453 research outputs found
Resistively Shunted NbN/AlN/NbN Tunnel Junctions for Single Flux Quantum Circuits
AbstractWe have developed resistively shunted NbN junctions to realize superconducting single flux quantum circuits operating at 10K and/or high speed. The junctions consist of epitaxial NbN/AlN/NbN tunnel junctions and molybdenum (Mo) resistors fabricated on single-crystal MgO substrates. The junction qualities are systematically investigated in a wide range of critical current density (Jc). The gap voltage and the ratio of Rsg/RN were about 5.6mV and 11 for the junctions with the Jc of 10 kA/cm2, respectively. The overdamped Josephson junctions with parallel Mo resistors having a nominal sheet resistance showed non-hysteretic current-voltage characteristics for the junctions with Jc of 10 kA/cm2
Collective quantum phase slips in multiple nanowire junctions
Realization of robust coherent quantum phase slips represents a significant
experimental challenge. Here we propose a new design consisting of multiple
nanowire junctions to realize a phase-slip flux qubit. It admits good
tunability provided by gate voltages applied on superconducting islands
separating nanowire junctions. In addition, the gates and junctions can be
identical or distinct to each other leading to symmetric and asymmetric setups.
We find that the asymmetry can improve the performance of the proposed device,
compared with the symmetric case. In particular, it can enhance the effective
rate of collective quantum phase slips. Furthermore, we demonstrate how to
couple two such devices via a mutual inductance. This is potentially useful for
quantum gate operations. Our investigation on how symmetry in multiple nanowire
junctions affects the device performance should be useful for the application
of phase-slip flux qubits in quantum information processing and quantum
metrology.Comment: 12 pages, 6 figure
Towards understanding two-level-systems in amorphous solids -- Insights from quantum circuits
Amorphous solids show surprisingly universal behaviour at low temperatures.
The prevailing wisdom is that this can be explained by the existence of
two-state defects within the material. The so-called standard tunneling model
has become the established framework to explain these results, yet it still
leaves the central question essentially unanswered -- what are these two-level
defects? This question has recently taken on a new urgency with the rise of
superconducting circuits in quantum computing, circuit quantum electrodynamics,
magnetometry, electrometry and metrology. Superconducting circuits made from
aluminium or niobium are fundamentally limited by losses due to two-level
defects within the amorphous oxide layers encasing them. On the other hand,
these circuits also provide a novel and effective method for studying the very
defects which limit their operation. We can now go beyond ensemble measurements
and probe individual defects -- observing the quantum nature of their dynamics
and studying their formation, their behaviour as a function of applied field,
strain, temperature and other properties. This article reviews the plethora of
recent experimental results in this area and discusses the various theoretical
models which have been used to describe the observations. In doing so, it
summarises the current approaches to solving this fundamentally important
problem in solid-state physics.Comment: 34 pages, 7 figures, 1 tabl
High Density Fabrication Process for Single Flux Quantum Circuits
We implemented, optimized and fully tested over multiple runs a
superconducting Josephson junction fabrication process tailored for the
integrated digital circuits that are used for control and readout of
superconducting qubits operating at millikelvin temperatures. This process was
optimized for highly energy efficient single flux quantum (ERSFQ) circuits with
the critical currents reduced by factor of ~10 as compared to those operated at
4.2 K. Specifically, it implemented Josephson junctions with 10 uA unit
critical current fabricated with a 10 uA/um2 critical current density. In order
to circumvent the substantial size increase of the SFQ circuit inductors, we
employed a NbN high kinetic inductance layer (HKIL) with a 8.5 pH/sq sheet
inductance. Similarly, to maintain the small size of junction resistive shunts,
we used a non-superconducting PdAu alloy with a 4.0 ohm/sq sheet resistance.
For integration with quantum circuits in a multi-chip module, 5 and 10 um
height bump processes were also optimized. To keep the fabrication process in
check, we developed and thoroughly tested a comprehensive Process Control
Monitor chip set.Comment: 10 pages, 5 figures, 1 tabl
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