Radio frequency operation of a quantum point contact charge detector

Quantum point contact (QPC) charge detectors are sensitive electrometers, and their ease of fabrication and integration into semiconductor-based qubit systems makes them an attractive candidate as a readout device for spin or charge based qubits in quantum dots. Nevertheless, QPC performance to date has been limited by relatively low operational speeds and 1/f noise. Here we report the operation of a QPC charge sensor realized in an GaAs/AlGaAs two dimensional electron gas at radio- frequencies (RF-QPC), in a mode analogous to rf operation of the single electron transistor [1]. For a typical QPC detector coupled to a quantum dot (QD), a charge oscillation of one electron in the QD corresponds to a change in the QPC conductance of 1-3 percent. We simulate these operating conditions by applying a small ac voltage to the QPC gate to cause a similar change in the zero bias QPC conductance. When operated this way the signal to noise ratio of the RF-QPC is about 30dB, which corresponds to a charge sensitivity of about $7x10^{-4}e/\sqrt{Hz}$ referred to the dot charge. The operational characteristics of the RF-QPC at 4.2K also will be discussed. [1] R. J. Schoelkopf et al., Science \b {280}, 1238–1242 (1998).