Quantum Voltage

Goals

To meet the nation’s voltage metrology needs by developing state-of-the-art voltage standard systems and precision measurement techniques that are founded on quantum-based superconducting circuit and system technology so that internationally consistent, accurate, and reproducible voltage measurements are readily and continuously available for U.S. industrial, governmental, and scientific applications.

Customer Needs

The demands of modern technology for accurate voltage calibrations have exceeded the capability of classical artifact standards. To meet current needs, an international agreement signed in 1990 redefined the practical volt in terms of the voltage generated by a superconductive integrated circuit developed at NIST and the Physikalisch-Technische Bundesanstalt in Germany. This circuit contains thousands of superconducting Josephson junctions, all connected in a series array and biased at a microwave frequency. The voltage developed by each junction depends only on the frequency and a fundamental physical constant; thus, the circuit never needs to be calibrated. This allows any standards or commercial laboratory to generate highly accurate voltages without the need to calibrate an artifact standard. This advance has improved the uniformity of voltage measurements around the world by about a hundredfold. These systems are now essential for meeting legal and accreditation requirements in commercial, governmental, and military activities.

Burm Baek, Nicolas Hadacek, Paul Dresselhaus, Charlie Burroughs, Sam Benz, Michio Watanabe, and Yonuk Chong, at NIST-Boulder labs.

The U.S. electronics instrumentation industry maintains its world position through the development and deployment of increasingly accurate, flexible, easy-to-use instruments. Providing U.S. industry with quantum voltage standard systems gives these customers immediate realization of the highest possible in-house accuracy. These customers also benefit dramatically by eliminating their dependence on less accurate reference standards that require frequent calibration.

We also support the standards community by applying quantum-based voltage metrology to new areas and by developing voltage standard systems with new capabilities, including lower cost, increased functionality, and ease of use. This is especially true in the area of ac voltage metrology where we have developed the world’s first ac Josephson voltage standard system for arbitrary waveform synthesis. Other customers are the superconductive electronics community and the U.S. military, which we support through development of novel superconductive circuits, cryogenic packaging, and high-performance systems, and also by providing technical expertise.

A 1 centimeter × 1 centimeter superconducting integrated circuit with over 132,000 triple-stacked SNS Josephson junctions for the 5.0 volt high-resolution programmable voltage standard operating at 18.5 GHz.

Technical Strategy

Over the past 25 years, we have developed superconductive Josephson junction array technology for quantum voltage standard systems. Groundbreaking work at NIST led to commercialization of the first practical dc Josephson voltage standard system. Recent improvements in system design and operation have led to a traveling Josephson voltage standard system that is compact, low cost, and transportable for calibration of Zener reference standards. The technology for this conventional Josephson voltage standard system has been completely transferred to the private sector, where systems are produced and supported by a number of small companies.

In order to create a new generation of voltage standards, we developed a novel superconductor-normal metal-superconductor (SNS) junction technology that adds the features of stability and programmability to the accuracy of conventional Josephson voltage standards. This gave rise to the first Programmable Josephson Voltage Standard (PJVS) systems with 1 V output voltage. This was the first practical system to have the unique feature that the quantum-defined voltage steps are intrinsically stable, which is essential for some calibrations and many applications. As a result, PJVS systems have been delivered to, and installed in, a number of metrology experiments – namely the Watt-balance experiments at NIST and Switzerland’s Federal Office of Metrology (OFMET), where their accuracy, stability and noise immunity have reduced the uncertainty of the experimental measurements. In order to support the maintenance and dissemination of the volt, a one-volt programmable voltage standard system is also now in use in the NIST primary voltage calibration laboratory.

Schematic diagram depicting a 4-junction stacked series array with molybdenum-disilicide normal metal barriers, superconducting niobium electrodes, and silicon dioxide insulating dielectric.

