I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
06
Mä
2026
A persistent-current-biased and current-actuated switch for superconducting circuits
Broadband and low-loss superconducting switches can facilitate large-scale quantum information processors and cryogenic detectors by dynamically reconfiguring the connectivity of their
circuits. The time dependent connectivity is enabled by the nonlinearity of lossless Josephson junctions, which are often wired into superconducting loops to be controlled by magnetic flux. However, this approach needs a power-consuming constant flux bias and dynamic flux actuation, both of which are hard to isolate from other switches or flux sensitive elements, limiting their integration density. Here, we design and characterize a microwave switch that implements a persistent current bias and direct current actuation to reduce static power consumption, actuation energy and potential crosstalk to other devices. We show that persistent current associated with tens of flux quanta is stable and long-lived, reducing the need for on-the-fly tuning. We further demonstrate that our switch has desirable performance for superconducting-circuit-based quantum information processing, including an off mode with more than 20 dB isolation comparable to commercial ferrite isolators, power handling larger than 100 pW sufficient for resonator readout tones and amplifier pumps, and modulation bandwidth broader than 600 MHz useful for multiplexing schemes.
05
Mä
2026
In-situ Characterization of Light-Matter Coupling in Multimode Circuit-QED Systems
Multimode cavity-QED systems can be leveraged to explore a wide range of physical phenomena; however, a complex multimode environment makes systematic characterization of light-matter
interactions challenging. Here we present a general measurement protocol, applicable to both atomic and synthetic cavity-QED systems, that enables the determination of coupling to individual photonic modes. The method leverages measurements of the AC-Stark and Kerr effects, along with known detuning dependencies, to eliminate the need for single-photon resolution, independent photon-number calibration, or insertion-loss calibration. We demonstrate the method using a superconducting transmon qubit coupled to a one-dimensional microwave resonator lattice. We validate the consistency of the extracted light-matter couplings g determined at multiple qubit detunings, and from the self-Kerr and cross-Kerr shifts for three photon modes, which provide separate measurements of g for each of the three modes.
Temperature-Dependent Dielectric Function of Tantalum Nitride Formed by Atomic Layer Deposition for Tunnel Barriers in Josephson Junctions
We report the dielectric functions of insulating tantalum nitride (TaN) films, deposited using atomic layer deposition (ALD) on 300 mm Si/SiO2 substrates, to demonstrate their suitability
as tunnel barriers in tantalum-based Josephson junctions (JJ) for superconducting quantum circuits. The temperature-dependent ellipsometric angles were measured using ALD TaN films with nominal thicknesses of 13 nm and 25 nm at an incidence angle of 70 degrees, across photon energy ranges of 0.03 eV to 0.7 eV (80-300 K) and 0.5 eV to 6.5 eV (80-600 K). This data was used to develop a dispersion model for insulating ALD TaN films that incorporates a Tauc-Lorentz oscillator with a band gap of 1.5-1.8 eV to model the interband optical transitions. The extracted dielectric function of ALD TaN films shows an insulating behavior (mid-infrared transparency) at all temperatures and for both film thicknesses tested. ALD TaN does not exhibit infrared absorption due to free carriers, even at elevated temperatures, demonstrating its insulating nature, which is required for the tunnel barrier of the JJ in quantum applications. The results of transmission electron microscopy, including selected area electron diffraction, and X-ray diffraction are also discussed. Sputter depth-profile X-ray photoelectron spectroscopy (XPS) shows an N/Ta ratio of ~1.2 throughout the film. The lower band gap, low roughness, and thermal stability of ALD TaN compared to AlOx suggest the possibility of fabricating JJs with thicker barriers while achieving critical current densities required for qubits, better control of thickness and composition, reduced topography, and resistance to aging.
04
Mä
2026
Universal Hamiltonian control in a planar trimon circuit
Multimode circuits provide an avenue for flexible control of single and multi-qubit gates. In this work we implement a multimode circuit known as a trimon integrated in a planar geometry.
The trimon features three transmon-like modes with strong all-to-all ZZ coupling. We demonstrate high fidelity operations on the trimon, achieving flexible control of its rich state space. This includes qubit rotations conditioned on one or both other qubits, unconditional single-qubit rotations, and both excitation-conserving and double-excitation two-qubit entangling gates. Through multi-tone driving we are able to implement all 16 two-qubit Pauli operators in the two-qubit space. We further demonstrate using the trimon as a qudit with up to 8 states and higher coherence than typical transmon-based implementations. Our results show a compact, highly controllable device that can potentially replace transmons in standard superconducting processor architectures.
03
Mä
2026
Analytical Quantum Full-Wave Analysis of Few-Photon Transport Through a Superconducting Cavity Qubit
A promising way to scale up superconducting quantum computers is to link different devices together using propagating photons. Correspondingly, accurately modeling the quantum information
transfer in such quantum interconnects is critical to advancing this emerging technology. To accomplish this, a full-wave quantum numerical model is essential for describing the few-photon transport characteristics of various components. Unfortunately, validating the accuracy of such numerical models remains a difficult challenge due to the lack of appropriate analytical solutions for standard component types. Recently, progress has been made on creating the first-ever analytical quantum full-wave solutions for a superconducting circuit quantum device. These efforts considered the case of two-photon transport through an empty rectangular waveguide cavity and the interactions of photons inside a closed rectangular waveguide cavity with a transmon qubit formed by a Josephson junction connected across the terminals of a small wire dipole antenna. Here, we advance these efforts by considering the one- and two-photon transport properties through a rectangular waveguide cavity containing a qubit in this form when the cavity is interfaced with via two coaxial ports. Such devices can be used in various ways for quantum interconnects, such as to form parts of a quantum memory or a photon source. We perform this analysis leveraging a quantum input-output theory formalism to derive the relevant single- and two-photon transport characteristics of interest. We then examine the signatures of the nonlinear quantum scattering effects in the good and bad cavity regimes of cavity quantum electrodynamics. In the future, these analytical results can be used to validate numerical full-wave quantum solvers for modeling quantum interconnects.
