Quantum rings as phase coherent detectors

Aharonov–Bohm oscillations are studied in the magnetoconductance through two micron-sized quantum rings. The structures are defined in a two-dimensional electron gas of a Ga[Al]As heterostructure by way of local oxidation with an atomic force microscope. In the experiments, the rings are used as phase-coherent detectors of the charge state of quantum dots coupled to the rings in two specific arrangements. In the first case, two quantum dots are induced in each of the arms of an open four-terminal ring geometry. This allows to measure the evolution of the relative transmission phase when the number of electrons in each of the dots is tuned using appropriate gate voltages. The experimental findings are compared to expectations from single-particle theory and deviations are discussed. In the second case, a quantum dot is coupled capacitively to one arm of a ring. The amplitude of the Aharonov–Bohm oscillations in the transconductance depends strongly on the charge state of the quantum dot. It is demonstrated that the effect is due to a single-electron screening effect. This shows that Aharonov–Bohm oscillations in a quantum ring can be used for the detection of single electronic charges.