In order to increase the junction density of arrays and thereby the output voltage for all our quantum-based voltage standard circuits, we invented a nanoscale junction fabrication technology in which the spacing between junctions is reduced from 7 µm to less than 50 nm. Over many years we explored various fabrication methods and materials before determining the optimal method of vertically stacking the junctions on top of each other in three dimensions. We found a number of useful junction barrier materials, in particular molybdenum-disilicide and niobium-silicide, that could be etched contiguously with niobium electrodes. To form the junction stacks we developed a deep vertical etching process using an inductively coupled plasma etcher to make tall stacks. Stacks as tall as 15 junctions have been demonstrated and uniform electrical characteristics of arrays have been measured using Nb-electrodes as thin as 5 nm. The stacked-junction technology has now matured to the point where we have made useful PJVS circuits with over 130,000 junctions operating uniformly on a single chip. Such nano-stacked junction arrays are now the basis for all of our next generation dc and ac Josephson voltage standard systems.

Our present goals are to further improve NIST electrical measurement capabilities by applying quantum voltage standards to both existing and new areas, in particular, in dc and ac metrology, 60 Hz power applications, and electronic-based precision thermometry. Our most challenging and long-term goal is to demonstrate a 10 V PJVS system that is essentially a turn-key system that does not require an expert to operate. Such a system would be preferable for many applications that are not fulfilled using the existing 10 V conventional Josephson voltage standards. This challenging task requires significant development of array technology, microwave integrated circuits, electronic instrumentation, and cryogenic packaging. We have demonstrated circuits that have reached voltages as high as 5 V. This was accomplished by increasing the number of junctions in each array through the development of new junction barrier materials and nano-stacked junction technology. Achieving 10 V PJVS circuits requires improved microwave designs.

Scanning electron microscope image of an 8-junction stack with niobium-silicide barriers and niobium electrodes. The image shows that the nano-stacked junctions can be vertically etched which is critical for achieving uniformity of the electrical characteristics. (Image by Burm Baek, NIST)

Over the past 10 years we also have been developing the necessary technology to make a practical and automated ac Josephson voltage standard (acJVS) system. We call this same system the Josephson arbitrary waveform synthesizer (JAWS) when it is used to synthesize non-sinusoidal waveforms. The concept for this new device was co-invented by NIST and Northrop-Grumman researchers in 1996, and is essentially a digital-to-analog converter that is capable of synthesizing arbitrary waveforms and, like the previously described dc-only systems, exploits the perfectly quantized pulses of Josephson junctions. Present ac voltage calibrations are done using ac-dc thermal voltage converters, which thermally compare the root-mean-square voltage of an ac signal with a known and accurate dc signal. The new quantum-based acJVS is a perfect voltage source and will provide an entirely new instrument and methodology for ac voltage metrology. The precision, stable and accurate arbitrary waveforms will also be useful for calibrating other scientific instruments, such as ac voltmeters, spectrum analyzers, amplifiers, and filters. The biggest challenge has been to reach practical output voltages of at least one-quarter volt.

In 2006, we completed the first acJVS system, which is now installed in the NIST Voltage Calibration Lab and can generate arbitrary waveforms up to 100 mV rms and frequencies up to 100 kHz. This is the world’s first quantum-mechanically accurate ac voltage standard source with practical output voltage at audio freqeuncies. Precision measurements with the acJVS demonstrated the accuracy of these synthesized voltage waveforms from dc up to 100 kHz over the full voltage range. This new measurement capability was achieved by using superconducting integrated circuits with nano-stacked Josephson junction arrays, better on-chip filters, and a well-controlled output transmission line. This work was undertaken specifically to improve NIST’s low-voltage ac calibration service, and we anticipate a 10- to 100-fold reduction in calibration uncertainties in this 1 mV to 100 mV range.

Charles Burroughs with the Programmable Josephson Voltage Standard system showing (left to right) the low thermal probe, the microwave and high-speed bias electronics, and the computer.

