Single-electron transistor as a radio-frequency mixer

We demonstrate the use of the single-electron transistor as a radio-frequency mixer, based on the nonlinear dependence of current on gate charge. This mixer can be used for high-frequency, ultrasensitive charge measurements over a broad and tunable range of frequencies. We demonstrate operation of the mixer, using a lithographically defined thin-film aluminum transistor, in both the superconducting and normal states of aluminum, over frequencies from 10 to 300 MHz. We have operated the device both as a homodyne detector and as a phase-sensitive heterodyne mixer. We demonstrate a charge sensitivity of ${\text{<4×10}}^{\text{−3}}\mathrm{\hspace{0.17em}e/}\sqrt{\mathrm{Hz}},$ limited by room-temperature electronics. An optimized mixer has a theoretical charge sensitivity of ${\text{≲1.5×10}}^{\text{−5}}\mathrm{\hspace{0.17em}e/}\sqrt{\mathrm{Hz}}.$