1. INL Standards and Calibration Laboratories (S&CL) - Our Road To the Josephson Junction. MIKE STEARS

2. INL S&CL – Brief History Located approximately 45 miles West of Idaho Falls on the Idaho National Laboratory (DOE test site). S&CL was opened in 1969. Provide support to the multiple areas located on the INL as well as some outside companies. Support most areas of metrology in electrical and physical parameters. NVLAP accredited in most of our primary standard parameters.

3. Three generations of DC reference standards Saturated Cells Zener References Josephson Junction

4. Saturated Cells

6. Output approximately 1.018V. Intolerant of current drain. –<0.1mA surge maximum. –Measured with potentiometric device. –If short circuited, the cell was useless as a standard. Saturated cells are stable over time, but have a relatively large temperature coefficient. Saturated cells used as laboratory standards where temperature is controlled.

7. Saturated Cells at the S&CL Earliest calibration data in 1968. One-year stability was about 10ppm. Calibration of cells was performed through intercomparison with a specialized cell from Sandia. –Specialized cell was hand carried to and from Sandia in order to calibrate our cells in place. Uncertainty of later saturated cells was improved to around 2ppm.

8. First S&CL Saturated Cells

9. S&CL Standard Cells

10. Zener References

11. Zener References – Fluke 732B

12. 10V Reference Circuit Consists of an NPN transistor in series with a zener diode. Oven controlled for temperature stability. Temperature coefficient (TC) is the sum of the TC of the zener voltage and the transistor base-emitter voltage. –The TC of the zener is negative and the TC of the transistor is positive. –Each amplifier is tested to determine the collector current at which the TCs cancel each other out and yield an overall TC very close to zero. The reference can be adjusted about ±220ppm.

13. Zener References

16. The first zeners used at the S&CL as transfer standards were Fluke 732A’s. –Calibrated with standard cells until 1993. –Calibration uncertainty started at 5ppm in 1986 and eventually made it to 2ppm in 1991 as uncertainty of standard cell calibrations improved. The first zeners used as reference standards at the S&CL were Fluke 732B’s. –First NIST calibration was in 1993. The S&CL expanded uncertainty was estimated at 0.5ppm (K=2).

17. Zener References

18. Fluke 734A (bank of four 732Bs) has been used as the reference standard for zener intercomparisons at the S&CL since 1993. Reference zeners were calibrated by NIST every 6 months. Data Proof software and scanners are used to perform intercomparisons nightly. The S&CL is accredited to 0.3ppm for measurements of zener references through direct comparison at the 10V level.

19. Zener References - Uncertainty At the S&CL, we do not adjust our zeners. We plot the history of each zener and use the established slope to assign daily values to the reference bank.

20. Zener References - Uncertainty Uncertainty of reference zener bank calibration (NIST). Type B Uncertainty of the scanning system consisting of: Type B –Scanner –Null meter –Wiring History of reference (Prediction uncertainty). Type A Environmental effects. Type B Shipping. Type B Pressure Coefficients Type B Randomness of process. Type A Operator effects. Type B Software. Type B

21. Zener References - Uncertainty Increasing precision needs of customers = increased precision needs of reference standards. With the ever tighter specifications of standards such as 8.5 digit multimeters and precision calibrators, the uncertainty of our reference standards must continue to decrease to meet these calibration needs. We looked at the different contributors to our total uncertainty and considered how we could improve any of them to improve our overall process.

22. Zener References - Uncertainty One of the contributors we considered for improvement in our total uncertainty was shipping. –Step shifts in our zeners have been observed between pre-ship and post-ship measurements. –These step shifts are difficult to deal with and could affect our scope of accreditation if the shift is significant relative to our accreditation level. Step shifts can make the reliability of prediction data, based on history, questionable. –Another concern is loss of battery power due to unexpected delays in shipping or a faulty battery resulting in loss of calibration and a possible shift in value.

23. Zener References - Uncertainty Another contributor we considered for improvement was the pressure coefficient corrections made to the reference zeners. –The pressure coefficient data was provided by Fluke and stated that the zeners would change –0.29ppm with an altitude change of 5000 ft. The 2-sigma uncertainty of the correction was estimated at 0.1ppm. Improvements in our processes for these contributors should also improve our history data and prediction uncertainty.

24. Zener References - Uncertainty The most obvious way to reduce some of our uncertainties was to calibrate our reference zeners in place. The desire to calibrate our references in-house lead to adding the Josephson Junction to our wish list of equipment.

25. Josephson Junction (JJ)

26. What is a JJ?

27. Josephson Junction The Josephson Junction DC reference standard is based upon the Josephson effect predicted by Brian Josephson in 1962. Josephson derived equations for the current and voltage across a junction consisting of a thin insulating barrier separating two superconductors (cryogenic environment). His equations predicted that if a junction is driven at a frequency f, then its current-voltage (I-V) curve will develop regions of constant voltage at the values hf/2e. (h = Planck’s constant, e = elementary charge)

28. Josephson Junction When a DC voltage is applied across the junction, the current will oscillate at a frequency f j =2eV/h where 2e/h ≈ 484 GHz/mV. The very high frequency and low level of this oscillation make it difficult to observe directly.

