Self-referenced quantum current source for the SI base unit ampere
30.12.2014
Figure 1: Electron micrograph of a self-referenced quantum current source.Three single-electron pumps (A) are connected in series along a semiconductor wire (B). During the operation of the pumps, the two single-electron detectors (C) can check how many electrons have really been transferred.
Figure 2: The scientists involved in this project received the Helmholtz Prize 2014: (from left to right) Dr. Nathalie v. Siemens (treasurer of the Helmholtz Fund), Dr. Hans Werner Schumacher, Dr. Frank Hohls, Dr. Bernd Kästner, Dipl.-Phys. Lukas Fricke, Dr. Ralf Dolata, Prof. Dr. Joachim Ullrich (President of PTB and Chairman of the Helmholtz Fund).
In the new International System of Units (SI), the definition of the unit of current, the ampere, is to be defined by fixing the value of the electron charge e. In order to realize a primary standard of the redefined base unit, the flux of single electrons can be clocked by means of a dynamic semiconductor quantum dot which is, in this case, called a “singleelectron pump”. The electrons are transferred one by one, at regular intervals set by an accurate clock. Such pumps are, however, not absolutely flawless: a transfer cycle might fail; the electron current then “loses the beat”. At PTB, the occurrences of such pump errors have now been successfully detected and recorded for the first time during the generation of the nominally quantized current. In this way, the exact number of electrons having been transferred at each cycle – and thus the electric current in units of the elementary charge e – could be stated precisely. The self-referenced quantum current source developed at PTB is based on three single-electron pumps that are serially connected via two intermediary conducting islands. If the three pumps are operated synchronously, then the charge on each island remains constant. If, however, an error occurs, for example, at the first pump so that no electron is transferred by this pump, then the charge on the succeeding island is reduced by exactly one elementary charge e. Even such a tiny charge difference can be reliably detected by the single-charge detectors that are coupled to the islands. A systematic analysis of the change in charge on all islands during the operation of the pumps allows conclusions to be drawn on the errors that have occurred, and thus allows a precise correction of the single-electron current generated. The quantum current source is currently still operated at low pump frequencies and generates small currents of only a few attoamperes. In future, however, it is planned to generate considerably stronger quantized currents reliably by increasing the pump frequency as well as the detector bandwidth.