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. An 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.
1. DC Voltage Standards: Worldwide access to quantum-based DC voltage standards took a major step forward in 2014 with the introduction of NIST's first Standard Reference Instrument, the cryocooled (liquid cryogen free) 10 V Programmable Josephson Voltage Standard (PJVS). Quantum Voltage Project researchers have successfully implemented operation of the superconducting integrated circuits at the heart of the PJVS on a cryogenic refrigerator that cools the device to 4 K. This was accomplished by improving the fabrication of the circuits that contain nearly 300,000 Josephson junctions, developing new packaging that allowed the devices to be easily operated in either liquid helium or on a cryocooler, and optimizing the system performance with improved software algorithms that control the quantum states as a function of the cryostat’s cold-head temperature. Quantized voltages can now be programmed over a voltage range of -10 V to +10V with a stable current range of 1.32 mA and at an operating temperature of 4.3 K. The quantum accuracy of the voltage output has been demonstrated for temperature setpoints up to 4.8 K. Since Jan. 2014 the Project has delivered three new PJVS systems, upgraded 3 more systems, and provided seven 10 V and ten 2 V PJVS chips to metrology laboratories around the world.
2. AC Voltage Standards: The quantum standard for ac voltage is the NIST Josephson Arbitrary Waveform Synthesizer (JAWS). The Synthesizer produces sine waves, multi-tone signals, and arbitrary waveforms with quantum accurate and calculable voltages at frequencies up to 1 MHz by digitally programming the quantum states of 25,600 Josephson junctions connected in series. Over the past 3 years improvements in circuit design and fabrication, packaging, and pulse-bias techniques have quadrupled the output voltage to 1 V root-mean-square (RMS) (1.4 V peak amplitude) over an operating current range greater than 2 mA. Quantum Voltage Project researchers recently demonstrated the capabilities of the system by synthesizing a 1 Hz sine wave useful for ac voltage calibrations and a two-tone waveform at kilohertz frequencies that can be used for inter-modulation measurements of analog-to-digital converters. Based on these advances, researchers are completing development of the new 1 V JAWS system, which contains a single cryopackaged 1V chip with 51,200 Josephson junctions, operated by customized pulse-bias electronics. This first in the world system is slated for delivery to the ac voltage calibration laboratory at the U.S. Army Primary Standard Laboratory in September, 2015.
3. Precision Electronic Measurement of Boltzmann's Constant: The Quantum Voltage Project is applying quantum-based voltage waveform synthesis to a precision measurement of Boltzmann’s constant kB by developing a primary temperature standard based on the electrical measurement of the Johnson-Nyquist voltage noise of a resistor at the triple-point temperature of water. Just as the measurement of Planck’s constant h by the NIST watt-balance experiment will soon replace the international kilogram artifact standard with an “electronic kilogram”, the NIST Johnson Noise Thermometer (JNT) may eventually replace the large collection of temperature fixed points (temperature artifact standards) that define the ITS-90 thermodynamic temperature scale. Most importantly, JNT is providing a unique electronic measurement of the Boltzmann constant, which is important for the ‘New SI’ (New International System) redefinition that is anticipated in 2017. The value of kB is presently determined exclusively by acoustic gas thermometry measurements, so it is very important to have an alternative determination using an entirely different physical measurement technique, such as the electronic technique of JNT. NIST published the first electrical measurement of Boltzmann’s constant in 2010, which contributed to CODATA and agreed with the previous SI-assigned temperature of +0.5 μK/K ± 12 μK/K. The most recent JNT measurement resulted from a 2014 NIST collaboration with NIM China, which reduced the combined measurement uncertainty to ~4 μK/K. This lower-uncertainty result was accomplished by a new pulse-bias technique for the quantum voltage waveform synthesizer that reduces some frequency-dependent systematic error signals and improved modelling of the frequency-dependent fits. We anticipate further improvement in the uncertainty on kB based on new measurements at NIST within the year.
4. High-end Digital Computing: The Quantum Voltage Project recently joined the three year, IARPA-funded, Cryogenic Computing Complexity (C3) program, whose goal is to demonstrate an energy-efficient, single flux quantum (SFQ) based computer clocked at 10 GHz. NIST’s role in this program is primarily to develop metrology for SFQ logic and cryogenic memory, while the commercial partners develop the device and circuit design expertise. NIST researchers are actively engaged with these commercial partners, providing valuable measurement feedback on initial circuits and diagnostic chips as we continue to augment our measurement infrastructure for the later phases of the program. In addition, NIST has been working with the government-funded C3 chip foundry (MIT-Lincoln Laboratory) to independently verify individual Josephson junction device yields by exploiting our voltage-standard design, fabrication, and testing expertise. Finally, NIST has provided valuable technical evaluation and support to IARPA for both the logic and cryogenic memory thrusts of this project.
5. High-speed Single Flux Quantum Digital Circuits: NIST researchers are pushing the speed of Josephson junctions (JJs) through material science research of the junction barriers. We fabricated static divider SFQ circuits with a-NbxSi1-x and a-Si barriers that increased the critical current density from Jc=3.5 to 85 kA/cm2. For the lower Jc range (3.5 to 17 kA/cm2) we used optical lithography, and for the 85 kA/cm2 JJs we used e-beam lithography. We measured SFQ circuits with a maximum operating speed of 300 GHz by use of intrinsically shunted junctions having sub-micrometer dimensions and an 85 kA/cm2 current density. These results demonstrate that silicide-barrier junctions have the potential to realize high-speed SFQ circuits and higher density circuits because the junctions are small and don’t require external lithographically defined shunt resistors.
Set up the direct JVS comparison between the NRC JVS, Canada, and NIST compact JVS.
Lead Organizational Unit:pml
Samuel Benz, Project Leader