Quantum Voltage System Development and Dissemination

This project is developing new standards using Josephson junctions, superconductor-based devices whose quantum behavior makes them perfect frequency-to-voltage converters. Project scientists exploit this property to create extremely accurate voltage standards and also to develop novel methods for the precise measurement of other fundamental electrical quantities.  Accurate representation of the unit of electric potential difference, the volt, and precise techniques for measuring voltage are essential to the electrical and electronics industries. A historic NIST responsibility is defining such standards and disseminating them to U.S. and international measurement institutes and companies, which use them to define other standards and build accurate electrical devices.

In 1990, an international agreement redefined the volt in terms of the voltage generated by a superconducting integrated circuit developed jointly at NIST and the Physikalisch-Technische Bundesanstalt (PTB), Germany's national metrology institute. The circuit contains thousands of Josephson junctions, each one a sandwich consisting of an insulating layer between two superconducting segments and having a typical thickness of a few hundred nanometers.

Current will flow across a Josephson junction despite the insulating layer. When it's an alternating current (AC), a voltage develops across the junction that is exactly proportional to the AC frequency. This relationship depends only on fundamental parameters of quantum physics, and does not depend on the physical properties of the junction, such as its dimensions, or environmental conditions, like temperature. Josephson junctions, as perfect frequency-to-voltage converters, provide an excellent basis for a voltage standard because frequencies can be defined with enormous precision.

Josephson voltage standard systems have been deployed around the world since 1990, greatly improving the uniformity of voltage measurements. One key development at NIST is the programmable Josephson voltage standard (PJVS) system, which can provide desired voltages with an uncertainty better than a few parts in a billion. PJVS replicas are used at NIST and throughout the world to provide voltage calibrations for a variety of applications, particularly experiments that measure other electrical units. The system is regularly used in the NIST voltage calibration lab and is also being implemented in a novel electric power calibration system that will provide the electric power industry with the world's most precise electrical standards, tests and services and, in turn, support the reliable operation of the electrical power grid.

Project scientists have also made substantial improvements to the process of accurately transferring a fundamental standard to end users. For example, we have developed a portable, compact Josephson voltage standard (CJVS) that scientists can carry with them to compare Josephson voltage systems in different geographic locations. The CJVS decreased end-user uncertainty by a factor of 10 or better.

As well as making junctions and performing voltage measurements, the Quantum Voltage project at Boulder, a subgroup of the Josephson voltage project, applies quantum-based voltage metrology to new areas and develops voltage standard systems that are more functional and easier to use. For example, project scientists developed the world’s first AC Josephson voltage standard system that generates made-to-order voltage waveforms, that is, a voltage that changes with time in a desired way. This system is currently in use in the NIST AC voltage calibration laboratory and several have been installed in other national metrology institutes.

The project has also created a quantum-based electronic temperature standard that uses the ACJVS in conjunction with measurement of the Johnson noise -- random electrical noise caused by thermal agitation in a conductor -- in resistors at arbitrary temperatures. This Johnson noise thermometry system is in use at NIST's Chemical Science and Technology Laboratory in support of work aimed at reducing uncertainty in temperature measurements and improving the understanding of the international temperature scale, the standard for measurements made in the Celsius and Kelvin, as well the development of a novel electronic method for determining the Boltzmann constant, a fundamental constant of nature that relates temperature and energy.

• Developed an ac Josephson voltage standard (ACJVS) that provides the most accurate ac voltage measurements ever made. This standard has the potential to change the basis of measurement of all ac voltage measurements in the world and to significantly reduce the uncertainties provided to customers.
• Developed a protocol to successfully compare the NIST Compact Josephson Voltage Standard (CJVS) with the JVS of the National Research Council (NRC) of Canada. The two JVS systems were found to be equivalent to an uncertainty level of a few parts in 1010, one of the smallest uncertainties achieved in direct JVS comparisons.
• Performed a direct JVS comparison with Lockheed Martin Mission Services in Denver, and found a difference between the two systems of a few parts in 1010, a result that supports traceability of SI units for U.S. industry and other government agencies.
• Developed the protocol to approve the use of a new voltmeter to be used in NIST's Manufacturing Metrology Division for force metrology, using the programmable JVS to calibrate the new instrument. A preliminary test by the division has verified consistency and continuity from the old instrument to the new one.
• Increased the output voltage of the ACJVS system, which could make possible smaller Josephson junction arrays.
• Achieved five-fold reduction in the measurement uncertainty quantum-based Johnson noise, demonstrating that Johnson noise thermometry has the potential to contribute to the redefinition of the Boltzmann constant that is expected in 2010.
• Demonstrated operation at 400 GHz of a high-speed junction with a novel niobium-silicide barrier. This has the potential to improve operation speed and yield of superconducting digital electronics.