A tunable rf SQUID manipulated as flux and phase qubit

We report on two different manipulation procedures of a tunable rf SQUID. First, we operate this system as a flux qubit, where the coherent evolution between the two flux states is induced by a rapid change of the energy potential, turning it from a double well into a single well. The measured coherent Larmor-like oscillation of the retrapping probability in one of the wells has a frequency ranging from 6 to 20 GHz, with a theoretically expected upper limit of 40 GHz. Furthermore, here we also report a manipulation of the same device as a phase qubit. In the phase regime, the manipulation of the energy states is realized by applying a resonant microwave drive. In spite of the conceptual difference between these two manipulation procedures, the measured decay times of Larmor oscillation and microwave-driven Rabi oscillation are rather similar. Due to the higher frequency of the Larmor oscillations, the microwave-free qubit manipulation allows for much faster coherent operations.Comment: Proceedings of Nobel Symposium "Qubits for future quantum computers", Goeteborg, Sweden, May 25-28, 2009; to appear in Physica Script

Deep-well ultrafast manipulation of a SQUID flux qubit

Superconducting devices based on the Josephson effect are effectively used for the implementation of qubits and quantum gates. The manipulation of superconducting qubits is generally performed by using microwave pulses with frequencies from 5 to 15 GHz, obtaining a typical operating clock from 100MHz to 1GHz. A manipulation based on simple pulses in the absence of microwaves is also possible. In our system a magnetic flux pulse modifies the potential of a double SQUID qubit from a symmetric double well to a single deep well condition. By using this scheme with a Nb/AlOx/Nb system we obtained coherent oscillations with sub-nanosecond period (tunable from 50ps to 200ps), very fast with respect to other manipulating procedures, and with a coherence time up to 10ns, of the order of what obtained with similar devices and technologies but using microwave manipulation. We introduce the ultrafast manipulation presenting experimental results, new issues related to this approach (such as the use of a feedback procedure for cancelling the effect of "slow" fluctuations), and open perspectives, such as the possible use of RSFQ logic for the qubit control.Comment: 9 pages, 7 figure

Tracking ground state Ba+ ions in an expanding laser–plasma plume using time-resolved vacuum ultraviolet photoionization imaging

We report results from a study of the integrated column density and expansion dynamics of ground-state-selected Ba+ ions in a laser–plasma plume using a new experimental system—VPIF (vacuum-ultraviolet photoabsorption imaging facility). The ions are tracked by recording the attenuation of a pulsed and collimated vacuum ultraviolet beam, tuned to the 5p–6d inner-shell resonance of singly ionized barium, as the expanding plasma plume moves across it. The attenuated beam is allowed to fall on a CCD array where the spatial distribution of the absorption is recorded. Time-resolved ion velocity and integrated column density maps are readily extracted from the photoionization images

The relevance of surface impurities on the effect of temperature on powder flow behavior

Cohesive interparticle forces may have a relevant role in several industrial process operations involving particulate materials, such as fluidization, granulation and drying, as well as storage and solids handling units. Several of these operations require process conditions which involve high temperatures which, in turn, may affect the intensity of interparticle forces such as van der Waals, capillary and electrostatic forces. The mean by which the system temperature can affect all these forces is the change of particle hardness, the generation of liquid phases, which determines the formation of liquid bridges, and the modification of the particle dielectric properties. A direct measure of interparticle forces is possible but can be affected by large fluctuations and require a great number of repetitions. Interparticle forces, instead, play in averaged ensembles in bulk properties such as powder cohesion. It is of interest, therefore, to have the possibility to measure powder cohesion at the process temperature and to observe possible changes due to temperature variations to infer possible changes at the particle level. Powder shear testing is one of the available methods able to measure powder cohesion and it has the great advantage of measuring well established physical properties and of being able to produce highly repeatable results. It has to be remarked, however that to date no established procedure exists to relate powder cohesion measured at the bulk level to powder fluidization properties. In this paper a systematic study on the effect of the process conditions on the fluidization quality of ceramic powders is presented. Tests were carried out on powders of industrial interests, characterized by different particle size distributions and by different amounts of surface impurities, ranging from easy-flowing to cohesive flow behaviour.Two different experimental facilities were used: a modified ring shear tester available at the University of Salerno and aX-ray high temperature fluidization facility available at University College of London. The first apparatus was used to characterize powder cohesion at different temperatures between ambient and 500°C. Experimental results have been interpreted in terms of possible changes in interparticle forces as a function of temperature. The powder samples without impurities show an increase of cohesion with temperature as a result of an increase of interparticle van der Waals forces. A larger increase of cohesion was observed in the case of the powder samples with chemical impurities. The behaviour can be explained only by considering a cooperative effect of both van der Waals and capillary forces. It is noteworthy that the amount of surface impurities that is able to determine significant changes of powder flow properties is still so small that no evidence of phase transition could be detected by means of sample thermal analysis.The same powders have been characterized in terms of fluidization quality by using the x-ray fluidization facility available at University College of London under the same temperature range. Moreover the changes by temperature on the flow properties of the bulk solid evaluated with the shear cell and the behaviour of particles under fluidization conditions are analysed and discussed. Though a direct quantification of the particle-particle interactions in fluidized beds and of their changes under process conditions is very difficult, this paper suggests a method by which powder rheology can be used to indirectly evaluate the effects of the interparticle forces on fluidization

