We present time-domain electron counting studies on charge fluctuations in a mesoscopic system. In this measurement, a radio-frequency single-electron transistor (RF-SET) acting as a fast electrometer is capacitively coupled to a quantum dot (QD) electrostatically defined in a GaAs/AlGaAs heterostructure. Random telegraph signals (RTSs) were observed on the RF-SET and were interpreted as resulting from individual electrons tunneling into/out of the QD hence switching the QD charge state between N and N+1 electrons near the charge degeneracy point. Periodic behavior of the switching events is observed as the number of average electrons in the electron box is decreased one by one. The occupation time of the excess electron is directly measured and changes from a few microseconds to a few milliseconds as the tunneling resistance of the QD is increased. Histogram of the occupation time obtained from the time-domain electron counting measurements agrees well with the frequency-domain power spectrum analysis, both suggesting a Poissonian process at the charge degenerate point, in agreement with a model consisting of two charge states. Real time electron counting techniques can play a powerful role in studies on temporal electronic correlations, as well as quantum information processing schemes. Techniques that will improve the charge sensitivity of the RF-SET will also be discussed.
By coupling a radio-frequency single-electron transistor (RF-SET) to a quantum dot (QD) in a GaAs/AlGaAs heterostructure, we have succeeded in detecting the tunneling of individual electrons on and off the QD on time scales as short as one microsecond. Using charge detection to probe the state of the QD allows us to nearly isolate the dot from its leads, thereby minimizing decoherence-inducing effects of the environment. We have extended these charge detection techniques to double quantum dots (DQDs) that can simultaneously be used to characterize the backaction of the RF-SET. The combined RF-SET/DQD system is well-suited to the development of charge- or spin-based quantum bits, and to investigation of the quantum measurement problem.