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The quantum ampere is more precise than the conventional ampere


In the current International System of Units (SI), the electrical base unit – the ampere – is defined as the force between two current-carrying conductors. This “conventional ampere” can be realized with a measurement uncertainty of around 0.3 μA/A. In the revised SI, whose introduction is planned for 2018, the ampere is to become more precise. In the new system of units, the ampere – the unit of electric current – will be defined by means of the value of the elementary charge e. To directly realize this definition, single electrons are propelled through an extremely narrow conductor whose dimensions are in the nanometer range. Over the past several years, components known as single-electron pumps have been developed at PTB on the basis of semiconductor materials and manufactured in its Clean Room Center. If the pumps are operated at a clock frequency f, it is expected that a current I = ef will be generated. For f = 1 GHz, the current is around 160 pA. For many years, such small currents were measurable only with uncertainties of 1 μA/A. Theory predicts that quantized currents from semiconductor single-electron pumps will have uncertainties well below 1 μA/A.

Semiconductor single-electron pump (left), connected to a high-precision current/voltage transformer ULCA (center) combined with a Josephson voltage standard for measuring voltage (right). All components and devices were developed by PTB.

Schematic representation of the measurement set-up, with two current amplifiers (ULCA A and ULCA B) that are used as current/voltage transformers in the two current supply lines that lead to the singleelectron pump (SET pump). In combination with a programmable Josephson voltage standard, the voltmeter measures the difference between the two current amplifiers’ output voltages UA and UB. A frequency standard controls both the voltagepulse generator that drives the single-electron pump, and the microwave source of the Josephson voltage standard. The systematic contribution of this set-up to the uncertainty of the current measurement is only 0.084 μA/A.

To verify this prediction, a special measurement value amplifier was designed and realized over several years of development work; this measurement value amplifier initially amplifies the small, quantized current by around 1000 times and then transforms it into a current that can be measured more easily by a transimpedance amplifier. Traceability to a quantum Hall resistance ensures the precision of this part of the measuring chain. To measure the voltage, another quantum standard is used: a Josephson quantum voltmeter.

The image below contains a schematic representation of the measurement of the quantized current from a semiconductor single-electron pump by means of the newly developed measurement technology. After optimizing all elements of the measuring chain, it was possible to show that a quantized current I = 96 pA agreed with the expected value I = ef within a measurement uncertainty of only 0.16 μA/A. The accuracy obtained in a relatively short measuring time of 21 hours is a new record for semiconductor single-electron pumps and is impressive proof of the quality of all of the quantum standards that were used. The result confirms earlier measurements that had an uncertainty of 0.20 μA/A. This work has allowed PTB to successfully prove that the “quantum ampere” can be realized with a smaller measurement uncertainty than the conventional ampere in the current SI – a milestone on the path to the planned revision of the International System of Units.