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Feldhoff, Frank; Töpfer, Hannes
Short- and long-term state switching in the superconducting niobium neuron plasticity. - In: IEEE transactions on applied superconductivity, ISSN 1558-2515, Bd. 34 (2024), 3, 1300105, insges. 5 S.

Bio-inspired algorithms and architectures are considered superior to classical architectures for certain applications. An important aspect with regard to the function of the human memory is the sorting according to important and unimportant experiences. Certain important experiences are stored significantly longer than less important ones. One criterion to make this distinction is the frequency of occurrence of a property. In this work an RSFQ circuit is presented, which performs this weighting in the learning process of a synapse. In a simulation study, the principle of the selective learning mechanism is shown to work and a variant of permanent memory is demonstrated.



https://doi.org/10.1109/TASC.2024.3355876
Schmelz, Matthias; Mutsenik, Evgeniya; Bravin, Sofia; Sultanov, Aidar; Ziegler, Mario; Hübner, Uwe; Peiselt, Katja; Mechold, Stephan; Oelsner, Gregor; Kunert, Jürgen; Stolz, Ronny
Wafer-scale Al junction technology for superconducting quantum circuits. - In: IEEE transactions on applied superconductivity, ISSN 1558-2515, Bd. 34 (2024), 3, 1701005, insges. 5 S.

Josephson tunnel junctions represent a key element in superconducting electronics and quantum circuits. For many years, shadow evaporation by means of Dolan-type bridges has been the state-of-the-art for deep sub- micrometer sized structures. Increasing demand in the number of Josephson junctions, e.g., in qubit circuits and travelling wave parametric amplifiers, requests for a wafer-scale fabrication process with precise control of junction parameters and have led to an advanced lift-off technique called Manhattan-type junction technology in recent years. Herein, we report on the development of a 100 mm wafer-scale fabrication technology for deep sub-micrometer sized Al Josephson junctions with linear dimensions down to 180 nm. The critical current IC of the junctions ranges from about 10 to 120 nA scaling with their linear dimensions. Low temperature transport measurements as well as room-temperature characterization has been used for IC and process homogeneity determination of series arrays of up to 50 Josephson junctions. We discuss technology parameters such as yield, on-chip and on-wafer reproducibility of the junction's critical currents as well as main process limitations. Moreover, we present experimental results on the characterization of first transmon-type qubits fabricated using this technology.



https://doi.org/10.1109/TASC.2024.3350580
Mechold, Stephan; Peiselt, Katja; Schmelz, Matthias; Oelsner, Gregor; Ziegler, Mario; Hübner, Uwe; Kunert, Jürgen; Stolz, Ronny
Towards fabrication of sub-micrometer cross-type aluminum Josephson junctions. - In: IEEE transactions on applied superconductivity, ISSN 1558-2515, Bd. 34 (2024), 3, 1101205, insges. 5 S.

The performance of superconducting electronic devices such as superconducting quantum bits (qubits) and superconducting quantum interference devices (SQUIDs) strongly relies on high-quality Josephson junctions (JJ) and their integration into surrounding circuit elements. Therefore, a corresponding fabrication technology should allow for the fabrication of all required elements including the JJs, inductances, capacitances and waveguides. For a long time, shadow evaporation technique was the state of the art for the implementation of sub-µm sized JJs based on aluminum for qubits of high coherence times. Although, the use of a single lithographic step represents a major advantage of this technique. However, shadowing effects limit sample size, device complexity, and thus scalability of the circuitry. To overcome these limitations and to meet the demands of next generation scalable quantum circuits, in this work we introduce our cross-type JJ aluminum technology, where JJs are defined by the overlap of two narrow perpendicular stripes. We discuss the technological challenges, with a focus on our newly developed dry etching process for patterning of the aluminum thin film. Compared to a lift-off based process, this advanced wafer-scale fabrication technology offers a high integration density and the required design flexibility. We will present first results on cross-type aluminum JJs.



https://doi.org/10.1109/TASC.2023.3343681
Kunert, Jürgen; Schmelz, Matthias; Peiselt, Katja; Oelsner, Gregor; Reddy, Soundarya Gopala; Ortlepp, Thomas; Stolz, Ronny
Advanced FLUXONICS process CJ2 based on sub-µm-sized cross-type Nb/AlOx/Nb Josephson junctions for mixed signal circuits. - In: IEEE transactions on applied superconductivity, ISSN 1558-2515, Bd. 34 (2024), 3, 1101105, insges. 5 S.

