  Measuring single-electron charge is one of the most fundamental quantum
technologies. Charge sensing, which is an ingredient for the measurement of
single spins or single photons, has been already developed for semiconductor
gate-defined quantum dots, leading to intensive studies on the physics and the
applications of single-electron charge, single-electron spin and
photon-electron quantum interface. However, the technology has not yet been
realized for self-assembled quantum dots despite their fascinating quantum
transport phenomena and outstanding optical functionalities. In this paper, we
report charge sensing experiments in self-assembled quantum dots. We choose two
adjacent dots, and fabricate source and drain electrodes on each dot, in which
either dot works as a charge sensor for the other target dot. The sensor dot
current significantly changes when the number of electrons in the target dot
changes by one, demonstrating single-electron charge sensing. We have also
demonstrated real-time detection of single-electron tunnelling events. This
charge sensing technique will be an important step towards combining efficient
electrical readout of single-electron with intriguing quantum transport physics
or advanced optical and photonic technologies developed for self-assembled
quantum dots.
