Superconducting circuits are one of the most promising realizations for quantum technologies especially for quantum computing. Corresponding research must consider both: the quantum bit (so-called qubit) itself, which is often implemented as a transmon-type qubit for large scale applications, and the qubit's on-chip environment. Therein, coplanar waveguide resonators and coplanar transmission lines are mostly used for addressing and reading out the transmons. The coplanar resonators need to have high intrinsic quality factors which significantly depend on the material and the fabrication process. Consequently, a profound knowledge of the fabrication process is mandatory. For an increasing number of qubits on a chip and, thus, more complex circuit designs, beyond-2D integration technologies are required to suppress unwanted slotline modes due to anharmonicities and discontinuities within the circuits ground planes. Superconducting low-inductance air bridges can ensure this feature by equalizing the ground planes' potential. The aim of this work is to describe our recent progress in wafer-scale fabrication of beyond-2D superconducting quantum circuits. We show a combined study of resonator circuits and aluminum air bridges. We discuss our optimized fabrication process for resonator circuits with intrinsic quality factors up to approx. Qi = 2,000,000 at high photon numbers as well as the implementation of air bridges into resonator circuits. Finally, we demonstrate the successful implementation of air bridges in context of open quantum systems.
The pursuit of higher magnetic fields is a key objective in particle accelerators, nuclear magnetic resonance (NMR) spectroscopy, and fundamental physics research. The canted-cosine-theta (CCT) magnet is well suited for insert magnets owing to its high magnetic field quality, favorable mechanical properties, and reduced conductor peak stress. These advantages are critical for the reliable operation of strain-sensitive superconductors such as Nb3Sn. This study presents the magnetic design and mechanical design of an Nb3Sn CCT dipole insert magnet with a 7.5 T central magnetic field under a 6.5 T background magnetic field. Stress analyses under a 6.5 T background field are performed using finite element method (FEM). Subsequently, the Nb3Sn CCT dipole magnet was fabricated and cold tested at 4.2 K in liquid-helium. During the first cold test, the magnet reached a current of 1465 A without quench. This study presents the Nb3Sn CCT dipole magnet fabrication process and the cold test results.
Although the ripple loss of self-shielding high-temperature superconducting (HTS) DC cables have been studied previously, their thermal stability in comparison with conventional coaxial bidirectional DC cables has not been systematically analyzed. For further investigating the operation stability of self-shielding HTS cables, a three-dimensional (3D) coupled electromagnetic‑thermal model is established to compare the thermal stability characteristics of the two configurations. Both cables have a four‑layer structure and are wound with 48 HTS tapes, each 2 mm wide, 0.1 mm thick, and with the critical current of 80 A at 77 K. The results indicate that the self‑shielding cable not only exhibits a higher overall critical current (e.g., 1700 A for self‑shielding cable and 1600 A for conventional cable), but also shows higher minimum quench energy (e.g., at 1350 A, its MQE is about 12.5% higher than that of the conventional cable) and slower quench propagation velocity (e.g., at 1350 A, its QPV is about 35.3% lower), thus exhibiting a clear advantageous trend in thermal stability. This work provides critical technical insights for designing and optimizing high-stability, high-safety HTS DC cables, addressing a key need in the field.
We report the design, fabrication, and characterization of a cryogenic blackbody calibrator intended for laboratory-based detector characterization, providing multi temperature calibration of millimeter- and submillimeter-wave detectors over a temperature range of 4 K to ∼ 10 K. The device employs a Stycast/SiC absorber shaped for uniform array illumination and mounted on a phosphor-bronze substrate with optimized thermal links. Measured thermal conductance and heat capacity show the expected temperature dependence, although the conductance was lower than estimated, yielding a thermal time constant of 8.5 s at 10 K versus the 3.7 s design estimate. The response remains practical, demonstrating the calibrator's effectiveness as a stable reference source for precise calibration of detector arrays.
In the frame of the SPICA (ESA/JAXA) telescope project, we investigated the feasibility of a polarimetric camera, the B-BOP instrument, designed to study the magnetic field at different scales in the universe in the far infrared band (50 - 500 μm). A polarimetric detector array prototype was developed to address the 100-μm band. It is made of 16x16 pixels with a pitch of 750 μm and is fabricated on top of a Read Out Integrated Circuit (ROIC) CMOS wafer. Each pixel consists in four active and four reference bolometers arranged in a double Wheatstone bridge configuration. Two kinds of polarimetric pixels (0°/90° and -45°/+45°) were integrated into the array in order to retrieve the linear Stokes coefficients without the need for a rotating polarizer or half-wave plate. At a wavelength of 100 μm, the detectors exhibit strong electromagnetic absorption (>90%), high responsivity (1–3x1011 V/W), and a total measured Noise Equivalent Power (NEP) of 8x10-18 W/Hz0.5. The electrical cut-off frequency is approximately 2 Hz (-3 dB), measured under an optical load of a few femtowatts at an operating temperature of 50 mK.
