  Aharonov-Bohm oscillations are studied in the magnetoconductance of a
micron-sized open quantum ring coupled capacitively to a Coulomb-blockaded
quantum dot. As the plunger gate of the dot is modulated and tuned through a
conductance resonance, the amplitude of the Aharonov-Bohm oscillations in the
transconductance of the ring displays a minimum. We demonstrate that the effect
is due to a single-electron screening effect, rather than to dephasing.
Aharonov-Bohm oscillations in a quantum ring can thus be used for the detection
of single charges.
