the quantitative assessment of the efficiency of the PPMC. Besides the key parameters that can help to improve this efficiency are pointed out. In certain cases the efficiency of the PPMC is acceptable under the condition that a generator parameter (its leakage reactance) remains under a given limit. This work ends with a list of suggestions for future works related to the improvement of the PPMC and related to the AG project

Single-Atom Addressing in Microtraps for Quantum-State Engineering using Rydberg Atoms

We report on the selective addressing of an individual atom in a pair of single-atom microtraps separated by $3\;\mu$m. Using a tunable light-shift, we render the selected atom off-resonant with a global Rydberg excitation laser which is resonant with the other atom, making it possible to selectively block this atom from being excited to the Rydberg state. Furthermore we demonstrate the controlled manipulation of a two-atom entangled state by using the addressing beam to induce a phase shift onto one component of the wave function of the system, transferring it to a dark state for the Rydberg excitation light. Our results are an important step towards implementing quantum information processing and quantum simulation with large arrays of Rydberg atoms.Comment: 4 pages, 3 figure

Single-atom trapping in holographic 2D arrays of microtraps with arbitrary geometries

We demonstrate single-atom trapping in two-dimensional arrays of microtraps with arbitrary geometries. We generate the arrays using a Spatial Light Modulator (SLM), with which we imprint an appropriate phase pattern on an optical dipole trap beam prior to focusing. We trap single $^{87}{\rm Rb}$ atoms in the sites of arrays containing up to $\sim100$ microtraps separated by distances as small as $3\;\mu$m, with complex structures such as triangular, honeycomb or kagome lattices. Using a closed-loop optimization of the uniformity of the trap depths ensures that all trapping sites are equivalent. This versatile system opens appealing applications in quantum information processing and quantum simulation, e.g. for simulating frustrated quantum magnetism using Rydberg atoms.Comment: 9 pages, 10 figure

Hydroelectric System Response to Part Load Vortex Rope Excitation

The prediction of pressure and output power fluctuations amplitudes on Francis turbine prototype is a challenge for hydro-equipment industry since it is subjected to guarantees to ensure smooth and reliable operation of the hydro units. The European FP7 research project Hyperbole aims to setup a methodology to transpose the pressure fluctuations induced by the cavitation vortex rope on the reduced scale model to the prototype generating units. A Francis turbine unit of 444MW with a specific speed value of ν = 0.29, is considered as case study. A SIMSEN model of the power station including electrical system, controllers, rotating train and hydraulic system with transposed draft tube excitation sources is setup. Based on this model, a frequency analysis of the hydroelectric system is performed to analyse potential interactions between hydraulic excitation sources and electrical components

Forced response analysis of hydroelectric systems

At off-design operating points, Francis turbines develop cavitation vortex rope in the draft tube which may interact with the hydraulic system. Risk resonance assessment by means of eigenmodes computation of the system is usually performed. However, the system response to the excitation source induced by the cavitation vortex rope is not predicted in terms of amplitudes and phase. Only eigenmodes shapes with related frequencies and dampings can be predicted. Besides this modal analysis, the risk resonance assessment can be completed by a forced response analysis. This method allows identifying the contribution of each eigenmode into the system response which depends on the system boundary conditions and the excitation source location. In this paper, a forced response analysis of a Francis turbine hydroelectric power plant including hydraulic system, rotating train, electrical system and control devices is performed. First, the general methodology of the forced response analysis is presented and validated with time domain simulations. Then, analysis of electrical, hydraulic and hydroelectric systems are performed and compared to analyse the influence of control structures on pressure fluctuations induced by cavitation vortex rope

Підручник зі слов’янського фольклору в Канаді

Рецензія на підручник: Kononenko N. Slavic Folklore: a Handbook. - Westport (Connecticut); London: Greenwood Press, 2007. - 209 pp