Observation of Quantum Fluctuations of Charge on a Quantum Dot

We have incorporated an aluminum single electron transistor directly into the defining gate structure of a semiconductor quantum dot, permitting precise measurement of the dot charge. Voltage biasing a gate draws charge from a reservoir into the dot through a single point contact. The dot charge increases continuously for large point contact conductance and in single electron steps with the contact nearly closed, and we measure the corresponding capacitance line shapes. The line shapes are not typical of lifetime or thermal broadening but fit well to predictions of perturbation theory.