29. Josephson Junction However, if an ac current at frequency f is applied to the junction, the junction oscillation tends to phase lock to the applied frequency and the average voltage across the junction equals hf/2e. It is also possible for the junction to phase lock to harmonics of f resulting in a series of steps at voltages V=nhf/2e, where n is an integer.

30. Josephson Junction

31. The constant 2e/h is known as the Josephson constant (K j ). In 1990, the constant K j was assigned the value 483597.9 GHz/V through international agreement. The equation for the Josephson voltage can now be written as V=nf/ K j. The accuracy of the above relationship has been tested and the upper limit of the voltage difference set at 3 parts in 10 19.

32. Josephson Junction A frequency of approximately 75 GHz is a common operating frequency for the junction. Applying 75 GHz to the equation V=nf/K j and assuming n=1, to calculate the voltage for a single quantum step, the Josephson voltage equates to about 155 microvolts. Single junction Josephson standards were introduced in the early 1970’s but they could only generate voltages in the range of 1-10mV. More series junctions were needed to increase the voltage.

33. Josephson Junction The problems that needed to be overcome were how to place numerous junctions (an array) on an integrated circuit, source microwave frequency to all of the junctions, provide bias to the junctions, etc, all in a cryogenic environment.

34. Josephson Junction The problems were overcome through a joint effort between the U.S. and Germany and the first practical 1V Josephson standard was produced in 1985. Advances in superconductive integrated circuit technology made much larger arrays possible and by 1989 all of the hardware and software were commercially available for a complete Josephson voltage metrology system.

35. Josephson Junction

36. Josephson Junction 20,208 junction, 10V chip 10mm x 19mm

37. Josephson Junction Josephson voltage standard (JVS) components –A cryogenic system that includes a cryoprobe, which provides dc and microwave interface to the array and enables it to be cooled to about 4°K (-452°F, -269°C) in a liquid helium dewar. –A bias system that controls the array step number and enables the array I-V curve to be observed on an oscilloscope for setup and debugging. –A frequency-stabilized microwave source that supplies approximately 75 GHz power to the array.

38. Josephson Junction JVS system components continued –A measurement loop that enables calibration of secondary standards against the array. –A computer for system control to automate calibrations and other system functions.

39. Josephson Junction

40. Measurement Process –Preliminary measurement of unit under test with system DVM. –Computer calculates appropriate step number for measurement. Example: n=VK j /f n = (9.9999865V)(483597.9GHz/V) / 75GHz n = 64,480 –Computer biases the array to the appropriate step number using DVM measurement of array voltage **(DVM uncertainty must be < ½ step voltage).

41. Josephson Junction Measurement Process continued –System measures difference between array voltage and unit under test (multiple measurements). –Polarities are reversed and measurements repeated (largely eliminates errors due to thermal voltages). –System computes array voltage based upon known step number and known frequency. –V uut calculated using difference readings and computed array voltage.

42. Josephson Junction Measurement uncertainty –Evaluation of the total uncertainty involves frequency errors, leakage, detector errors, uncorrected thermal errors, etc. –The dominant uncertainty contributor in measurements using the JVS system is the short- term stability and noise from the unit under test itself.

43. Josephson Junction Measurement uncertainty continued –The expanded uncertainty for a single measurement of a typical zener reference using our JVS is 0.013ppm at the 10V level. –Measurement taken of a zener over a two-week period would have an expanded uncertainty typically around 0.06ppm at the 10V level.

44. Josephson Junction Benefits of the JJ –We are measuring our reference zeners with the same method used by NIST. –We have eliminated the risk of losing irreplaceable history as a result of shipping damage to our references. –The uncertainty due to pressure corrections has been eliminated due to in-house calibration. –Step shifts in voltage due to handling during shipping have been eliminated and our prediction uncertainty and overall uncertainties are lower.

45. Josephson Junction The INL participated in the 2002 NCSLI JVS comparison Results of 2002 NCSLI JVS comparison (10V) –INL - SNL = -39 nV± 201 nV (95% level of confidence) –INL - NIST = -29 nV ± 205 nV (95% level of confidence) –Differences of 0.0039 ppm and 0.0029 ppm respectively The INL participated in the 2005 NCSLI Josephson Voltage Standard comparison. –Results were satisfactory but not as good as 2002 due to a loss of zener power during shipping. (0.016 ppm) –Results were still well within our accredited uncertainty.

52. Josephson Junction References –Fluke 732B Manual (1994) –C.A Hamilton NIST JVS (2000) –NCSLI JVS RISP-1 (2002)

53. Josephson Junction Questions?