  We demonstrate high speed manipulation of a few-electron double quantum dot.
In the one-electron regime, the double dot forms a charge qubit. Microwaves are
used drive transitions between the (1,0) and (0,1) charge states of the double
dot. A local quantum point contact charge detector measures the photon-induced
change in occupancy of the charge states. Charge detection is used to measure
T1~16 ns and also provides a lower bound estimate for T2* of 400 ps for the
charge qubit. In the two-electron regime we use pulsed-gate techniques to
measure the singlet-triplet relaxation time for nearly-degenerate spin states.
These experiments demonstrate that the hyperfine interaction leads to fast spin
relaxation at low magnetic fields. Finally, we discuss how two-electron spin
states can be used to form a logical spin qubit.
