Josephson voltage standard

This effect found immediate application in metrology because it relates the volt to the second through a proportionality involving only fundamental constants.

based on voltage values derived from the SI volt realization as maintained by Weston cells.

[8] In the most precise of these experiments, two Josephson devices are driven by the same frequency source, biased on the same step, and connected in a series opposition loop across a small inductor.

Since this loop is entirely superconductive, any voltage difference leads to a changing magnetic field in the inductor.

This field is detected with a SQUID magnetometer and its constancy has set an upper limit on the voltage difference of less than 3 parts in 1019.

The lack of stable regions between the first few steps means that for small DC bias currents, the junction voltage must be quantized.

After several preliminary experiments,[14][15][16] a joint effort in 1984 between the National Bureau of Standards in the U.S. and the Physikalisch-Technische Bundesanstalt in Germany resolved the problems of junction stability and microwave distribution and created the first large Josephson array based on Levinsen's idea.

[18][19] Advances in superconductive integrated circuit technology, largely driven by the quest for a Josephson junction computer,[20] soon made possible much larger arrays.

and is ultimately set by a trade-off between stability and the economics of providing a very high frequency microwave source.

Several precautions are required to avoid reflections that would lead to standing waves and the consequent nonuniform power distribution within the subarrays: Voltage standard chips are typically fabricated on silicon or glass substrates.

The integrated circuit has eight levels: A block diagram of a modern Josephson voltage standard system is shown in Fig.

The Josephson array chip is mounted inside a high-permeability magnetic shield at the end of a cryoprobe that makes the transition between a liquid helium Dewar and the room temperature environment.

The box, the filters, and the Dewar itself form a shield that protects the Josephson array from electromagnetic interference that could cause step transitions.

Microwave power is delivered through a waveguide consisting of a 12 mm diameter tube with WR-12 launching horns on each end.

A phase-locked oscillator (PLO) operating at a frequency near 75 GHz provides the microwave power to the chip.

The primary requirements for the 75 GHz source are: (1) its frequency must be known with high accuracy (1 part in 1010) and (2) it should produce a stable output power of at least 50 mW (+17 dBm).

The PLO may be constructed using a commercial microwave counter with feedback capability or it may be a custom built phase-locked loop.

The frequency reference for the system is usually a 10 MHz sine wave derived from a GPS receiver or an atomic clock.

Computer control of this three-step process enables the system to find and stabilize the array voltage on a particular step within a few seconds.

These algorithms differ in the amount of averaging used, the type and placement of reversing switches, and the statistical methods used to reduce the data and compute uncertainty.

This eliminates data that may be corrupted by the transient that occurs when there is a spontaneous transition between quantum voltage steps.

remains constant thus making the data collection process relatively immune to step transitions.

The scatter in the data that results from noise in the unknown and in the null meter can generally be modeled by a Gaussian process with one standard deviation on the order of 20 to 100 nV.

Except in the case of data with nonuniform delays between the reversals, a simple average of the absolute values of the full set of

7 are used to calibrate secondary standards, such as Weston cells, Zener references, and precise digital voltmeters.

The ability to set the Josephson array to a wide range of discrete voltages also makes it the most accurate tool for measuring the linearity of high-accuracy digital voltmeters.

The standard error resulting from sources 3–8 is just the root mean square (RMS) value of the set of short circuit measurements.

[33] Typically, the total uncertainty contribution of a Josephson system in a measurement averaging time of a few minutes is a few nanovolts.

Since the success of the first Josephson array voltage standards in 1984, their use has proliferated to more than 70 national measurement institutes (NMIs), military, and commercial laboratories around the world.

This has resulted in some confusion about the traceability of non-NMIs that are in possession of a JVS that is, in principle, as good as the national standard.

Fig. 1 Constant voltage steps in the I–V curve of a junction driven with microwave radiation for (a) a low capacitance junction and (b) a high capacitance junction.
Fig. 2 The approximate level of agreement in DC voltage measurements among standards laboratories through the years 1930 to 2000.
Fig. 3. The structure of a superconductor–insulator–superconductor Josephson junction typically used in DC voltage standards.
Fig. 4. A three-dimensional visualization of the region of stable voltage operation as a function of , , and .
Fig. 5. (a) A series of Josephson junctions arranged to form a stripline and (b) the circuit of a typical Josephson voltage standard integrated circuit chip.
Fig. 6. The layout of a 20 208 -junction, 10 V Josephson array voltage standard chip.
Fig. 7 Block diagram of a voltage standard system.
Fig. 8 (a) The bias circuit for a JVS and (b) a graphical solution of the operating points for the Josephson array.
Fig.9 The measurement loop used to determine the voltage of an unknown device relative to the Josephson standard.
Josephson junction array chip developed by the National Bureau of Standards as a standard volt