SUPERCONDUCTING INSTRUMENTATION FOR PRECISION MEASUREMENT AND CONTROL

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• Document Number (FOIA) /ESDN (CREST): CIA-RDP96-00789R003300190002-4
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• Document Creation Date: November 4, 2016
• Document Release Date: May 18, 2000
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Superconducting Instrumentation
for Precision Measurement and Control
R.V. Duncan
Sandia National Laboratories, Albuquerque, NM 87185, USA
Abstract. The Josephson series-array voltage standard (JAVS) has
dramatically improved the level to which high-accuracy electrical
equipment may be monitored and stabilized. For example, a small
pressure dependence in the output of some solid-state voltage standards
has. been measured with the JAVS. The use of two such series-arrays
in a superconductive loop promises to be a new method of ultra-small
frequency difference measurement. The accuracy of these new
measurement configurations is set by the superconductive loop induct-
ance and the Josephson junction parameters.
Series-arrays [1] of Josephson junctions, all biased by a common
microwave source, are in use within approximately twenty laboratories
throughout the world. as an intrinsic voltage standard. At Sandia
National Laboratories (SNL), and similarly at many other laboratories,
this standard is routinely used to track the drift, accuracy, and linearity
of digital volt meters (DVM), voltage reference standards, and voltage
calibrators to an uncertainty better than 0.1 ppm. In collaboration with
the National Institute of Standards and Technology (NIST, formerly
NBS) we have used this technology to detect small systematic
variations in the output of same solid.-state voltage standards with
elevation. Such refinements in measurement technology would be
impossible without this wide-spread use of the JAVS. Recently a
powerful new measurement technique utilizing these series-arrays of
Josephson junctions has been developed [2]. Here two such arrays
have been placed in series-opposition in a superconductive measure-
ment loop which is inductively coupled to a SQUID. We [3] have
analyzed this interferometer circuit within the framework of the
Steward-McCumber model [4] to determine the measurement accuracy
[5]. This circuit, which we refer to as a macroscopic quantum circuit
(MQC) due to its superconductive coherence throughout the loop, may
prove useful as an ultra-accurate interferometric readout at millimeter
wavelengths for such applications as gravity wave detection and
Sagnac-effect gyroscopes [6]. The full utilization of a MQC requires
446 Springcr Proccccfiags in Physiati, Vol. G4 SopcrconducUug Devices anJ Thclr Appllcsdions
Editors: H. Koch, H. Liibbig ?Springer-Vcrlag Berlin Itciddbcrg 1992
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the development of a superconductive positioner to fine-tune cavity
oscillators used to bias the Josephson junctions and possibly to tune the
MQC frequency response directly.
The SNL Josephson voltage standard is similar to that used by NIST
[7] except that it incorporates cryogenic (rather than room-temperature)
passive filtering [8], and it is cooled continuously by a refrigerated
Dewar [9] which maintains the standard at 4K with na helium loss or
routine operator intervention. In the near future this refrigerated Dewar
will be used to simultaneously support the Josephson array and to re-
liquify the 4He boil-off from a nearby quantum Hall effect resistance
standard.
2. Environmental effects on solid-state voltage standards
Solid-state valtage standards (SSVSs) utilizing Zener diode references
are in common use today for do valtage metrology [10]. The JAVS
provides an extremely accurate measurement system against which
these SSVSs may be critically evaluated. The thermal coefficient of
many SSVS have been measured against the JAVS and found to be
typically 17 ppm/K. Since the set temperature of the SSVS enclosure is
stable to better than 2 mK, and when intenrianally over-heated it returns
to its set point to within 5 mK, thermal effects create inconsequential
errors in all the SSVSs observed to date. A systematic variation in
some SSVS with elevation (and hence pressure) has been detected.
One such SSVS, designated SSVS#1, was transported between four
locations within the United States during 1989. This device was
operated at elevations ranging from sea-level to 1,500 meters and no
elevation or pressure dependence was observed in its output. SSVS#1
displayed excellent repeatability and stability as measured against the
JAVS from location to location (once its drift rate was taken into
account, SSVS#1 typically deviated by no mare than 0.1 ppm during
shipment). Na variation of the JAVS has been observed during
controlled temperature and pressure changes [11].
In August, 1990 another SSVS (called SSVS#2) was shipped round-
trip between SNL in Albuquerque and NIST in Gaithersburg. At each
location the device was measured against a Josephson voltage standard.
The average of its four 1.018V taps was 0.98 ppm higher at
Gaithersburg than at Albuquerque - a factor of ten larger in variation
than the SSVS#1 result! A comparison of the value of SSVS#2 after its
return from NIST to its value before it was shipped agreed to within
0.15 ppm. In order to check for a pressure dependence in the SSVS#2
output, bath SSVS#1 and SSVS#2 were measured in a mobile
laboratory at an elevation of 3,260 meters (on the summit of Sandia
Crest, which is located 50 krn from SNL) in collaboration with S. L.