Quantum computers represent a prominent example of technology harnessing quantum phenomena for practical applications. Implementations based on superconducting solid-state qubits play a leading role. These have facilitated the implementation of the first commercially viable quantum computers through the use of well-established and scalable fabrication technologies. As the number of qubits in these systems is continuously increasing, there is an urgent need to advance wiring and integration methods. Specifically, the demand for high-frequency control and readout lines within the millikelvin coolers introduces unwanted heat load. As a scalable alternative, superconducting digital electronics has been proposed as a promising candidate for direct interfacing with superconducting quantum circuits. We, therefore, advanced our well-established cross-type, sub-micrometer-sized Nb-based Josephson junction technology for analog circuits to allow for the implementation of digital circuits. Thus, an advanced mixed signal process CJ2 hosts both, analog and digital circuits, on a single chip, using Josephson junctions of a wide critical current range. We discuss the technology and the realization of first circuits as well as results of basic logic gate and dc-SQUID operations. This advanced technology CJ2 enables the development of digital interfaces for quantum circuits by academic and industrial partners in the framework of the European FLUXONICS foundry.



https://doi.org/10.1109/TASC.2024.3355024
Stolz, Ronny; Schiffler, Markus; Becken, Michael; Schneider, Michael; Chubak, Glenn
The hunt for mineral resources with quantum magnetometers. - In: Technisches Messen, ISSN 2196-7113, Bd. 91 (2024), 1, S. 41-50

Quantum sensing provides advanced technologies which significantly improve sensitivity and accuracy for sensing changes of motion, gravity, electric and magnetic field. Therein, quantum sensors for the detection of magnetic fields, so-called quantum magnetometers, are one of the most promising technological realizations. We firstly will provide a brief overview on methods in geophysical exploration benefitting from quantum magnetometers with resolution at the physical and technical limit. We will introduce recent developments on SQUID and OPM based sensors as specific implementations of a quantum magnetometer systems and application examples.



https://doi.org/10.1515/teme-2023-0116
Pikushina, Alena; Centeno, Luis Fernando; Stehr, Uwe; Jacobs, Heiko O.; Hein, Matthias
Electrical lengths and phase constants of stretchable coplanar transmission lines at GHz frequencies. - In: Flexible and printed electronics, ISSN 2058-8585, Bd. 9 (2024), 1, 015005, S. 1-12

Elastic, bendable and stretchable electronics establish a new and promising area of multi-physics engineering for a variety of applications, e.g. on wearables or in complex-shaped machine parts. While the area of metamorphic electronics has been investigated comprehensively, the behavior at radio frequencies (RFs), especially in the GHz range, is much less well studied. The mechanical deformation of the soft substrates, for instance, due to stretching, changes the geometrical dimensions and the electrical properties of RF transmission lines. This effect could be desirable in some cases, e.g. for smart devices with shape-dependent transmission or radiation characteristics, or undesirable in other cases, e.g. in feed and distribution networks due to the variable electrical lengths and thus phase variations. This contribution describes the results of a systematic study of the broadband RF properties of coplanar transmission lines on Ecoflex® substrates, based on numerical simulations and experimental data. Two types of stretchable transmission line structures were studied: Meander- and circular ring-segmented lines. Modeling and simulation were performed combining a 2D circuit simulation software with electromagnetic full-wave simulations. The experimental part of the work included the fabrication of metamorphic substrates metallized with thin copper layers and systematic measurements of the electrical lengths and phase constants of coplanar waveguides in the frequency range from 1 to 5 GHz based on vector network analysis for different stretching levels. With the given substrate technology, we succeeded in demonstrating stretchability up to a level of 21%, while the theoretical limit is expected at 57%. The meander- and circular-shaped line structures revealed markedly different sensitivities to the stretching level, which was lower for circular structures compared to the meander structures by approximately a factor of three.



