Observation of Quantum Fluctuations of Charge on a Quantum Dot

TL;DR

This paper reports the direct measurement of quantum charge fluctuations on a semiconductor quantum dot using an integrated single electron transistor, revealing both continuous and step-like charge behaviors.

Contribution

It demonstrates a novel integrated measurement technique that captures quantum charge fluctuations on a quantum dot across different conductance regimes.

Findings

Charge increases continuously at high conductance

Charge increases in steps at low conductance

Capacitance lineshapes match perturbation theory predictions

Abstract

We have incorporated an aluminum single electron transistor directly into the defining gate structure of a semiconductor quantum dot, permitting precise measurement of the charge in the dot. Voltage biasing a gate draws charge from a reservoir into the dot through a single point contact. The charge in the dot increases continuously for large point contact conductance and in a step-like manner in units of single electrons with the contact nearly closed. We measure the corresponding capacitance lineshapes for the full range of point contact conductances. The lineshapes are described well by perturbation theory and not by theories in which the dot charging energy is altered by the barrier conductance.