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Electrical quantum metrology

electrons are tunneling through a SET device
Counted: electrons are tunneling through a SET device.

Historically speaking, not much time has passed since Nicola Tesla dazzled audiences with his controlled bursts of lightning and ghostly seeming light effects and a certain Thomas Alva Edison electrified the industrialized world with his inventions. The discovery and technical utilization of electricity took off at the end of the 19th century and conquered more and more conventional technical terrain until, in the late 1940s, the transistor was invented at Bell Labs in New Jersey. The transistor gave electricity its first quantum-mechanical form.

  • International units: Today, electric currents and voltages and electrical resistances can be best measured by exploiting quantum-mechanical effects. For many years, national metrology institutes have used quantum effects to disseminate these units, which are named after several pioneers of electricity such as André Marie Ampère, Georg Simon Ohm and Alessandro Volta. The fundamental reorganization of the International System of Units (SI) by means of natural constants means that our entire system of units (with the exception of the candela) is now based on quantum- mechanical principles.
  • Practical quantum standards: While, for many years, electrical quantum standards were available only in highly specialized metrological laboratories, such standards are being applied to practical situations with increasing success. For example, at PTB, a programmable “quantum voltmeter” has been commercialized, thus making it available to industry. The Josephson Arbitrary Waveform Synthesizer (JAWS) was developed as a universal tool for generating and measuring arbitrary and spectrally pure alternating voltages.

For many decades, PTB has consistently been among the world’s top addresses in the world of electrical quantum metrology. At PTB, metrologists develop the scientific foundations for extremely high-precision measurements of the electric quantities, specialists manufacture and characterize the hardware components necessary for this purpose in PTB’s own Clean Room Center, and links are forged to industry for practical applications.

For the ohm, the unit of electrical resistance, the freedom of movement of the electrons in a semiconductor is restricted and the resulting abrupt change in the resistance in an applied magnetic field is exploited – this is the so-called quantum Hall effect, for which Klaus von Klitzing received the Nobel Prize in Physics and which was used metrologically at PTB from “the hour of its birth”. For voltages, the Josephson effect of superconductivity is used, for which two electrons always group together to form a couple. Finally, for currents, it is possible at PTB to count the individual electrons carrying the electricity while these electrons, being quantum particles, tunnel through barriers that are normally insurmountable.

Professional Information:Professional Information:

The Electrical Quantum Metrology Department develops quantum-based methods and systems for the traceability of electric base units to natural constants. Most of the activities are pursued together with external partners within the scope of cooperation projects. These projects aim to further develop and simplify the complex setups to realize the quantum-based units. For one thing, this is beneficial to service and customer-oriented areas of metrology institutes such as PTB; for another, it allows quantum-based standards to be used in industrial calibration laboratories.

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The quantum-based realization of the unit of voltage, the volt, is based on the Josephson effect in superconductors. Integrated circuits made of tens of thousands of Josephson junctions allow the generation of DC voltages with "quantum accuracy" and low-frequency AC voltages with amplitudes of up to 10 volts. Josephson junctions driven by high-frequency pulse sequences allow AC voltages to be precisely synthesized with arbitrary wave forms at higher frequencies.

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In the new International System of Units (SI), which came into force on 20 May 2019, the unit of electric current, the ampere, is defined based on the value of the elementary charge e. To directly realize this definition, single electrons are transported through a quantum dot that is defined by means of gate electrodes in an extremely narrow conductor. At PTB, components known as single-electron pumps are being developed on the basis of semiconductor materials. Single-electron pumps represent an important contribution to the realization of the SI base unit ampere offer a good potential for developing shot-noise-free electronics and allow fundamental consistency tests in electrical quantum metrology.

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