28
Feb
2026
Optimizing CMOS-compatible, superconducting Titanium Nitride Resonators: Deposition Conditions and Structuring Processes
We report on the fabrication and characterization of superconducting coplanar waveguide (CPW) resonators based on titanium nitride (TiN) thin films deposited on 200,mm diameter high-resistivity
Si(100) substrates. We systematically investigate how deposition conditions, dry-etch power and in-situ resist strip temperature affect morphology, superconducting properties and dielectric losses. By tuning reactive sputtering conditions, three distinct preferred crystal orientations – (111), (200), and mixed are achieved. Our results demonstrate that all films exhibiting similar minimal two-level system (TLS) losses, with TiN111 exhibit the lowest median TLS losses δ̃ TLS, and greater robustness against reoxidation. The applied structuring process, in contrast, had a far greater influence on the TLS loss than the crystal orientation of the TiN film and, consequently, the intrinsic material properties of the superconducting layer. The lowest TLS losses for all TiN depositons were achieved with a low power etch and low temperature resist strip. An additional buffered oxide etch (BOE) treatment could remove high-loss interfacial oxides at the metal-air (MA) and substrate-air (SA) interface and recover the etch-induced TLS losses. Consequently, TiN resonators exhibiting δ̃ TLS values as low as 9.67×10−7 were realized. The corresponding median low-power loss, δ̃ LP, amounts to 11.04×10−7, which translates to an internal quality factor approaching one million. These findings highlight the critical role of process induced oxide formation at the MA and SA interfaces in limiting the performance of TiN resonators and provide a scalable, low-loss process compatible with industry-grade 200 mm CMOS qubit fabrication workflows.
27
Feb
2026
Characterization of Josephson Junction Aging and Annealing Under Different Environments
Understanding the aging behavior of Josephson junctions and the effect of annealing on junction resistances is important in building large-scale superconducting quantum processors.
Here we study the effects of aging of Josephson junctions under different storage conditions from immediately after fabrication up to 2 to 3 months. We find that the aging curve follows a logarithmic curve, with the aging amplitude mainly determined by fabrication conditions and the aging speed determined by storage conditions. Junctions stored at ambient laboratory conditions aged faster compared to junctions stored in a nitrogen atmosphere or vacuum, with the aging speed appreciably changes when the storage condition changed. We also compared the effect of thermal annealing under nitrogen environment with annealing under ambient conditions up to 250∘ C. We find that under nitrogen environment, the resistances decreased at all temperatures tested, while under ambient environment the resistances increased at 200∘ C and decreased at 250∘ C instead. We were unable to decrease the resistance below the initial-time resistance, suggesting a lower limit on the range of resistance tuning.
Coherent Control of Population and Quantum Coherence in Superconducting Circuits
Quantum mechanics, with its counterintuitive principles and probabilistic nature, has long been confined to the microscopic realm of atoms and photons. Yet, recent breakthroughs have
pushed the boundaries of quantum behavior into the macroscopic world, where objects are visible to the naked eye and governed by classical physics. This review article traces the extraordinary progress toward achieving coherent control of population distributions among multiple quantum levels, as well as manipulation of absorption and refractive index, in such large-scale quantum systems, a feat once considered beyond reach.
3D Integrated Embedded Filters for Superconducting Quantum Circuits
Microwave filtering for superconducting qubits is a key element of quantum computing technology, enabling high coherence and fast state detection. This work presents the design and
implementation of novel microwave Purcell filters for superconducting quantum circuits, integrated within a multilayer printed circuit board (PCB). The off-chip design removes all filter components from the qubit substrate, reducing device complexity, improving layout footprint and allowing better scalability to large qubit counts. Each embedded filter can couple up to nine readout resonators, enabling efficient multiplexed readout. Electromagnetic simulations of the filter predict a thousand-fold improvement in qubit isolation from the readout port. The design was experimentally validated under cryogenic conditions in conjunction with a 35-qubit device, demonstrating compatibility of the PCB-based filter with high-coherence superconducting qubits. The comparison of the measured qubit median T1 of 84 μs with the expected radiative limit from electromagnetic simulations validated the presence of Purcell filtering in the system.
A frequency-agile microwave-optical interface for superconducting qubits
Superconducting quantum processors operate at microwave frequencies in millikelvin environments, making it challenging to interconnect distant nodes using conventional microwave wiring.
Coherent microwave-to-optical (M2O) transduction enables superconducting quantum networks by interfacing itinerant microwave photons with low-loss optical fiber. However, many state-of-the-art transducers provide efficient conversion only over a narrow frequency span, complicating deployment with heterogeneous superconducting devices that are detuned by gigahertz-scale offsets. Here we demonstrate a frequency-agile microwave-optical interface that overcomes this bandwidth mismatch by cascading an electro-optic M2O transducer with a multimode microwave-to-microwave (M2M) frequency converter, with in situ tunability of the microwave resonances in both stages. Using this architecture, we realize continuous frequency coverage from 5.0 to 8.5 GHz within a single system. As an application relevant to superconducting-qubit networking, we use the cascaded M2M-M2O interface to optically read out a superconducting qubit whose readout resonator is detuned by 1.7 GHz from the native M2O microwave resonance, demonstrating a scalable route toward fiber-linked superconducting quantum nodes.