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The ampere


In the new SI, the base unit the ampere is directly realized with quantum current sources which generate a single-electron current I at a frequency f : I = ef. PTB is developing a so-called self-referenced quantum current source.

Prototype of a self-referenced quantum current source developed at PTB with four semiconductor single-electron current sources (“single-electron pumps“) connected in series and three metallic singleelectron detectors.

This integrated quantum circuit consists of semiconductor single-electron current sources which are connected in series, and metallic single-electron detectors of as high a bandwidth as possible. The detectors are used to determine the errors of the single-electron transport which are unavoidable due to the stochastic character of the transport mechanism. If these errors are taken into account in the determination of the current, a smaller uncertainty can be achieved with the series connection than with an individual single-electron current source. The aim of the work is the direct realization of the new ampere by generating a quantized current in the range of 100 pA with a relative uncertainty which is smaller than the uncertainty of the ampere’s realization in today’s SI (i.e. approx. 10–7).

For this purpose, the uncertainty of the individual single-electron current sources must lie in the range of 10–6. This requirement is met, as has recently been shown by a direct measurement of the quantized current with a newly developed, highly sensitive current amplifier. In addition, the bandwidth of the single-electron detectors must be increased to approx. 100 kHz. The so-called RF-SET detection method required for this purpose, in which a single-electron transistor is integrated into an oscilFalating circuit, is currently being established at PTB. Initial RF-SET measurements have already been demonstrated. The improved determination of the quantized current has also been demonstrated (taking the measured errors into account) with an integrated quantum circuit composed of three current sources and two single-electron detectors. The detectors had, however, a small bandwidth and the circuit was operated at a frequency of 30 Hz. To achieve the above-mentioned goal, five single-electron current sources and four RF-SET detectors must be integrated in one circuit. This places considerable demands on the reproducibility of the nano-technology used for circuit manufacturing. Intensive work is being performed on the optimization of the manufacturing technology.