It is shown that the universal set of quantum logic gates can be realized using solid-state quantum bits based on coherent electron transport in quantum wires. The elementary quantum bits are realized with a proper design of two quantum wires coupled through a potential barrier. Numerical simulations show that (a) a proper design of the coupling barrier allows one to realize any one-qbit rotation and (b) Coulomb interaction between two qbits of this kind allows the implementation of the CNOT gate. These systems are based on a mature technology and seem to be integrable with conventional electronics.

This paper reports an experimental and theoretical analysis of the diffusivity of electrons in Si as function of temperature, field strength, and field direction. Results for the longitudinal diffusion coefficient have been obtained experimentally for fields applied along (111) and (100) directions with time-of-flight and noise measurements. Calculations have been performed with the Monte Carlo procedure. The theoretical analysis, which includes an extensive discussion of the intervalley diffusion process, has yielded a revised version of the silicon model which correctly interprets both the new diffusion data and other well-established electron transport properties. The revision of th~ model is mainly concerned with the relative weights of/ and g intervalley scattering mechanisms. In fact the interpretation of the anisotropy of the diffusion allows separate estimates of the two types of scattering through their different effects on the intervalley diffusion which comes about when electrons have different drift velocities in different valleys.

A new Monte Carlo method is presented for the evaluation of the density matrix from the solution of the Liouvillevon Neumann equation for an ensemble of noninteracting electrons in a semiconductor crystal. The method is applied to the study of the electron transient response to a high external electric field in Si and to the relaxation of photoexcited electrons in GaAs in absence of external electric fields. The phonon population is always assumed at equilibrium, but no assumptions are made about the strength of the electron-phonon interaction. Results show that typical quantum features such as energy-nonconserving transitions, intracollisional field effect, and multiple collisions change the very first transient of the system with respect to a semiclassical description.

A solid-state implementation of a universal set of gates for quantum computation is proposed and analysed using a time-dependent 2D SchroÈ dinger solver. The qubit is de®ned as the state of an electron propagating along a couple of quantum wires. The wires are suitably coupled through a potential barrier with variable height and/or width. It is shown how a proper design of the system allows the implementation of any one-qubit transformation. The two-qubit gate is realized through a Coulomb coupler able to entangle the quantum states of two electrons running in two wires of two di erent qubits. The simulated devices are GaAs±AlGaAs heterostructures that should be on the borderline of present semiconductor technology. An estimate of decoherence e ects due to phonon scattering is also presented.

The calculation of free-energy landscapes in proteins is a challenge for modern numerical simulations. As to the case of potassium ion channels is concerned, it is particularly interesting because of the nanometric dimensions of the selectivity filter, where the complex electrostatics is highly relevant. The present study aims at comparing three different techniques used to bias molecular dynamics simulations, namely Umbrella Sampling, Steered Molecular Dynamics, and Metadynamics, never applied all together in the past to the same channel protein. Our test case is represented by potassium ions permeating the selectivity filter of the KcsA channel.