PTB's own world record topped!

Nanostructured electric circuits which allow the controlled transport of single electrons (so-called “single-electron pumps”) can be used to realize a future quantum current standard and are therefore the object of intense investigations at PTB. These circuits generate electric currents on the order of 100 pA that are measured with great accuracy by current/ voltage transformers (so-called ULCAs – short for Ultrastable Low-noise Current Amplifiers – see PTB News 1/2015) which were developed at PTB specifically for this purpose. The ULCA's transresistance is calibrated by means of a quantized Hall resistor with a relative measurement uncertainty of less than 0.1 μΩ/Ω.

Thanks to a new experiment, it was possible to further improve the measurement uncertainty and, thus, to break the previous year's world record (see PTB News 1/2016). This was made possible by an enhanced measuring arrangement where the input and output currents of the single- electron pump are transformed into voltages via two ULCA channels; the difference between those voltages was then measured using a quantum standard (a Josephson voltage standard). In this way, the influence of the systematic uncertainty in the voltage measurement was significantly reduced. In addition, the measuring time was exploited more efficiently due to optimizing the computeraided data acquisition.

The new measurement uncertainty record value of 0.16 μA/A at a current of 96 pA was attained in a measuring time of 21 hours. In comparison, 95 hours were necessary to achieve the previous record value of 0.2 μA/A. In both cases, the current generated by the single-electron pump was in agreement with the expected quantized value I = e · f within the limits of the measurement uncertainty.

PTB has thus successfully demonstrated that the “quantum ampere” can be realized with a smaller measurement uncertainty than the conventional ampere in the current SI, which represents a milestone on the path to the planned revision of the International System of Units. In addition, the experimental improvements allow single-electron pumps to be characterized much faster – a crucial practical advantage for metrological applications.