Compact high turns ratio superconducting trans formers are used to transfer power between low current copper/semiconductor based circuits and high current superconductor based circuits. This type of transformer is a key component in devices such as superconducting transformer rectifiers and superconducting current source inverters. Typically the copper winding and the superconducting winding are arranged with a turn ratio above 100:1 respectively and are situated entirely within a cryogenic environment. The ohmic losses associated with the high count copper winding is problematic for most cryogenic applications and demands attention even when low currents on the order of 1- 10 A are used. To address this problem, a transformer has been made using REBCO High Temperature Superconductor (HTS) coated conductor for both the primary and secondary windings. The coupling coefficient and power consumption of this transformer were measured and compared to an equivalent transformer consisting of a copper primary winding and HTS secondary winding. The HTS primary was found to provide a significant reduction in transformer power consumption and a slight improvement to transformer coupling at frequencies below ∼8 Hz. An increased transformer core loss and winding AC loss began to negate these efficiency improvements above ∼8 Hz. Additionally, co winding the primary and secondary windings provided a higher coupling coefficient and lower power consumption for all winding combinations and frequencies tested. Finally, a short discussion on the implications of these findings respect to superconducting transformer rectifiers contextualizes the findings.
π-Josephson junctions based on ferromagnetic barriers enable energy- and area-efficient superconducting digital circuits. The critical current density Jc of superconductor-ferromagnet-superconductor (SFS) junctions is highly sensitive to barrier thickness, magnetic scattering, and temperature, making reliable π state operation dependent on fabrication and thermal variations. In this paper, the sensitivity of Jc to temperature and fabrication process parameters is quantified for SFS junctions with weak ferromagnetic barriers. A methodology is developed to determine allowable parameter margins for junctions used as phase shifters or active switching elements. An algorithm is introduced to generate design guidelines in terms of acceptable ranges of thickness and spin-flip scattering for a target Jc. Nominal barrier thickness regions are identified in which strong sensitivity to variation and second harmonic contribution in the current-phase relationship can compromise stable circuit operation. These results provide practical guidelines for robust π-junction operation in superconducting digital circuits.
CUPID (CUORE Upgrade with Particle IDentification) is a next-generation experiment searching for neutrinoless double beta decay ($0\nu 2\beta$) in $^{100}$Mo to probe the Majorana nature of neutrinos. It employs Li$_{2}$MoO$_{4}$ scintillating crystals at millikelvin temperatures, coupled with thin Ge bolometric light detectors for simultaneous heat and light readout. This work presents recent progress in the development of Ge light detectors enhanced via the Neganov-Trofimov-Luke (NTL) effect, which substantially improves the signal-to-noise ratio in the light channel. This improvement is crucial for suppressing background from pile-up events arising due to the slow bolometric response and the relatively fast $^{100}$Mo $2\nu 2\beta$ decay. We report on the fabrication and preliminary testing of NTL-enhanced HPGe light detectors, detailing process steps, surface preparation methods, and strategies implemented to reduce leakage current. Future integration of NTL-enhanced light detectors is important to assist in CUPID's goal of uncovering the fundamental nature of neutrinos.
Lanzhou Ion Therapy Company, Ltd. (LANITH) began the research and development of miniaturized heavy ion therapy devices since 2022. The Gantry superconducting magnet, which composed of a curved dipole magnet and two quadrupole magnets at both ends of the dipole coil, is the core component of the whole device. At present, this project has completed manufacturing and cryogenic tests for a prototype and a batch of gantry magnets. In order to test magnet winding and impregnation process, a gantry prototype magnet without iron core was processed and cryogenic tested at first, and followed with a batch of gantry magnets with iron core. This paper first provides a detailed introduction to the measurement system for the magnetic field homogeneity of Gantry superconducting magnets, along with the measurement results and analysis of integral field homogeneity at room temperature. It then describes the superconducting magnet testing and quench protection system, conducts a comprehensive evaluation of the performance of multiple magnets during testing, and proposes improvement measures. After completed cryogenic test in liquid helium, the Gantry magnets are planned to be loaded into a conduction cooling cryostat for extra testing of cooling process, AC loss and magnetic field uniformity.
