  We report on the realization of a few-electron double quantum dot defined in
a two-dimensional electron gas by means of surface gates on top of a
GaAs/AlGaAs heterostructure. Two quantum point contacts (QPCs) are placed in
the vicinity of the double quantum dot and serve as charge detectors. These
enable determination of the number of conduction electrons on each dot. This
number can be reduced to zero while still allowing transport measurements
through the double dot. Microwave radiation is used to pump an electron from
one dot to the other by absorption of a single photon. The experiments
demonstrate that this quantum dot circuit can serve as a good starting point
for a scalable spin-qubit system.
