Coupled quantum dots as quantum gates

We consider a quantum-gate mechanism based on electron spins in coupled sem iconductor quantum dots. Such gates provide a general source of spin entang lement and can be used for quantum computers. We determine the exchange cou pling J in the effective Heisenberg model as a function of magnetic (B) and electric fields, and of the interdot distance a within the Heitler-London approximation of molecular physics. This result is refined by using sp hybr idization, and by the Hund-Mulliken molecular-orbit approach, which leads t o an extended Hubbard description for the two-dot system that shows a remar kable dependence on B and a due to the long-range Coulomb interaction. We f ind that the exchange J changes sign at a finite field (leading to a pronou nced jump in the magnetization) and then decays exponentially. The magnetiz ation and the spin susceptibilities of the coupled dots are calculated. We show that the dephasing due to nuclear spins in GaAs can be strongly suppre ssed by dynamical nuclear-spin polarization and/or by magnetic fields. [S01 63-1829(99)01003-6].