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Kupferman. These measurements were made against ahigh-accuracy
8.5-digit DVM which was calibrated against the JAVS immediately
before and immediately after the mountain measurements. SSVS#2
experienced a -3.40 ppm deviation in the average of its four 1.018V
taps with the 1,760 meter elevation gain, while SSVS#1 displayed an
incansequeritial +0.8 ppm deviation. Given the previous performance
of SSVS#l, this deviation was likely due to a systematic change in the
DVM gain with elevations. Only the 1.018V taps of SSVS#2 displayed
this variation with elevation, indicating that a resistive voltage divider
within SSVS#2 was most likely the source of this systematic error.
These results serve as a warning that all transportable voltage standards
used in critical metrology applications must be individually qualified
through their full range of environmental conditions against a JAVS. In
the future we will use our JAVS and an automated pressure/
temperature/ humidity-controlled chamber to make these critical
environmental qualification measurements. Notice also that these very
small systematic effects could not have been detected without the JAVS.
3. Macroscopic Quantum Circuit Measurements
Consider two rf-biased. series-arrays connected in series-opposition
with superconductive wire, forming the MQC with a total self-
inductance L. The supercurrent is in the MQC which builds up over
time t due to the difference in electric potential 0V between the two rf-
biased devices is given by
(1)
This supercurrent is then read out by a SQUID which is magnetically
coupled to the MQC. This method of ultra-precise potentiometry was
developed by Clarke [ 12], and more recently used by Jain et al. [ 13] to
obtain measurements with a precision of OV/V ~ 3x10-19 using single
rf-biased junctions operating at about 300 ?V. Kautz and Lloyd [2]
have used. this technique to compare the voltage output of two series-
arrays, each containing 2,076 Josephson junctions and biased to V~ 1
volt, to a pre;cisian of OV/V ~ 2x 10-17. In all the above measurements
both Josephson devices were biased by the same rf source to check for
deviations fi?arn is = 0 when both Josephson devices were biased to the
same step (n). Other experirnents[14] have been performed which used
two different rf sources of known detuning (~cu) to bias the junctions
and thus to generate an increasing is consistent with Eq. 1 and the
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prediction that DV = ~Vo = nhOc:~/4~e, where h is Planck's constant,.
and a is the magnitude of the charge of the electron. This method. of
measurement may provide a powerful new method of detecting very
tiny frequency differences generated in gravity wave detectors and
Sagnac-effect gyroscopes [6].
Referring to Eq. 1, it is clear that an arbitrarily precise measurement
of 0V may be made by either decreasing L, increasing the length of the
measurement interval, or both. The value of the self-inductance (L) of
the MQC need not be limited by the SQUID's input inductance [13].
Due to the interaction of the two Josephson devices via the
superconductive loop, the measured 0V is not simply equal to OVo =
nh~co/4~e. Rather there exists a correction which depends on the value
of L, and which becomes large as L approaches the Josephson
inductance LJ = h/2eIc~ where Ic is the junction's critical current.
These corrections, which have been calculated within the framework of
the Steward-McCumber model [4] in the absence of external noise, are
detailed in the proceedings of ISEC'91 [5].
These JAVS may be used as an ultra-stable voltage source provided
that the load is very quiet and that the compliance current never exceeds
the voltage state's critical current (which is an the order of 100 ?A). In
such applications (which may include ultra-accurate drift tube
measurements at cryogenic temperatures) the voltage resolution and
stability is directly determined by the linewidth and center-frequency
stability of the biasing oscillator. Conventional microwave
locker/counters can servo the center frequency of Gunn diode
oscillators to within a few Hertz out of 7S - 95 GHz. Superconductive
cavity oscillators have been developed which have Q > 106 over their
full tuning range [15]. This cavity oscillator may be tuned with a
proposed ultra-stable superconductive positioner [ 16] in order to obtain
about a factor of one thousand improvement over conventional center-
frequency stability.
Acknowledgement
This work is supported by the U.S.A. Department of Energy under
Contract Number DE-AC04-76DP00789.
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References
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and R. E. Harris, IEEE Elec. Dev. Lett. 8, 449 (1987); R.
Popel, J. Niemeyer, R. Fromknecht, W. Meier, L. Grimm, and
F. W. Dunschede, IEEE Trans. Instrum. Meas. 40, 298 (1991);
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2. R.L. Kautz and F.L. Lloyd, Appl. Phys. Lett. 51, 2043 (1987).
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4. D. E. McCurnber, J. Appl. Phys. 39, 3113 (1968); W. C.
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5. D. H. Dunlap and R. V. Duncan, Procedings of ISEC'91,
Glasgow, (1991).
6. L. Z. Wang and R. V. Duncan, in preparation.
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8. R. V. Duncan, Bull. Am. Phys. Soc. 34, 1535 (1989).
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16. R. V. Duncan, in preparation.
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