  Excitonic transitions offer a possible route to ultrafast optical spin
manipulation in coupled nanostructures. We perform here a detailed study of the
three principal exciton-mediated decoherence channels for optically-controlled
electron spin qubits in coupled quantum dots: radiative decay of the excitonic
state, exciton-phonon interactions, and Landau-Zener transitions between
laser-dressed states. We consider a scheme to produce an entangling
controlled-phase gate on a pair of coupled spins which, in its simplest dynamic
form, renders the system subject to fast decoherence rates associated with
exciton creation during the gating operation. In contrast, we show that an
adiabatic approach employing off-resonant laser excitation allows us to
suppress all sources of decoherence simultaneously, significantly increasing
the fidelity of operations at only a relatively small gating time cost. We find
that controlled-phase gates accurate to one part in 10^2 can realistically be
achieved with the adiabatic approach, whereas the conventional dynamic approach
does not appear to support a fidelity suitable for scalable quantum
computation. Our predictions could be demonstrated experimentally in the near
future.
