Richard Kautz of NIST discovered why an array of 5,000 Josephson junctions shows no voltage steps at all—chaos theory. The junction response became chaotic when the external microwave frequency approached the resonance frequency of the Josephson junction. Kautz showed that stable, chaos-free voltages require a microwave frequency that is larger than the resonance frequency of the junction.
(Citation: R.L. Kautz, "The ac Josephson effect in hysteretic junctions: range and stability of phase lock," J. Appl. Phys. 52 (1981) 35283541)
NIST and its German counterpart, Physikalisch-Technische Bundesanstalt (PTB), developed the first quantum voltage demonstration chip. The 1-volt chip has 1,474 Josephson junctions made of lead-indium-gold alloy electrodes. This array is not fully practical, because it often switches spontaneously between voltage levels, and the junctions cannot be thermally cycled, which occurs when the chip is taken in and out of liquid helium at 4 K.
(Artifact 1, Citation: J. Niemeyer, J.H. Hinken & R.L. Kautz, Appl. Phys. Lett. Vol. 45, August 1984, pp. 478-80)
NIST made the world's first practical, stable and easy to use 1-volt standard, which had 1,484 junctions made of niobium and lead-indium-gold alloy superconducting electrodes and niobium oxide barriers. Each junction was 15 micrometers by 40 micrometers in size. The silicon chip was relatively large at 6.35 millimeters by 12.7 millimeters. This type of series-array microchip became the workhorse of voltage calibration around the world, with more than 50 systems operating in national, industrial, and military laboratories.
(Artifact 2, Citation: C.A. Hamilton, R.L. Kautz, R. L. Steiner, F. L. Lloyd, IEEE Elect. Dev. Lett. Vol. 6, Dec. 1985, pp. 623-625)
NIST made the world's first 10-volt chip, with 14,184 junctions. Each junction had one niobium electrode, a niobium-oxide barrier, and lead-indium-gold as the second electrode. The original photograph of oscilloscope traces shows the first time in the world that a Josephson device (or any other device) generated a stable quantized voltage above 10 volts, as measured by Clark Hamilton on Jan. 6, 1988. The photo is a double exposure showing the current-volt loop and superimposed step (short vertical line at lower left) at about 10.8 volts.
(Artifact 3, Citation: C.A. Hamilton , F. L. Lloyd, K. Chieh, & W.C. Goeke, IEEE Trans. Inst. Meas. Vol. 38, April 1989, pp. 314-316)
NIST researchers designed a waveguide holder and "spring-finger board" to deliver microwaves to the early volt standard chips. The chips are mounted in a brass holder. A pocket in the holder aligns the chip to a slot in the waveguide that delivers microwave power. The finger board presses down on the chip to hold it in place.