10 research outputs found
Robust quantum storage and retrieval in a hybrid system by controllable Stark-chirped rapid adiabatic passages
Evidence for interacting two-level systems from the 1/f noise of a superconducting resonator
Two-color second-order sideband generation in an optomechanical system with a two-level system
Correlating the nanostructure of Al-oxide with deposition conditions and dielectric contributions of two-level systems in perspective of superconducting quantum circuits
Introductory Notes on the Josephson Effect: Main Concepts and Phenomenology
Josephson predicted the existence of tunnel currents carried by Cooper pairs between
two superconductors SR and SL separated by a thin (typically about 1 nm) insulating
layer I [1], and paved the way to the study of a series of interesting phenomena associated
with this coherent flow of Cooper pair currents [2, 3]. The whole story from
the first tunnel Josephson junctions in the 1960ies using soft superconductors as Sn,
In, Pb, and thermal oxidation for the barrier, and the subsequent “lead-alloy technology”
with the first self-limiting sputter-oxidation process, to the more mature class
of devices based on “rigid” superconductors as Nb, is perfectly accounted by the two
main textbooks published in the 1980ies by Barone and PaternĂł [2] and Likharev
[3], respectively. Artificial barriers replacing Nb oxide barriers were the key towards
the development of the Nb technology. Al revealed as the perfect solution forming a
natural, self-limiting, high quality, insulating oxide [4]. Other rigid superconductors
were NbN, Nb3Sn, V3Si and Nb3Ge. All of them needed artificial barriers. From
the historical point of view, the use of rigid superconductors had definitely overcome
some problems of stability in thermal cycling of lead-alloy based junctions.
A detailed recent account on the history of the first developments of the Josephson
junctions is also given in [5]. It was not only the first search of suitable novel materials
and barriers, but also of the appropriate processing techniques and layouts
[2, 3, 6–8]. In the 1980ies the integrated thin film Superconducting QUantum Interference
Device (SQUID) was introduced [9]. Many junctions were integrated with
thin film resistors and thin film transmission-line interconnections into complex,
monolithic, integrated circuits (ICs). The higher critical current density (Jc) leads to the necessity of reducing the area of the junctions to meet requirements on junction
impedance, promoting special geometry such as edge-type junction or e-beam lithography
in sandwich or planar layout. The technology of NbN was the first attempt to
increase operating temperature of Josephson junctions [10]. Since early times it was
clear that the development of superconducting devices based on the Josephson effect
needed to proceed on three levels: basic physics, device and circuit innovation, and
materials science and processing development.
The impact of high critical temperature superconductors (HTS) was also impressive
for the development of activities on Josephson devices [11–14]. It was amazing
not only for the opening of new horizons in solid state physics but also for the development
of novel notions and ideas in superconducting electronics, possibly operating
at higher temperatures. HTS gave clear awareness of a new era where a more indissoluble
link between superconductivity and material science clearly appeared. All
unconventional materials after HTS have followed the same conceptual and experimental
workflow to codify their unconventional phenomenology, and specifically
also those notions that have been helpful for the realization of a Josephson device.
This obviously includes innovative methods of building barriers in intrinsically
non homogeneous materials. In the meanwhile the advent of mesoscopic physics
also in superconducting systems was changing some conceptual paradigms on how
to approach the problem of coherent transport in superconducting junctions, and
nanotechnologies started offering new experimental tools to build completely new
families of devices.
The modern era of Josephson devices is thus strongly influenced by the combined
continuous progress in material science and nanotechnologies applied to superconductivity.
These aspects are tightly connected. Progress in material science means
newmaterials and newsuperconductors, and novel abilities in building interfaces and
in the precise control of heterostructure in the growth process. Also barriers of tunnel
junctions are designed and fabricated with unprecedented precision, opening the
route tomore performing devices even for technologies based on well established low
critical temperature superconductors (LTS). Advances in nanotechnologies applied
to superconductivity is a necessary tool towards several material science solutions,
scaling barriers and interfaces, handling pre-built barriers as for instance nanowires
(NWs) and flakes. Hybrid junctions are an obvious consequence of the combined
progress of material science and nanotechnology.
In conclusions, we have never had so many different families of superconducting
materials and so many different types of Josephson junctions as nowadays, with so
many open questions on their nature and ultimate limits of their performances. Part of
their future depends on the ability of combining the unique features and advantages
of Josephson devices with the functionalities of the barriers