  In this thesis I find an analytic expression for the conductance of a single
electron transistor in the regime when temperature, level spacing, and charging
energy of an island are all of the same order. I also study the correction to
the spacing between Coulomb blockade peaks due to finite dot-lead tunnel
couplings. I find analytic expressions for both correction to the spacing
averaged over mesoscopic fluctuations and the rms of the correction
fluctuations.
  In the second part of the thesis I discuss the feasibility of quantum dot
based spin- and charge-qubits. Firstly, I study the effect of mesoscopic
fluctuations on the magnitude of errors that can occur in exchange operations
on quantum dot spin-qubits. Mid-size double quantum dots, with an odd number of
electrons in the range of a few tens in each dot, are investigated through the
constant interaction model using realistic parameters. It is found that the
number of independent parameters per dot that one should tune depends on the
configuration and ranges from one to four. Then, I study decoherence of a
quantum dot charge qubit due to coupling to piezoelectric acoustic phonons in
the Born-Markov approximation. After including appropriate form factors, I find
that phonon decoherence rates are one to two orders of magnitude weaker than
was previously predicted. My results suggest that mechanisms other than phonon
decoherence play a more significant role in current experimental setups.