Deliverables

Our present goal is to increase the output voltage and measurement bandwidth of the acJVS to 1 V and 1 MHz. We recently demonstrated 220 mV rms (311 mV peak) output voltage circuits that are even closer to realizing this goal and were possible through the development of high-density arrays and novel tapered transmission line circuits. Future research and creative breakthroughs in electronics and circuit design are required to expand the output range to 1 V and the output bandwidth to 1 MHz. A low-voltage version of the acJVS has also been developed as a precision pseudo-noise voltage source to calibrate the measurement electronics of a novel Johnson noise thermometry system. The thermometry application is described in detail in the Johnson noise thermometry part of this site.

Finally, in order to improve calibrations of 60 Hz power meters, we have renewed our interest in using the PJVS system as an ac standard for frequencies below 1 kHz. Since higher voltages are possible with this system, we can use the PJVS as a multi-bit digital-to-analog converter by rapidly switching array segments to appropriate constant voltage steps. The new quantum-based power standard will use two PJVS systems (one for voltage and one for current) to produce precision 60 Hz sinusoidal reference signals. Using step-wise approximation synthesis, the PJVS systems produce sine waves with calculable rms voltage and spectral content. This work is in collaboration with the Applied Electrical Metrology Group, and they are developing novel divider and amplifier techniques to implement comparison between the 120 V calibration and 1.2 V PJVS signals. Our goal is to reduce all error sources and uncertainty contributions from the PJVS-synthesized waveforms to be a few parts in 10⁷, so that the overall uncertainty in the power standard will be a few parts in 10⁶.

Paul Dresselhaus holds silicon wafer containing superconducting integrated circuits and shows the current-voltage characteristics of a Josephson junction array.

Accomplishments

• Demonstrated accurate 100 mVrms synthesized quantum-based voltage waveforms from dc up to 100 kHz. This new measurement capability was achieved by using superconducting integrated circuits with nano-stacked Josephson junction arrays, better on-chip filters, and a well-controlled output transmission line. This work was undertaken in order to improve our low-voltage ac calibration service.
• Investigated new JJ barriers, WSi₂, TiSi₂, Nb_xSi_1-x which have led to new high-speed Josephson applications.
• Further increased programmable voltage standard output with longer triple-stacked arrays. The record voltage is now 5 V with a useful 1 mA current range. This result gives us confidence that we have a practical path to reach 10 V, which is essential for making a turn-key quantum voltage standard system that will revolutionize voltage metrology.
• Duplicated, automated, and delivered a 100 mVrms acJVS system to voltage calibration lab in Gaitherburg. Performed the first direct quantum-based measurements on the two lowest voltage ranges of a Fluke transfer standard.
• Synthesized and measured 60 Hz 1.5 V rms sine waves with PJVS arrays for quantum-based power calibration EEEL Director’s Reserve program. Measured harmonic content agrees with the expected quantum-accurate synthesized voltages.
• Demonstrated precision measurements of a commercial analog-to-digital-converter with a record 220 mV rms (311 mV peak) sine waves and arbitrary waveforms.

Recent Publications

Download our publications.

S. P. Benz, C. J. Burroughs, P. D. Dresselhaus, T. E. Lipe and J. R. Kinard, “An AC Josephson Voltage Standard for AC-DC Transfer Standard Measurements,” to appear in IEEE Trans. Inst. Meas., vol. 56, no. 2, April 2007.

C. J. Burroughs, S. P. Benz, P. D. Dresselhaus, Y. Chong, B. Waltrip, T. Nelson, J. M. Williams, D. Henderson, P.Patel, L. Palafox, and R. Behr, “Development of a 60 Hz Power Standard using SNS Programmable Josephson Voltage Standards,” to appear in IEEE Trans. Inst. Meas., vol. 56, no. 2, April 2007.

S. P. Benz, C. J. Burroughs, P. D. Dresselhaus, T. E. Lipe and J. R. Kinard, “100 mV ac-dc transfer standard measurements with a pulse-driven ac Josephson voltage standard,” 2006 Conference on Precision Electromagnetic Measurements Digest, presented 10–14 July 2006, Torino, Italy, pp. 678-679.

