1/N-Expansion for the Dicke model and the decoherence program

An analysis of the Dicke model, N two-level atoms interacting with a single radiation mode, is done using the Holstein-Primakoff transformation. The main aim of the paper is to show that, changing the quantization axis with respect to the common usage, it is possible to prove a general result either for N or the coupling constant going to infinity for the exact solution of the model. This completes the analysis, known in the current literature, with respect to the same model in the limit of N and volume going to infinity, keeping the density constant. For the latter the proper axis of quantization is given by the Hamiltonian of the two-level atoms and for the former the proper axis of quantization is defined by the interaction. The relevance of this result relies on the observation that a general measurement apparatus acts using electromagnetic interaction and so, one can state that the thermodynamic limit is enough to grant the appearance of classical effects. Indeed, recent experimental results give first evidence that superposition states disappear interacting with an electromagnetic field having a large number of photons.     

An adaptive filter for gravitational-wave antennas

Summary We report on the development of an adaptive optimum filter for processing the data of a resonant bar gravitational-wave detector. This filter, based on the matched-filter theory, is adaptive in the sense that the function it realizes is derived from the actual noise spectrum of the data being analysed (instead from an idealized model of the noise). Its implementation is mostly based on frequency domain techniques. We also report on the application of the new filter to the data of the cryogenic antenna Explorer of the Rome group, with particular reference to the comparison between its performance and that of an otpimum filter with fixed values of the parameters.     

The photoconversion R-CH3 → R-CHO in indazole derivatives

The UV irradiation of 1-(p-nitrophenyl)-3-methyl-indazole leads to the formation of the corresponding 3-formyl derivative as product of the photooxidation R-CH₃ → R-CHO. This method was applied to a series of substituted indazoles and the same reaction was observed with methyl groups attached to the pyrazole and to the benzene ring of the indazole.     

Chaos control by using Motor Maps

In this paper a new method for chaos control is proposed, consisting of an unsupervised neural network, namely a Motor Map. In particular a feedback entrainment scheme is adopted: a chaotic system with a given parameter set generates the reference trajectory for another chaotic system with different parameters to be controlled: the Motor Map is required to provide the appropriate time-varying gain value for the feedback signal. The state of the controlled system is considered as input to the Motor Map. Particular efforts have been paid to the feasibility of the implementation. Indeed, the simulations performed have been oriented to design a Motor Map suitable for an hardware realization, thus some restrictive hypotheses, such as for example a low number of neurons, have been assumed. A huge number of simulations has been carried out by considering as system to be controlled a Double Scroll Chua Attractor as well as other chaotic attractors. Several reference trajectories have also been considered: a limit cycle generated by a Chua's circuit with different parameters values, a double scroll Chua attractor, a chaotic attractor of the family of the Chua's circuit attractors. In all the simulations instead of controlling the whole state space, only two state variables have been fed back. Good results in terms of settling time (namely, the period in which the map learns the control task) and steady state errors have been obtained with a few neurons. The Motor Map based adaptive controller offers high performances, specially in the case when the reference trajectory is switched into another one. In this case, a specialization of the neurons constituting the Motor Map is observed: while a group of neurons learns the appropriate control law for a reference trajectory, another group specializes itself to control the system when the other trajectory is used as a reference. A discrete components electronic realization of the Motor Map is presented and experimental results confirming the simulation results are shown. (c) 2002 American Institute of Physics.     

A modern review of the two-level approximation

The paradigm of the two-level atom is revisited and its perturbative analysis is discussed in view of the principle of duality in perturbation theory. The models we consider are a two-level atom and an ensemble of two-level atoms both interacting with a single radiation mode. The aim is to see how the latter can be actually used as an amplifier of quantum fluctuations to the classical level through the thermodynamic limit of a very large ensemble of two-level atoms [M. Frasca, Phys. Lett. A 283 (2001) 271] and how can remove Schrödinger cat states. The thermodynamic limit can be very effective for producing both classical states and decoherence on a quantum system that evolves without dissipation. Decoherence without dissipation is indeed an effect of a single two-level atom interacting with an ensemble of two-level atoms, a situation that proves to be useful to understand recent experiments on nanoscale devices showing unexpected disappearance of quantum coherence at very low temperatures.     

A Chaotic Circuit with Ferroelectric Nonlinearity

In this paper, the possibility of observing strange attractors in an electronic circuit including a nonlinear ferroelectric component has been investigated. The ferroelectric constitutes the medium interposed between the two plates of a capacitor. A circuit including the nonlinearity of the ferroelectric has been designed, while the parameters have been found by performing numerical integration with respect to different values of them. The circuit has been realized on a discrete components board. Experimental results, showing that for a suitable range of parameters a chaotic attractor emerges, are reported.     

Analogical device for a rough localization of gravitational-wave sources

Summary The response of a gravitational cilindrical antenna depends, besides on the values of the intensity of the wave and the sensitivity of the antenna, on the angle between the direction of the source and the axis of the antenna and on the polarization angle of the wave. In this paper a device (gravitational astrolabe) that can roughly compute the angular part of the response is presented. It gives the zone of the sky toward which a given antenna is “directed” at a certain time, the angle of polarization that is best “received” and solves easily a number of similar problems.

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Riassunto La risposta di un'antenna gravitazionale cilindrica dipende, oltre che dall'intensità dell'onda e dalla sensibilità dell'antenna, dall'angolo tra la direzione della sorgente e l'asse dell'antenna e dall'angolo di polarizzazione dell'onda. In quest'articolo si presenta uno strumento (astrolabio gravitazionale) che può calcolare approssimativamente la parte angolare della risposta. Con esso si possono facilmente calcolare la zona del cielo verso cui è “diretta” una data antenna a un dato istante, l'angolo di polarizzazione che è meglio “ricevuto” e si possono risolvere molti simili problemi.

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Резюме Отклик гравитационной цилиндрической антенны зависит, помимо зиачений интенсивности волны и чувствиельности антенны, от угла между направлением на источник и осью антенны и от угла поляризации волны. В этой статье предлагается прибор (гравитационная астролябия), который позволяет грубо определить угловую часть отклика. Результат задает зону неба, по направлению которой ?направлена? антенна в определенное время, определяется угол поляризации и решается ряд аналогичных проблем.

     

Fully reversible guest exchange in tetraphosphonate cavitand complexes probed by fluorescence spectroscopy

We report here the monitoring of reversible guest inclusion in phosphonate cavitands through a large increase in luminescence intensity caused by the modulation of the exoergonicity of an electron-transfer reaction.