Single-photon counting at microwave frequencies around 500 GHz is demonstrated by using a single-electron transistor (SET) formed by two capacitively coupled GaAs/AlxGa1−xAs parallel quantum dots (QDs). A point contact separating the double QDs allows the prompt escape of an excited electron from one of the QDs to another. The resulting long-lived photoinduced ionization of the QD is detected as a change in the SET current.

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