724 research outputs found
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
The role of surface chemical reactivity in the stability of electronic nanodevices based on two-dimensional materials "beyond graphene" and topological insulators
Here, we examine the influence of surface chemical reactivity toward ambient
gases on the performance of nanodevices based on two-dimensional materials
"beyond graphene" and novel topological phases of matter. While surface
oxidation in ambient conditions was observed for silicene and phosphorene with
subsequent reduction of the mobility of charge carriers, nanodevices with
active channels of indium selenide, bismuth chalcogenides and transition-metal
dichalcogenides are stable in air. However, air-exposed indium selenide suffers
of p-type doping due to water decomposition on Se vacancies, whereas the low
mobility of charge carriers in transition-metal dichalcogenides increases the
response time of nanodevices. Conversely, bismuth chalcogenides require a
control of crystalline quality, which could represent a serious hurdle for up
scaling
Resonant effects in a SQUID qubit subjected to non adiabatic changes
By quickly modifying the shape of the effective potential of a double SQUID
flux qubit from a single-well to a double-well condition, we experimentally
observe an anomalous behavior, namely an alternance of resonance peaks, in the
probability to find the qubit in a given flux state. The occurrence of
Landau-Zener transitions as well as resonant tunneling between degenerate
levels in the two wells may be invoked to partially justify the experimental
results. A quantum simulation of the time evolution of the system indeed
suggests that the observed anomalous behavior can be imputable to quantum
coherence effects. The interplay among all these mechanisms has a practical
implication for quantum computing purposes, giving a direct measurement of the
limits on the sweeping rates possible for a correct manipulation of the qubit
state by means of fast flux pulses, avoiding transitions to non-computational
states.Comment: 6 pages and 6 figures. The paper, as it is, has been accepted for
publication on PRB on March 201
Fujita modified exponent for scale invariant damped semilinear wave equations
The aim of this paper is to prove a blow-up result of the solution for a semilinear scale invariant damped wave equation under a suitable decay condition on radial initial data. The admissible range for the power of the nonlinear term depends both on the damping coefficient and on the pointwise decay order of the initial data. In addition, we give an upper bound estimate for the lifespan of the solution. It depends not only on the exponent of the nonlinear term and not only on the damping coefficient but also on the size of the decay rate of the initial data
Low temperature/uv-assisted composites as gas sensors for medical applications
The sensing of gas molecules is of fundamental importance for environmental monitoring, control of chemical processes, medical applications, and so on [1]. Furthermore, recent success in non-invasive medical diagnostics, based on human breath analysis, is pushing forward the development of extremely sensitive gas sensors for ppb detection of specific analytes (e.g. acetone) in a complex gas mixture [1,2]. In recent years, graphene-based gas sensors have attracted much attention and different structures have been developed showing high sensing performances and room temperature working conditions [2]. However, they still suffer from several problems, which could be overcome by covering the graphene surface with metal oxide semiconductors. Furthermore, studies regarding the detection of Volatile Organic Compounds (VOCs) are still at the beginning [3].
Hence, the present work will be aimed at: i) optimizing the synthetic routes of ad hoc composite VOCs sensing materials (based on graphene oxide/SnO2 hybrids); ii) engineering the gas sensor device; and iii) evaluating the sensing performances at both high and mild temperatures (also exploiting the UV light) towards gaseous ethanol, acetone and ethylbenzene. Starting from pure graphite, graphene oxide (GO) powder was synthesized by adopting the Hummer\u2019s modified method, in which the synthetic route was deeply investigated, and several parameters (such as H2O2 concentration) were modulated. Once optimized this step, SnO2 were grown on its surface by hydrothermal method, varying the starting salt precursor/GO weight ratio between 4 and 32. For comparison, pure commercial and home-made SnO2 were also tested. Several physico-chemical analyses were performed to characterize all the as-prepared nanopowders. Subsequently, a homogeneous film was deposited by spraying technique onto Pt-Interdigitated Electrodes (Pt-IDEs). Then, gaseous ethanol (Figure 1) and acetone were sensed, obtaining very promising results for both pure and hybrid materials at 350\ub0C, and at lower temperatures (150\ub0C to 30\ub0C, by exploiting the UV light) for the graphene-based samples
Superconducting tunable flux qubit with direct readout scheme
We describe a simple and efficient scheme for the readout of a tunable flux
qubit, and present preliminary experimental tests for the preparation,
manipulation and final readout of the qubit state, performed in incoherent
regime at liquid Helium temperature. The tunable flux qubit is realized by a
double SQUID with an extra Josephson junction inserted in the large
superconducting loop, and the readout is performed by applying a current ramp
to the junction and recording the value for which there is a voltage response,
depending on the qubit state. This preliminary work indicates the feasibility
and efficiency of the scheme.Comment: 10 pages, 5 figure
Insight on Glucose and Fructose Absorption and Relevance in the Enterocyte Milieu
Although epidemiological studies indicate a strong correlation between high sugar intake and metabolic diseases, the biological mechanisms underlying this link are still controversial. To further examine the modification and crosstalk occurring in enterocyte metabolism during sugar absorption, in this study we evaluate the diffusion and intestinal metabolism of glucose, fructose and sucrose, which were supplemented in equimolar concentration to Caco-2 cells grown on polyester membrane inserts. At different time points after supplementation, changes in metabolite concentration were evaluated in the apical and basolateral chambers by nuclear magnetic resonance (NMR) and gas-chromatography (GC). Sucrose was only minimally hydrolyzed by Caco-2 cells. Upon supplementation, we observed a faster uptake of fructose than glucose, the pentose sugar being also faster catabolized. Monosaccharide absorption was concomitant to the synthesis/transport of other metabolites, which occurred differently in glucose and fructose supplemented cells. Our results confirm the prominent role of intestinal cells in fructose metabolism and clearance after absorption, representing a further step forward in the understanding of the role of dietary sugars. Future research, including targeted analysis on specific transporters/enzymes and the use of labeled substrates, will be helpful to confirm the present results and their interpretation
Static flux bias of a flux qubit using persistent current trapping
Qubits based on the magnetic flux degree of freedom require a flux bias,
whose stability and precision strongly affect the qubit performance, up to a
point of forbidding the qubit operation. Moreover, in the perspective of
multiqubit systems, it must be possible to flux-bias each qubit independently,
hence avoiding the traditional use of externally generated magnetic fields in
favour of on-chip techniques that minimize cross-couplings. The solution
discussed in this paper exploits a persistent current, trapped in a
superconducting circuit integrated on chip that can be inductively coupled with
an individual qubit. The circuit does not make use of resistive elements that
can be detrimental for the qubit coherence. The trapping procedure allows to
control and change stepwise the amount of stored current; after that, the
circuit can be completely disconnected from the external sources. We show in a
practical case how this works and how to drive the bias circuit at the required
value.Comment: 5 figures submitted to Superconductor Science and Technolog
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