https://doi.org/10.1088/2058-8585/ad1efd
Gholamhosseinian, Ashkan; Seitz, Jochen
CAI2M2: a centralized autonomous inclusive intersection management mechanism for heterogeneous connected vehicles. - In: IEEE open journal of vehicular technology, ISSN 2644-1330, Bd. 5 (2024), S. 230-243

https://doi.org/10.1109/OJVT.2024.3354393
Rakhimov, Damir; Haardt, Martin
Analytical performance assessment of 2-D Tensor ESPRIT in terms of physical parameters. - In: IEEE open journal of signal processing, ISSN 2644-1322, Bd. 5 (2024), S. 122-131

In this paper, we present an analytical performance assessment of 2-D Tensor ESPRIT in terms of physical parameters. We show that the error in the r -mode depends only on two components, irrespective of the dimensionality of the problem. We obtain analytical expressions in closed form for the mean squared error (MSE) in each dimension as a function of the signal-to-noise (SNR) ratio, the array steering matrices, the number of antennas, the number of snapshots, the selection matrices, and the signal correlation. The derived expressions allow a better understanding of the difference in performance between the tensor and the matrix versions of the ESPRIT algorithm. The simulation results confirm the coincidence between the presented analytical expression and the curves obtained via Monte Carlo trials. We analyze the behavior of each of the two error components in different scenarios.



https://doi.org/10.1109/OJSP.2023.3337729
Müller, Erik; Petkoviâc, Bojana; Ziolkowski, Marek; Weise, Konstantin; Töpfer, Hannes; Haueisen, Jens
An improved GPU-optimized fictitious surface charge method for transcranial magnetic stimulation. - In: IEEE transactions on magnetics, ISSN 1941-0069, Bd. 60 (2024), 3, 5100104, insges. 4 S.

The fictitious surface charge method (FSCM) is used for the calculation of the induced electrical field in magnetic stimulation. The method was embedded and optimized in Python. It was designed to allow for the computation of large problems. An element-wise Jacobi method was combined with vectorized matrix operations to increase the parallelization capabilities and enable GPU computing. The induced fields are compared against an analytical solution for a homogeneous sphere and a FEM solution on a realistic head model. The results for both cases show that the normalized root mean square error of less than 0.5% can be achieved with the integral-free FSCM even on low-performance computer hardware.



https://doi.org/10.1109/TMAG.2023.3334747
Gherekhloo, Sepideh; Ardah, Khaled; Haardt, Martin
SALSA: a sequential alternating least squares approximation method for MIMO channel estimation. - In: IEEE transactions on vehicular technology, ISSN 1939-9359, Bd. 0 (2023), 0, S. 1-6

In this paper, we consider the channel estimation problem in sub-6 GHz uplink wideband multiple-input multiple-output (MIMO)-orthogonal frequency-division multiplexing (OFDM) communication systems, where a user equipment with a fully-digital beamforming structure is communicating with a base station having a hybrid analog-digital (HAD) beamforming structure. A novel channel estimation method called Sequential Alternating Least Squares Approximation (SALSA) is proposed by exploiting a hidden tensor structure in the uplink measurement matrix. Specifically, by showing that any MIMO channel matrix can be approximately decomposed into a summation of R factor matrices having a Kronecker structure, the uplink measurement matrix can be reshaped into a 3-way tensor admitting a Tucker decomposition. Exploiting the tensor structure, the MIMO channel matrix is estimated sequentially using an alternating least squares (ALS) method. Detailed simulation results are provided showing the effectiveness of the proposed SALSA method as compared to the classical least squares and linear minimum mean squared-error (LMMSE) methods.



https://doi.org/10.1109/TVT.2023.3347290