A rheological model for the flowability of aerated fine powders

A mechanically stirred fluid-bed rheometer (msFBR) was used to study the rheology of powders aerated below the fluidisation threshold. Glass ballotini (group B) and silica powders (group A) with different fine contents were tested. The torque necessary to rotate an impeller immersed in a bed of aerated powders was measured for different impeller depths and aeration rates. A model was developed: (a) to estimate the state of stress at the impeller depth, following Janssen’s approach for the evaluation of stresses in silos, and (b) to evaluate the torque, with the hypothesis that it is determined by the powder shear on a flat cylinder surface around the impeller. The model uses some powder properties, such as the dynamic and the wall yield loci of the powder used, which were estimated with a Peschl shear cell modified for small loads. The reasonable prediction of the torque at impeller depth larger than 3 cm provided by the model supports the hypothesis that the torque is defined by the plastic deformation of powders and can be explained within a simple Mohr–Coulomb approach to powder flow. The passive stress distribution that appears to set up during the shearing experiments leaves open some fundamental questions regarding the limiting conditions determining such behaviour. As in previous experiments found in the literature, aeration does not affect the rheology of powders but modifies the stress distribution within the bed. The content of fines turns out to be a key factor in the determination of powder rheology as measured both with the shear cell and with the fluid-bed rheometer

The effect of temperature on the minimum fluidization conditions of industrial cohesive particles

In order to understand the factors responsible for changes in the fluidization behaviour of industrial particles at high temperatures, an experimental campaign was performed using a 140 × 1000 mm heated gas fluidized bed. Five powder cuts sieved out of the same mother powder covering Group B, A and C of Geldart's classification were investigated over a range of temperatures from ambient to 500 °C. The results show that the mean size distribution affects significantly the fluidization behaviour of the materials investigated. In particular, significant differences were observed in the fluidization behaviour of the coarsest samples (Group B-A) and finest samples (Group A-C). The minimum fluidization conditions were compared with the prediction of the Ergun equation. The comparison was satisfactory only when accounting for the experimental values of the bed voidage. In fact, the non-monotonic trend of the minim velocity for fluidization with increasing temperature cannot be explained only with the effects of temperature on the bed fluid dynamics. But several others are the observed effects on the fluidization behaviour due to the temperature rise that can be ascribed to the enhanced interparticle forces: 1) the increase of the peak of pressure drops, close to the minimum for fluidization, in the fluidization curve at increasing gas velocities; 2) the increase for the finest samples of the hysteresis in the fluidization curves, considering the fluidization and defluidization branches of the curve; 3) a greater tendency of the bed to expand homogeneously; 4) the increasing difference between the parameters of the Richardson-Zaki equation found with a fitting procedure on the experiments and those found using the Richardson-Zaki correlations and the theoretical terminal velocity. Furthermore, in the cases where larger interparticle forces were expected, the X-Ray facility allowed to identify different internal structures within the bed. Mostly vertical channels but also, in the case of the finest powder tested, horizontal channels

Detection and estimation of capillary interparticle forces in the material of a fluidized bed reactor at high temperature by powder flow characterization

Two ceramic powder samples having different compositions of surface impurities and particle size distributions were considered. These two samples resulted from a high temperature fluidized bed reactor which in its operation showed changes of working condition that might be attributed to the onset of strong interparticle forces. The flow behaviour of these powders was characterized by the High Temperature Annular Shear Cell (HT-ASC), between ambient temperature and 500 °C. Furthermore, a model is developed to relate the change of the powder flowability to the formation of a liquid phase due to the melting of particle impurities present on the particle surface. In particular, the model is used to predict, on the base of the salt composition, the intensity of the interparticle forces at different temperatures. The interparticle forces predicted by the model can be compared with those that can be inferred from the powder flow properties measured with the HT-ASC. Therefore, it is demonstrated that it is possible to derive a theoretical model to predict interparticle forces in a particulate material relevant to fluidized bed reactor, on the basis of the impurities composition. Furthermore, it is demonstrated the possibility to correctly estimate the intensity of average interparticle forces in the same kind of material by the interpretations of bulk flow properties measured with a shear tester, even in the case in which capillary forces take the place of the much weaker van der Walls forces. More in general, the paper suggests a method by which powder rheology can be used to indirectly evaluate the effects of the interparticle forces on fluidization processes even in case in which strong capillary interaction occur

Analysis of industrial reactive powders flow properties at high temperature

Changes of bulk flow properties of two different types of titanium dioxide powders were measured at room temperature and 500 °C using the High Temperature Annular Shear Cell. A significant increase of the macroscopic bulk flow properties was observed with increasing temperature, in particular with regard to the unconfined yield strength. A theoretical modelling procedure was proposed with the aim to relate the measured properties to the microscopic interactions between particles. The results indicated that the model might provide a good match with the experimental data if proper values for the model's parameters are taken into account