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(a) Scanning electron micrograph of a device as used in the experiment (gates which are grounded are hidden). Gates , , and define the quantum dot (dotted circle), and gates and form the QPC. Gate is connected to a pulse sourcevia a coaxial cable (see Ref. 8 for a more detailed description). (b) Schematic of the experimental setup, including the most relevant noise sources. The QPC is represented by a resistor, . (c) Noise spectra measured when the convertor is connected to the sample (top solid trace), and, for reference, to an open-ended twisted pair of wires (lower solid trace). The latter represents a load, if we include the measured amplifier input capacitance. The diagram also shows the calculated noise level for the reference load, neglecting (dotted–dashed), and the shot noise limit (dashed). The left and right axes express the noise in terms of current through the QPC and electron charge on the dot respectively.
(Color online) Measured changes in the QPC current, , with the electrochemical potential in the dot and in the reservoir nearly equal. is “high” and “low” for 0 and 1 electrons on the dot respectively (; the steps in are ). Traces are offset for clarity. (a) The dot potential is lowered from top to bottom. (b) The tunnel barrier is lowered from top to bottom.
(a) Measured changes in the QPC current, , when a pulse is applied to gate , near the degeneracy point between 0 and 1 electrons on the dot . (b) Average of 286 traces as in (a). The top and bottom panel are taken with a different setting of gate . The damped oscillation following the pulse edges is due to the eighth-order filter.
Contributions to the noise current at the convertor input. By dividing the noise current by (the signal corresponding to one electron charge leaving the dot), we obtain the rms charge noise on the dot.
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