C. J. Burroughs, S. P. Benz, P. D. Dresselhaus, Y. Chong, B. Waltrip, T. Nelson, J. M. Williams, D. Henderson, P.Patel, L. Palafox, and R. Behr, “Development of a 60 Hz Power Standard using SNS Programmable Josephson Voltage Standards,” 2006 Conference on Precision Electromagnetic Measurements Digest, presented 10–14 July 2006, Torino, Italy, pp. 682-683.

B. Baek, Paul D. Dresselhaus, and Samuel P. Benz, “Co-sputtered Amorphous Nb_xSi_1-x Barriers for Josephson-Junction Circuits,” to appear in IEEE Trans. Appl. Supercond., 2006.

M. Watanabe, P. D. Dresselhaus, and S. P. Benz, “Resonance-free Low-pass Filters for the ac Josephson Voltage Standard,” IEEE Trans. Appl. Supercond., vol. 16, no. 1, pp. 49-53, March 2006.

Y. Chong, N. Hadacek, P. D. Dresselhaus, C. J. Burroughs, B. Baek, and S. P. Benz, “Josephson junctions with nearly superconducting metal silicide barriers,” Appl. Phys. Lett., vol. 87, 222511, December 2005.

P. D. Dresselhaus, Y. Chong, N. Hadacek, B. Baek, M. Watanabe, C. J. Burroughs, and S. P. Benz, “Silicide Barrier SNS Junctions for AC Josephson Voltage Standards,” in Proceedings of the 10th International Superconductive Electronics Conference, ISEC'05, September 5 - 9, 2005, Noordwijkerhout, Netherlands.

S. P. Benz, C. J. Burroughs, P. D. Dresselhaus, N. Hadacek, and Y. Chong, and, “Progress Toward Practical AC and DC Josephson Voltage Standards at NIST,” in Proceedings of the 6th International Seminar on Electrical Metrology, VI Semetro, 21-23 September 2005, Rio de Janeiro, Brazil, pp. 32-34.

Y. Chong, P. D. Dresselhaus, and S. P. Benz, “Electrical properties of Nb-MoSi₂-Nb Josephson junctions,” Appl. Phys. Lett., vol. 86, 232505, 6 June 2005.

N. Hadacek, P. D. Dresselhaus, Y. Chong, S. P. Benz, and J. E. Bonevich, “Fabrication and Measurement of Tall Stacked Arrays of SNS Josephson Junctions,” IEEE Trans. Appl. Supercond., vol. 15, no. 2, pp. 110-113, June 2005.

C. J. Burroughs, S. P. Benz, P. D. Dresselhaus, Y. Chong, and H. Yamamori, “Flexible Cryo-packages for Josephson Devices,” IEEE Trans. Appl. Supercond., vol. 15, no. 2, pp. 465-468, June 2005.

P. D. Dresselhaus, Y. Chong, and S. P. Benz, “Stacked Nb-MoSi₂-Nb Josephson Junctions for ac Voltage Standards,” IEEE Trans. Appl. Supercond., vol. 15, no. 2, pp. 449-452, June 2005.

Y. Chong, C. J. Burroughs, P. D. Dresselhaus, N. Hadacek, H. Yamamori, and S. P. Benz, “Practical High-Resolution Programmable Josephson Voltage Standards using Double- and Triple-Stacked MoSi₂-Barrier Junctions,” IEEE Trans. Appl. Supercond., vol. 15, no. 2, pp. 461-464, June 2005.

C. J. Burroughs, S. P. Benz, P. D. Dresselhaus, and Y. Chong, “Precision Measurements of AC Josephson Voltage Standard Operating Margins,”  IEEE Trans. Inst. Meas., vol. 54, no. 2, pp. 624-627, April 2005.

Y. Chong, C. J. Burroughs, P. D. Dresselhaus, N. Hadacek, H. Yamamori, and S. P. Benz, “2.6 V high-resolution programmable Josephson voltage standard circuits using double-stacked MoSi₂-barrier junctions,”  IEEE Trans. Inst. Meas., vol. 54, no. 2, pp. 616-619, April 2005.