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World record set with single-electron pump

Demonstration of the quantum ampere with previously unattained accuracy

PTB News 1.2016
Especially interesting for
  • metrology institutes
  • fundamental research

Single-electron pumps are promising candidates for the realization of a future quantum current standard, and are the subject of intensive metrological research. By means of a novel, high-precision current amplifier, PTB researchers have succeeded in proving the quantization of a current of 100 pA supplied by a single-electron pump with an accuracy which had previously been unattained: within the relative uncertainty of only 2 ⋅ 10–7, the electric current agreed with the expected quantized value. This means a measurement uncertainty which is smaller than all results previously attained by around a factor of five and is thus for the first time better than the best possible ampere realization in the currently valid International System of Units (SI).

Operating principle of the single-electron pump investigated: single electrons, depicted as red spheres, are captured while moving from one side of a conductor (in the figure, from the left-hand side) by a “dynamic quantum well” (between the “hills” of the potential landscape depicted here), and released toward the other side. The entire procedure is repeated at the clock frequency of the AC voltage, which raises and lowers the left-hand barrier of the quantum well.

Precision current measurement at a single-electron pump (f = 545 MHz, corresponding tof = e · f ≈ 87 pA) as a function of the voltage at a control gate, depicted as a relative deviation from the nominal value. The error bars at the points (which were measured with different integration times) correspond to statistical standard measurement uncertainties. Also shown is the progression of a fitted theory line.

Single-electron pumps are nanostructured electrical circuits which allow the clocked transport of individual charge quanta (electrons with elementary charge e). At PTB, such circuits are manufactured on the basis of semiconductor heterostructures and investigated with a view to future metrological applications. For clock rates ƒ which extend into the gigahertz range, quantized electric currents I of more than one hundred picoamperes can be generated in accordance with I = eƒ. This makes singleelectron pumps promising candidates for quantum current sources, by means of which the unit of current could be realized in future after the envisaged redefinition of the ampere (see also Opens internal link in current windowPTB News 2.2014).

PTB researchers used a high-precision current amplifier developed by PTB itself (ultrastable low-noise current amplifier, see also Opens internal link in current windowPTB News 1.2015) which is calibrated in a traceable way. This amplifier is around one hundred times more accurate than the picoamperemeters which have been available to date. The current generated by a single-electron pump operated at ƒ = 545 MHz was measured with this new instrument as a function of external control parameters. The relative systematic measurement uncertainty in these current measurements was only 1.3 ⋅ 10–7.

The figure on the left shows the measurement results obtained while varying the current at the gate electrode, which controls the right-hand barrier of the “quantum well” of the electron pump. Depending on the measurement duration per point, statistical standard measurement uncertainties of down to 6 · 10–7 are reached. Within a relatively large range of around 11 mV, the measurement values deviated from the value of the inflection point by less than two standard deviations. Averaging the measurement values of this plateau region (plotted in the figure as the dashed frame) ultimately provided the final result: the relative difference between the generated current and the quantized nominal value – i.e., (I - e⋅ ƒ)/ (e · ƒ) – was -0.94 ± 1.94 · 10–7. This proved that the generated current agrees, within a total measurement uncertainty of around 2 ⋅ 10–7, with the expected quantized value e · ƒ. At 545 million electrons transferred per second, this corresponds to an uncertainty of only around 100 electrons. This means that – when compared to measurement results obtained previously – the measurement uncertainty is reduced by a factor of around five.

Furthermore, for the first time, the accuracy achieved during this “quantum ampere” demonstration surpasses the best possible experimental realization of the ampere, which uses “traditional” experiments in the currently valid International System of Units (SI); this realization achieves a total measurement uncertainty of 2.7 ⋅ 10–7.


Hansjörg Scherer
Department 2.6 Electrical Quantum Metrology
+49 (0)531 592 2610


Scientific publication

F. Stein, D. Drung, L. Fricke, H. Scherer, F. Hohls, C. Leicht, M. Götz, C. Krause, R. Behr, E. Pesel, K. Pierz, U. Siegner, F.-J. Ahlers, H. W. Schumacher: Validation of a quantized-current source with 0.2 ppm uncertainty, Applied Physics Letters 107, 103501 (2015)