A novel mechanism for counterion-mediated attraction between like-charged spherical macroions is proposed, which originates from a depletion zone of counterions between nearly touching macroions that is induced by Coulomb interactions. Using computer simulations of the primitive model, we show that this depletion effect dominates over the electrostatic contribution in the case of strong Coulomb coupling when all the counterions form a quasi-two-dimensional layer on the spherical macroionic surface. Its range is given by the typical spacing of counterions on the macroion surface.[S0031-9007(98)06791-X]

A detailed analysis of the electro-optical response of single as well as coupled semiconductor quantum dots is presented. This is based on a realistic -i.e., fully tridimensional-description of Coulomb-correlated few-electron states, obtained via a direct-diagonalization approach. More specifically, we investigate the combined effect of static electric fields and ultrafast sequences of multicolor laser pulses in the few-carrier, i.e., low-excitation, regime. In particular, we show how the presence of a properly tailored static field may give rise to significant electron-hole charge separation; these field-induced dipoles, in turn, may introduce relevant exciton-exciton couplings, which are found to induce significant -both intra-and inter-dot-biexcitonic splittings. We finally show that such few-exciton systems constitute an ideal semiconductor-based hardware for an all optical implementation of quantum information processing. 73.21.La, 78.47.+p,

We present a solid-state implementation of ultrafast conditional quantum gates. Our proposal for a quantum-computing device is based on the spin degrees of freedom of electrons confined in semiconductor quantum dots, thus benefiting from relatively long decoherence times. More specifically, combining Pauli blocking effects with properly tailored ultrafast laser pulses, we are able to obtain sub-picosecond spin-dependent switching of the Coulomb interaction, which is the essence of our conditional phase-gate proposal. This allows us to realize a fast two qubit gate which does not translate into fast decoherence times and paves the road for an all-optical spin-based quantum computer. 03.67.Lx, 73.21.La, 71.35.Cc Typeset using REVT E X Recent advances in quantum-information science [1] have led to a number of schemes for implementing quantum information processing (QIP) devices. Several theoretical proposals have been made and admirable experimental progress has been made in quantum optics [2,3], NMR [4], and solid state proposals including Josephson junctions and quantum dots [5][6][7][8][9][10][11]. Quantum dot (QD) implementation schemes based on the electronic spin de-