Instrumentation & Measurement Magazine 23-2 - 4

Present and Future of HighTemperature Superconductor
Quantum-Based Voltage Standards
Alexander M. Klushin, Jérôme Lesueur, Marian Kampik, Felix Raso, Andrea Sosso,
Sergey K. Khorshev, Nicolas Bergeal, François Couëdo, Cheryl Feuillet-Palma, Paolo
Durandetto, Michal Grzenik, Krzysztof Kubiczek, Krzysztof Musiol, and Artur Skorkowski

his paper presents a brief overview of the current state-of-the-art of Josephson junctions for
Quantum-based Voltage Standards fabricated with
High-Temperature Superconductors (HTS). A short introduction on the history and technical evolution of Low Temperature
Superconductors (LTS) technology is provided for non-specialists. Then HTS technology is summarized and discussed in
the context of quantum voltage standard applications. Finally,
the two most promising technologies: bicrystal and Focused
Helium-Ion Beam junctions are discussed with more detail,
analyzing strength, limitations and perspectives in both cases.

Josephson Junctions in Metrology
The application of quantum effects to electrical metrology in the
last decades has brought an extraordinary improvement in the
accuracy of primary standards. As an example, the volt, the unit
of electromotive force, can be nowadays reproduced with an
accuracy better than one part per billion. This is a breakthrough
in the primary metrology of electrical units following the possibility of directly linking, through quantum phenomena, the
value of the two standards to the fundamental constants of
physics: Josephson effect for the volt and quantum Hall effect
for the unit of resistance, the ohm. At the same time, the latest
achievements in science and industry radically changed instrumentation technology. In modern production, high-precision
digital measuring systems are being widely used. Therefore,
the requirements for the accuracy of calibration equipment
have increased. Thus, it becomes very important to create standards and references of physical quantities on the basis of
quantum effects for the practical metrology, too.
The basis for using the Josephson effect [1] in quantum
metrology is the fact that under the influence of an external
electromagnetic field with a frequency f on the current-voltage
characteristic (IVC) of the junction, current steps occur at voltages:

Vn = n

f
(1)
KJ

where KJ ≡ 2e/h is known as Josephson constant and since
the revision of SI system of units in 2019 has value of
4

483 597.848 4 GHz/V [2], h is the Planck constant, e is the elementary charge and n is the integer.
The validity of equation (1), its independence from the
experimental conditions (materials, temperature, type of Josephson junctions) and, therefore, its fundamental nature is
a principal condition for the use of Josephson junctions (JJs)
in metrology. In the most accurate experiments [3], it was
shown that KJ in various low temperature superconductors
at the liquid helium temperature T = 4.2 K coincides with relative uncertainty better than 3×10−19. Thus, the dc voltage VJ
is determined only by quantum constants and the irradiation
frequency. Currently, other physical effects that could compete
with the Josephson effect in quantum metrology with the accuracy of reproducing a voltage unit are not known.
Large arrays of tens of thousands synchronized LTS JJs with
output voltages up to 10 V are used in different types of dc and
ac voltage standards. Readers interested in the detailed operation of such systems are referred to the review giving the
state-of-the-art performance, best practices, and current impact of these standards on voltage metrology [4].
The discovery of Bednorz and Müller in 1986 [5] of hightemperature superconductors and the subsequent discovery
of Chu and collaborators [6] of superconductivity in yttriumbarium ceramics YBa2Cu3O7-x (YBCO) at temperatures above
the boiling point of liquid nitrogen (77 K) initiated work on
the creation of the JJ from these materials [7]. Among them we
should highlight bicrystal junction technology invented by the
IBM team led by Chaudhari [8]. HTS JJs are of considerable interest for quantum voltage metrology dissemination, thanks to
their higher operating temperature. The state-of-the-art HTS
technology of bicrystal junction circuits allows fabrication of
arrays containing hundreds of JJs with an output voltage up
to 0.1 V [9].

Application of Low-temperature
Superconductor Josephson Junctions
Josephson Voltage Standards based on LTS JJs have evolved
significantly from the initial development in the early 1970s.
The first standards were based on a single junction, or a small

April 2020

Instrumentation & Measurement Magazine 23-2

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