Instrumentation & Measurement Magazine 23-2 - 10

series, that were well synchronized, which is mandatory for
voltage standards applications. Preliminary results on YBCO in
the ESPCI Paris group point towards the same direction.
Clearly, arrays of YBCO JJ made by the He FIB technique
have great advantages to make high-Tc AC voltage standard
such as PJVS and JAWS or Quantized Voltage Noise Source
(QVNS). The latter application requires a moderate number
of JJs, but they are demanding in terms of RF design, since
for their operation, bias currents produced by short pulses
instead of a sinusoidal RF signal are used. The He FIB JJ technique allows the fabrication of compact and complex 1D and
2D circuits that fit these specifications. The large flexibility
in the JJ parameters is an asset as well. For instance, QVNS
circuits require high critical current (up to 2 mA) with low
IcRn product (20 to 40 μV) at the same time, which is difficult
to achieve with other fabrication techniques such as Grain
Boundary JJ, for example. It is also possible to tune the parameters of each individual JJ in the array, which opens new
and unique perspectives for advanced design of short JJ arrays for metrological applications. Finally, the possibility of
placing JJ at very short distances gives access to new synchronization schemes that may have a great interest for those
applications.

References
[1]	 B. D. Josephson, "Possible new effects in superconductive
tunneling," Phys. Rev. Lett., vol. 1, pp. 251-253, 1962.
[2]	 "Fundamental Physical Constants," NIST. [Online]. Available:
https://physics.nist.gov/cgi-bin/cuu/Value?kjos.
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gravitational effects on charged particles," Phys. Rev. Lett., vol. 58,
pp. 1165-1167, 1987.
[4]	 A. Rüfenacht et al., "Impact of the new generation of Josephson
voltage standards in ac and dc electric metrology," Metrologia, vol.
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[5]	 J. Bednorz and K. Muller, "Possible high Tc superconductivity in
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[6]	 C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, and
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9, s. 911-912, 1987.
[7]	 P. Seidel, Ed., Applied Superconductivity: Handbook on Devices and

Conclusions

Applications. Hoboken, NJ, USA: John Wiley & Sons, 2015.

For the development of LTS Voltage standards, the tremendous effort spent on research has led from single junction
devices to the development of a sophisticated superconductive technology that made arrays with thousands of junctions
available in many laboratories as quantum accurate dc voltage
standards. Successively, the development of programmable
standards, and more recently of pulsed standards, ac quantum
volt metrology has become a reality at acoustic frequencies for
both sinewaves and arbitrary signals, and it is about to reach
the RF domain.
Recent results obtained with arrays of HTS JJs operating
at liquid nitrogen temperature make it possible to overcome
issues related to liquid helium cryogenics and can even work
with closed-cycle refrigeration systems of limited size, low
consumption and low cost. Thanks to these characteristics,
an HTS Josephson quantum voltage standard being integrated in widely-used equipment, like digital multimeter, is
now foreseeable. A first prototype of compact cryocooled dc
Josephson standard is in fact currently available on the market [39], yet many relevant problems are still to be solved
to fully take advantage of the potentialities of HTS voltage
standards.
It seems that further research should be concentrated on
two fronts: the investigation of possible effects of the superconducting materials and novel technologies at unprecedented
accuracies; and the development of new sophisticated methods to apply HTS standards in fundamental metrology. An
example of such devices, which look promising for metrological applications, are the high-Tc JJs made using the low energy
helium FIB microscope. The accurate control of the individual
junction parameters on the one hand, and the large freedom
10	

to design complex and compact 1D or 2D short arrays on the
other hand, open new perspectives to fabricate ac voltage standards and noise sources with high-Tc superconductors. Also,
the future applications of the HTS arrays in PJVS are broad.

[8]	 P. Chaudhari et al., "Direct measurement of the superconducting
properties of single grain boundaries in YBa2Cu3O7−δ," Phys. Rev.
Lett., vol. 60, no. 16, pp. 1653-1656, 1988.
[9]	 A. Klushin, M. He, M. Levitchev, V. Kurin and N. Klein,
"Optimization of the coupling of mm wave power to arrays of
high-Tc Josephson junctions," J. Physics: Conf. Series, vol. 97, pp.
012268, 2008.
[10]	M. T. Levinsen, R. Y. Chiao, M. J. Feldman, B. A. Tuker: "An
inverse ac Josephson effect voltage standard," Appl. Phys. Lett.,
vol. 31, 776, 1977.
[11]	M. Gurvitch, M. A. Washington, and H. A. Huggins, "High
quality refractory Josephson tunnel junctions utilizing thin
aluminum layers," Appl. Phys. Lett., vol. 42, pp. 472-474, 1983.
[12]	J. Niemeyer, J. H. Hinken, and R. L. Kautz, "Microwave-induced
constant-voltage steps at one volt from a series array of Josephson
junctions," Appl. Phys. Lett., vol. 45 pp. 478-480, 1984.
[13]	"Primary Voltage Standard Josephson Junction Arrays," Hypres.
[Online]. Available: https://www.hypres.com/products/
primary-voltage-standard-josephson-junction-arrays/.
[14]	W. H. Parker, D. N. Langenberg, A. Denenstein, and B. N. Taylor,
"Determination of e/h using macroscopic phase coherence in
super-conductors," Phys. Rev., vol. 177, p. 639, 1969.
[15]	T. F. Finnegan, A. Denenstein, and D. N. Langenberg,
"AC-Josephson-effect determination of e/h: a standard of
electrochemical potential based on macroscopic quantum phase
coherence insuper conductors," Phys. Rev. B, Condens. Matter, vol.
4, p. 1487, 1971.
[16]	"Recommendation 1 (CI-2002): Revision of the practical
realization of the definition of the metre," 91st Meeting of the
CIPM, pp. 194- 204. [Online]. Available: https://www.bipm.org/
utils/en/pdf/CI-2002-1-EN.pdf.

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