High-Temperature Superconducting (HTS) cable is a promising technology for high-capacity and low-loss power transmission in future grids. Intermediate joints are critical components in HTS cable systems because they must simultaneously provide low-resistance conductor connection, reliable electrical insulation, mechanical support, cryogenic continuity, and vacuum sealing. This paper presents the design, fabrication, and testing of a 10 kV/2.5 kA three-phase concentric HTS cable joint prototype. The joint was developed for a practical concentric HTS cable system and was designed to satisfy the constraints of field installation. An internal bridging scheme was adopted for the former to improve alignment, prevent contamination of the flow channel, and enhance local mechanical stiffness. For the HTS phase conductors, a two-step pre-soldering and bundle-soldering method was developed to achieve compact, smooth, and mechanically reinforced multi-tape connections. Stress cone based Polypropylene Laminated Paper (PPLP) restoration and half-shell cryostat connectors were designed for electrical insulation and cryostat integration, respectively. A complete prototype system was fabricated and tested according to IEC 63075. The prototype passed the pressure test, vacuum leak rate test, power frequency and lightning voltage withstanding tests, and current carrying test. These findings firmly validate the feasibility and robustness of the proposed joint design, providing a practical foundation for the HTS cable system maintenance and longer distance deployment of future three-phase concentric HTS cable systems.
The cosmic microwave background (CMB), relic light emitted after the Big Bang, is a key probe of the early universe and inflationary theory. The Cosmic Microwave Background Stage Four (CMB-S4) project was intended as a ground-based telescope survey to map the temperature and polarization of the CMB with over 500,000 detectors. We report measurements from the first end-to-end test of prototype CMB-S4 detector and readout modules. The detector wafers consisted of 90 and 150 GHz transition-edge sensor (TES) detectors coupled to conical feedhorns via ortho-mode transducers (OMTs) and operated at 100mK inside a dilution refrigerator. Signals were read out using a superconducting quantum interference device (SQUID)-based time-division multiplexing (TDM) system. To support CMB-S4's plans for eight different types of detector wafers, the cryogenic readout electronics were modular. We report on the prototype design and performance, including characterization of TES response, readout performance, and optical response measured with a cryogenic cold load (readout here meaning everything that moves signals from detectors to external computers). We also report on the performance of a convolutional neural network to identify valid loadcurves for TES parameter characterization as part of TES characterization software; this software was used to characterize TESs to test the readout chain and identify problems in the test bed. We discuss identifying heating in and heatsinking a large detector module assembly. These tests show that the prototype CMB-S4 detector wafer and readout chain function as intended. While CMB-S4 will not move forward as planned, its design, prototyping, and test beds will benefit the development of future experiments.
Conduction-cooled no-insulation (NI) ReBCO magnets are attractive for compact high-field systems because they combine cryogen-free operation with intrinsic quench protection. Their field temporal stability, however, is limited by the coupled effects of screening-current-induced field (SCIF) relaxation and charging delay caused by turn-to-turn current sharing. This paper investigates a 50 K, 2 T desktop ReBCO magnet composed of 13 double-pancake (DP) coils and proposes an overshooting-current-profile optimization method for suppressing post-excitation field drift. A 2-D axisymmetric homogenized model, based on the T-A formulation and a radial voltage-control equation, is developed to calculate both screening-current evolution and NI current redistribution. A surrogate-model-based optimization procedure, combining Latin hypercube sampling and Gaussian process regression, is then used to analyze the effects of overshoot ratio, overshoot ramp rate, and plateau duration. The results show that the overshoot ratio is the dominant parameter controlling the sign and magnitude of the center-field drift, whereas a sufficiently long plateau can significantly reduce the overshoot required for low-drift operation. The optimized charging profile reduces the center-field drift rate to below 50 ppm/h in the 3000–5000 s post-excitation window. The study provides a practical charging strategy for improving the field temporal stability of conduction-cooled NI ReBCO magnets.
High temperature superconducting (HTS) tapes are increasingly being used in high-power electrical applications, demonstrating significant potential in high-intensity fields. In practical operation, HTS magnets are typically biased by a DC transport current, while the dynamic magneto-resistance effect induces substantial magnetization AC losses within the magnets under external magnetic fields. However, due to the highly non uniform current density distributions inherent to HTS coil geometries, accurately calculating AC losses in large-scale systems remains challenging. In this work, we propose a semi analytical homogenized (SAH) modeling method to improve model accuracy. The proposed method implements a robust two step integration. The integral method submodel containing only superconducting tapes is employed to calculat the governing parabolic critical boundary. The results are coupled into a coil submodel by introducing a gradient auxiliary function P, allowing effective homogenized division. To validate the proposed method, we analyzed a case study based on theoretical considerations and established a reference model. Experimental analysis and simulation results indicate that the proposed model enables a more precise discretization of stack configurations with non-uniform current density, effectively calculating the magnetization loss in large-scale HTS systems under external fields.
