Luca Bonci

Simulation of time evolution of clocked and nonclocked quantum cellular automaton circuits

We have studied, by means of a Monte Carlo simulation, data propagation in quantum cellular automaton circuits, comparing the performance of nonclocked and clocked implementations. Based on two choices of fabrication parameters, we have evaluated the maximum achievable clock speed for different operating temperatures, concluding that better performance can be obtained with the clocked architecture, which, however, involves significant technological difficulties. The large discrepancy existing between a simple estimate of the cell switching time, based on the RC time constant, and our results is discussed and explained by means of an intuitive argument. Two different circuits have been investigated, a binary wire and a majority voting gate, obtaining analogous results, with a slightly smaller maximum clock rate for the majority voting gate, due to the larger number of cells involved.

Analysis of power dissipation in clocked Quantum Cellular Automaton circuits

We investigate power dissipation in quantum cellular automaton (QCA) circuits, focusing on the clocked version, and including also the contribution from the nonunitary efficiency of the charging process. The dissipated energy is computed from an integration on the V - Q plane over a complete clock cycle. Results show that power dissipation in a QCA cell, also including the efficiency of the refrigerator needed to cool down the QCA circuit to the operating temperature, appears to be a few orders of magnitude smaller than those typical of traditional technologies. A fairer comparison, however, must take into account also the number of devices that are needed to accomplish a given functionality and the possibility of operating traditional devices with power saving adiabatic schemes

Brownian motion generated by a two-dimensional mapping

Abstract A completely deterministic derivation of the Fokker-Planck equation based on a simple 2D map is illustrated. Both friction and diffusion are derived from the properties of the chaotic booster, i.e. the map, with no ad hoc assumptions. Diffusion is shown to depend on the fact that chaos implies a sensitive dependence on initial conditions. More remarkably, friction is derived from a linear response approach, distinct from the conventional one by Kubo and taking advantage of the stability properties of the invariant distribution.

Monte-Carlo Simulation of Clocked and Non-Clocked QCA Architectures

We present a Monte Carlo simulation of two implementations of Quantum Cellular Automaton (QCA) circuits: one based on simple ground state relaxation and the other on the clocked cell scheme that has recently been proposed by Tóth and Lent. We focus on the time-dependent behavior of two basic circuits, a binary wire and a majority voting gate, and assess their maximum operating speed and temperature requirements for different sets of fabrication parameters.

Numerical investigation of energy dissipation in Quantum Cellular Automaton circuits

We present a numerical study of energy dissipation in the switching process of Quantum Cellular Automaton (QCA) circuits, for both the non clocked and the clocked case. As a reference circuit, we consider binary wires of different lengths, investigating the relationship between the energy delivered by the external voltage sources and the switching speed, the number of cells, and the maximum operating temperature. Finally, we compare the achieved performance with that of different technological approaches.

Analysis of shot-noise suppression in disordered quantum wires

We present a numerical model for shot-noise suppression in a semiconductor quantum wire, based on parameters obtained from a purposely fabricated and characterized device. Shot-noise suppression is studied as a function of the voltage applied to the depletion gates forming the wire in a GaAs/AlGaAs heterostructure, of the dopant concentration in the ! -doping layer, and of the length of the wire. Results provide an understanding of why a conclusive experimental demonstration of di!usive transport with 1 suppression of shot noise in mesoscopic semiconductor devices has so far proved to be elusive.

SPONTANEOUS LOCALIZATION, ENVIRONMENT-INDUCED DECOHERENCE AND INDIVIDUAL-SYSTEM OBSERVATIONS

Abstract We show that the modified version of quantum mechanics proposed by Ghirardi et al. [Phys. Rev. A 40 (1990) 78] is virtually equivalent to ordinary quantum mechanics, if a statistical perspective is adopted. However, this does not conflict with the possibility of an experimental assessment via individual-system observations.

Numerical Study of Weak Localization Effects in Disordered Cavities

We present a study of magnetoconductance through a cavity obtained by laterally shifting a section of a Si-Ge quantum wire, focusing on how randomly distributed impurities may affect its conductance behavior as a function of magnetic field.

Superdiffusive Behavior in Weakly Chaotic Systems: A New Avenue to Quantum Macroscopic Manifestations

We study the anomalous diffusion resulting from the standard map. The quantum mechanical breakdown of this property occurs in a time depending logarithmically on the inverse of the Planck constant and implies a coherence among so small regions of phase-space as to be robust against environment fluctuations.

Quantum Effects in the Nonlinear Nonadiabatic Dimer

Kenkre and Wu1 have studied the nonadiabatic effects on the nonlinear dimer, assuming a finite relaxation time for the vibrations.