Physikalisch-Technische Bundesanstalt (PTB)

Switchyard for single electrons – would you like one electron more?

Single-electron devices are based on electric circuits with ultra-small electrical tunnelling contacts. They allow the controlled transfer of single charge quanta and their detection. This enables the investigation of fundamental questions in quantum metrology. At PTB we now achieved to transfer very small charge "packets", comprising a well-defined number of few electrons, between metallic electrons precisely by using a single-electron pump. A single-electron transistor, being able to resolve charge variations of a single electron or less, served as a charge detector to monitor the charge movement. The successful experiment is an important milestone on the way to the setup of a new standard for capacitance, where a capacitor is charged by a well-known number of electrons. The corresponding voltage can be measured using a Josephson voltage standard. Tracing the capacitance to a resistance via the quantum-Hall effect finally allows the realisation of the so-called "Quantum Metrological Triangle", which establishes a link between all three electrical quantum effects. The precision aimed at in the experiment requires the demonstrated manipulation of charge on the scale of a single electron.

Figure 1: The single-electron pump used in the experiment (top left-hand side of the figure) consists of a series array of five ultra-small metallic tunnel junctions. On its right side, the chain of tunnel junctions is connected to a metallic electrode, the "island", which has a small total capacitance of about C_∑= 20 fF. With a fast train of voltage pulses on the gate electrodes of the pump (V_1-4, see bottom of figure) within 0.25 µs an electron is pumped through the chain onto the island. Here, the excess electron causes a change in potential of about 8 µV, which is detected by the single-electron transistor being capacitively coupled to the island (right-hand side of the figure). After a wait time t_w the excess electron is removed from the island by a pulse sequence in opposite direction.
In the "shuttle pumping mode" demonstrated, the successive charging and discharging of the island is repeated periodically. In the "hold mode" the gate voltages on the single-electron pump are not modulated. Then, in the ideal case, the charge state of the island remains constant since tunnel processes through the junction chain are suppressed by the Coulomb blockade effect.

Figure 2: Measurements of the output signal of the single-electron transistor, converted to the electrical potential of the island.
Panel (a) shows the time series of the signal when the single-electron pump is kept in the "hold mode": Apart from noise intrinsic to the single-electron detector the signal is constant for several seconds. Unwanted tunnel processes of single electrons through the chain of junctions cause step-like quantized fluctuations of the island potential. Changes of the island charge by one electron corresponds to a potential change of 8 µV. The mean time interval between these unwanted "error" events on the island was 40 s.
Panel (b) shows the signal time series in the "shuttle pumping mode", when charge packets of one up to five electrons where shuttled between the island and the opposite pump side with a clock time of t_w = 1 s (curves are offset vertically for clarity). Obviously the detected potential changes follow the second beat of island charging and discharging, and the signal amplitude is proportional to the number of excess electrons on the island.

Contact:

Hansjörg Scherer

Department 2.6

Electrical Quantum Metrology

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