  Gate-defined quantum dots define an attractive platform for quantum
computation and have been used to confine individual charges in a planar array.
Here, we demonstrate control over vertical double quantum dots confined in a
double quantum well, silicon-germanium heterostructure. We sense individual
charge transitions with a single-hole transistor. The vertical separation
between the quantum wells provides a sufficient difference in capacitive
coupling to distinguish quantum dots located in the top and bottom quantum
well. Tuning the vertical double quantum dot to the (1,1) charge state confines
a single hole in each quantum well beneath a single plunger gate. By
simultaneously accumulating holes under two neighbouring plunger gates, we are
able to tune to the (1,1,1,1) charge state. These results motivate quantum dot
systems that exploit the third dimension, opening new opportunities for quantum
simulation and quantum computing.
