Technique for lowering the noise figure in RF amplifiers

Low-level RF amplifiers are operated at noise figures at or near the theoretical minimum, while maintaining high-gain stability. Formulas for the minimum amplifier noise figure value are derived from the analytical noise-figure expression involving the parameters of the equivalent circuit representing the actual system

Quantum information as a non-Kolmogorovian generalization of Shannon's theory

In this article we discuss the formal structure of a generalized information theory based on the extension of the probability calculus of Kolmogorov to a (possibly) non-commutative setting. By studying this framework, we argue that quantum information can be considered as a particular case of a huge family of non-commutative extensions of its classical counterpart. In any conceivable information theory, the possibility of dealing with different kinds of information measures plays a key role. Here, we generalize a notion of state spectrum, allowing us to introduce a majorization relation and a new family of generalized entropic measures

Approximate transformations of bipartite pure-state entanglement from the majorization lattice

We study the problem of deterministic transformations of an \textit{initial} pure entangled quantum state, $|\psi\rangle$, into a \textit{target} pure entangled quantum state, $|\phi\rangle$, by using \textit{local operations and classical communication} (LOCC). A celebrated result of Nielsen [Phys. Rev. Lett. \textbf{83}, 436 (1999)] gives the necessary and sufficient condition that makes this entanglement transformation process possible. Indeed, this process can be achieved if and only if the majorization relation $\psi \prec \phi$ holds, where $\psi$ and $\phi$ are probability vectors obtained by taking the squares of the Schmidt coefficients of the initial and target states, respectively. In general, this condition is not fulfilled. However, one can look for an \textit{approximate} entanglement transformation. Vidal \textit{et. al} [Phys. Rev. A \textbf{62}, 012304 (2000)] have proposed a deterministic transformation using LOCC in order to obtain a target state $|\chi^\mathrm{opt}\rangle$ most approximate to $|\phi\rangle$ in terms of maximal fidelity between them. Here, we show a strategy to deal with approximate entanglement transformations based on the properties of the \textit{majorization lattice}. More precisely, we propose as approximate target state one whose Schmidt coefficients are given by the supremum between $\psi$ and $\phi$. Our proposal is inspired on the observation that fidelity does not respect the majorization relation in general. Remarkably enough, we find that for some particular interesting cases, like two-qubit pure states or the entanglement concentration protocol, both proposals are coincident.Comment: Revised manuscript close to the accepted version in Physica A (10 pages, 1 figure

Loss of coherence and dressing in QED

The dynamics of a free charged particle, initially described by a coherent wave packet, interacting with an environment, i.e. the electromagnetic field characterized by a temperature $T$, is studied. Using the dipole approximation the exact expressions for the evolution of the reduced density matrix both in momentum and configuration space and the vacuum and the thermal contribution to decoherence, are obtained. The time behaviour of the coherence lengths in the two representations are given. Through the analysis of the dynamic of the field structure associated to the particle the vacuum contribution is shown to be linked to the birth of correlations between the single momentum components of the particle wave packet and the virtual photons of the dressing cloud

Distillation by repeated measurements: continuous spectrum case

Repeated measurements on a part of a bipartite system strongly affect the other part not measured, whose dynamics is regulated by an effective contracted evolution operator. When the spectrum of this operator is discrete, the latter system is driven into a pure state irrespective of the initial state, provided the spectrum satisfies certain conditions. We here show that even in the case of continuous spectrum an effective distillation can occur under rather general conditions. We confirm it by applying our formalism to a simple model.Comment: 4 pages, 2 figure

Dynamics of Entanglement and Bell-nonlocality for Two Stochastic Qubits with Dipole-Dipole Interaction

We have studied the analytical dynamics of Bell nonlocality as measured by CHSH inequality and entanglement as measured by concurrence for two noisy qubits that have dipole-dipole interaction. The nonlocal entanglement created by the dipole-dipole interaction is found to be protected from sudden death for certain initial states

Loss of coherence and dressing in QED

The dynamics of a free charged particle, initially described by a coherent wave packet, interacting with an environment, i.e., the electromagnetic field characterized by a temperature T, is studied. Using the dipole approximation, the exact expressions for the evolution of the reduced density matrix both in momentum and configuration space and the vacuum and the thermal contribution to decoherence are obtained. The time behavior of the coherence lengths in the two representations are given. Through the analysis of the dynamic of the field structure associated with the particle the vacuum contribution is shown to be linked to the birth of correlations between the single momentum components of the particle wave packet and the virtual photons of the dressing clou

Reconstruction of time-dependent coefficients: a check of approximation schemes for non-Markovian convolutionless dissipative generators

We propose a procedure to fully reconstruct the time-dependent coefficients of convolutionless non-Markovian dissipative generators via a finite number of experimental measurements. By combining a tomography based approach with a proper data sampling, our proposal allows to relate the time-dependent coefficients governing the dissipative evolution of a quantum system to experimentally accessible quantities. The proposed scheme not only provides a way to retrieve full information about potentially unknown dissipative coefficients but also, most valuably, can be employed as a reliable consistency test for the approximations involved in the theoretical derivation of a given non-Markovian convolutionless master equation.Comment: 11 pages, 4 figures, revised version published on PR

Geochemistry and mineralogy of travertine deposits of the SW flank of Mt. Etna (Italy): Relationships with past volcanic and degassing activity

Travertine deposits outcropping in the lower SW flank of Mt. Etna were studied for their mapping, as well as for their chemical, mineralogical and isotopic compositions. These deposits are dated to about 24 to 5 ka in the Adrano area, located at the western limit of the study area. In this area travertines show high Mg contents and are composed mostly of dolomite, thus apparently ruling out any primary deposition in favour of a diagenetic origin. Travertines outcropping near Paternò, in the east part of the study area, should be younger than 18 ka. Those located to the SSW of Paternò (Paternò–Diga) show high Sr contents and aragonite as dominant mineralogical phase, thus suggesting primary deposition. Those located to the Wof Paternò (Paternò Simeto–Stazione) are instead poor both in Mg and in Sr and show calcite as dominant phase. Carbon isotope composition of travertines indicates a magmatic origin of CO2 that formed them. Based on the estimated volume of travertines, between 10 and 20 Gg/a of CO2 were involved in their formation. The time-span of travertine formation coincided with the eruptive cycles of Ellittico and the first part of Mongibello, which were probably characterised by a greater amount of CO2 transported through groundwater circulation. Widespread travertine deposition probably ceased after the opening of the Valle del Bove depression that modified the volcanologic and hydrologic conditions in the summit crater area