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PTB's own world record topped!

Current measurement with single-electron pumps with even smaller uncertainty

PTB-News 2.2017
02.05.2017
Especially interesting for

metrology institutes

fundamental research

PTB has topped its own world record established a year ago where electric currents were generated with singleelectron pumps and their quantization was verified with the greatest accuracy by means of a special measuring amplifier. The new record (which was made possible by improving the experimental setup) is a relative measurement uncertainty of only 0.16 μA/A. Moreover, this result was achieved within a considerably shorter measuring time.

Relative deviation of the current generated by the single-electron pump from the quantized value I = e · f. Performing measurements while varying the control voltage (blue symbols) serves to determine the optimal operating point at which the following 21 measurements will be carried out (red symbols). The statistical uncertainty of each of the measurement values (each determined in a measuring time of one hour) corresponds to 0.6 μA/A. The insert shows the histogram of the measuring points, together with the corresponding normal distribution. From this, together with the systematic uncertainty contributions, it is possible to calculate the total measurement result at the operating point by averaging: (I/e · f) – 1 = ( 0.10 ± 0.16) μA/A.

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.

Contact

Frank Hohls
Department 2.5 Semiconductor Physics and Magnetism
Phone: +49 (0)531 592-2530
Opens window for sending emailfrank.hohls(at)ptb.de

Scientific publication

F. Stein, H. Scherer, T. Gerster, R. Behr, M. Götz, E. Pesel, C. Leicht, N. Ubbelohde, T. Weimann, K. Pierz, H. W. Schumacher, F. Hohls: Robustness of single-electron pumps at subppm current accuracy level. Metrologia 54, 1–8 (2017)