We have measured single-electron tunneling transistors with superconducting islands and conclude that they may be used as ultrasensitive detectors of microwave radiation for frequencies greater than or equal to 80 GHz. These devices contain a small superconducting Al island that is weakly coupled to a bias circuit through two small-capacitance tunnel junctions and a capacitive gate. At low bias voltages and temperatures, a single quasiparticle may only be introduced to the island through photon-assisted tunneling, Once this occurs, the quasiparticle is trapped on the island for similar to 1 mu s because it takes a relatively long time for this specific quasiparticle to tunnel off. While it is trapped, charge is transported through the system two electrons at a time, Since the photon-assisted transition merely switches the detector current on, this device is not limited to one electron tunneled through the system per absorbed photon. Measurements indicate that at least 100 electrons can tunnel for every absorbed photon, which corresponds to a noise-equivalent power of 3 x 10(-20) W/root Hz at 80 GHz if the current is measured with a commercial current amplifier.
