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Robust single electron pump

A novel quantum current source was realized with the aid of nanotechnology in which single electrons are transported with external voltage pulses. A special feature of this “electron pump” is the great robustness of the physical transport process for which the exact strength of the driving voltage pulse no longer plays a role. As the pump, furthermore, only needs a single driving voltage, for the first time it is possible to operate many pumps in parallel. The current could thus be raised to a level which is usable in practice.

With a 3 meter long probe (left of centre) the single electron pump is led into the vacuum system which is underground and in which the necessary operating temperature of 1 Kelvin is produced.

The pumping of single electrons is being investigated at PTB with respect to a future quantum current standard, that traces back the electrical current to the value of the elementary charge and to the frequency. However, such a type of quantum current standard need not only possess a low uncertainty. In order to have practical relevance, it also has to supply currents which at least lie in the nanoampere range. For this, single electrons would have to be pulsed considerably faster than they have been to date which, however, due to a fundamental relation, leads to a loss of controllability and thus to a higher uncertainty. The alternative strategy of operating many pumps in parallel for a larger current has, however, so far not been possible because of the high complexity and low reproducibility of the single pumps.

While looking for a suitable principle for parallelism, a novel quantum circuit has now been developed on the basis of a semiconductor structure. In the circuit, exactly one electron per pulse is moved with only a single periodic voltage signal. Hereby, the strength of the voltage pulse does not play a role for the correct operation, similar to a dispenser for artificial sweeteners: any amount above a minimum level of pressure on the button will release a tablet, independent of the total amount of pressure applied. This special feature, coupled with the simplicity of the circuit, ultimately allows many pumps to be produced with a common parameter range and thus to arrive at parallel operation.

Corresponding components are to be integrated in parallel onto a chip in the future to generate currents which are useful in practice. This might lead to a new definition and realisation of the ampere.

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