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
17
Jul
2025
Topology-Enhanced Superconducting Qubit Networks for In-Sensor Quantum Information Processing
We investigate the influence of topology on the magnetic response of inductively coupled superconducting flux-qubit networks. Using exact diagonalization methods and linear response
theory, we compare the magnetic response of linear and cross-shaped array geometries, used as paradigmatic examples. We find that the peculiar coupling matrix in cross-shaped arrays yields a significant enhancement of the magnetic flux response compared to linear arrays, this network-topology effect arising from cooperative coupling among the central and the peripheral qubits. These results establish quantitative design criteria for function-oriented superconducting quantum circuits, with direct implications for advancing performance in both quantum sensing and quantum information processing applications. Concerning the latter, by exploiting the non-linear and high-dimensional dynamics of such arrays, we demonstrate their suitability for quantum reservoir computing technology. This dual functionality suggests a novel platform in which the same device serves both as a quantum-limited electromagnetic sensor and as a reservoir capable of signal processing, enabling integrated quantum sensing and processing architectures.
Two-photon coupling via Josephson element II: Interaction renormalizations and cross-Kerr coupling
We study the interactions mediated by symmetric superconducting quantum interference device (SQUID), their renormalizations, and applicability of the anharmonic oscillator model for
a coupled phase qubit. The coupling SQUID can switch between single- or two-photon interaction in situ. We consider a coupled resonator and an rf SQUID. The latter dwells in the vicinity of its metastable well holding a number of anharmonic energy states and acts as an artificial atom known as the phase qubit. Apart from the linear and two-photon couplings, interactions of optomechanical type and a cross-Kerr coupling arise. Near the two-photon resonance, we calculate the renormalizations due to nonresonant interactions, which are more prominent with the higher Josephson energy of the coupler. We interpret the renormalizations by depicting some of the virtual processes involved. That also allows us to determine the minimal amount of metastable states in the phase qubit for the renormalization formulas to hold.
16
Jul
2025
Material Loss Model Calibration for Tantalum Superconducting Resonators
Material research is a key frontier in advancing superconducting qubit and circuit performance. In this work, we develop a simple and broadly applicable framework for accurately characterizing
two-level system (TLS) loss using internal quality factor measurements of superconducting transmission line resonators over a range of temperatures and readout powers. We applied this method to a series of α-Ta resonators that span a wide frequency range, thus providing a methodology for probing the loss mechanisms in the fabrication process of this emerging material for superconducting quantum circuits. We introduce an analytical model that captures the loss behavior without relying on numerical simulations, enabling straightforward interpretation and calibration. Additionally, our measurements reveal empirical frequency-dependent trends in key parameters of the model, suggesting contributions from mechanisms beyond the standard tunneling model of TLSs.
14
Jul
2025
Thermal rectification in a qubit-resonator system
A qubit-oscillator junction connecting as a series two bosonic heat baths at different temperatures can display heat valve and diode effects. In particular, the rectification can change
in magnitude and even in sign, implying an inversion of the preferential direction for the heat current with respect to the temperature bias. We perform a systematic study of these effects in a circuit QED model of qubit-oscillator system and find that the features of current and rectification crucially depend on the qubit-oscillator coupling. While at small coupling, transport occurs via a resonant mechanism between the sub-systems, in the ultrastrong coupling regime the junction is a unique, highly hybridized system and the current becomes largely insensitive to the detuning. Correspondingly, the rectification undergoes a change of sign. In the nonlinear transport regime, the coupling strength determines whether the current scales sub- or super-linearly with the temperature bias and whether the rectification, which increases in magnitude with the bias, is positive or negative. We also find that steady-state coherence largely suppresses the current and enhances rectification. An insight on these behaviors with respect to changes in the system parameters is provided by analytical approximate formulas.
Manarat: A Scalable QICK-Based Control System for Superconducting Quantum Processors Supporting Synchronized Control of 10 Flux-Tunable Qubits
A scalable control architecture for superconducting quantum processors is essential as the number of qubits increases and coherent multi-qubit operations span beyond the capacity of
a single control module. The Quantum Instrumentation Control Kit (QICK), built on AMD RFSoC platforms, offers a flexible open-source framework for pulse-level qubit control but lacks native support for multi-board synchronization, limiting its applicability to mid- and large-scale quantum devices. To overcome this limitation, we introduce Manarat, a scalable multi-board control platform based on QICK that incorporates hardware, firmware, and software enhancements to enable sub-100 ps timing alignment across multiple AMD ZCU216 RFSoC boards. Our system integrates a low-jitter clock distribution network, modifications to the tProcessor, and a synchronization scheme to ensure deterministic alignment of program execution across boards. It also includes a custom analog front-end for flux control that combines high-speed RF signals with software-programmable DC biasing voltages generated by a low-noise, high-precision DAC. These capabilities are complemented by a software stack capable of orchestrating synchronized multi-board experiments and fully integrated with the open-source Qibo framework for quantum device calibration and algorithm execution. We validate Manarat on a 10-qubit superconducting processor controlled by two RFSoC boards, demonstrating reliable execution of synchronized control sequences for cross-board CZ gate calibration. These results confirm that sub-nanosecond synchronization and coherent control is achievable across multiple RFSoC boards, enabling scalable operation of superconducting quantum computers.
Optimal Calibration of Qubit Detuning and Crosstalk
Characterizing and calibrating physical qubits is essential for maintaining the performance of quantum processors. A key challenge in this process is the presence of crosstalk that
complicates the estimation of individual qubit detunings. In this work, we derive optimal strategies for estimating detuning and crosstalk parameters by optimizing Ramsey interference experiments using Fisher information and the Cramer-Rao bound. We compare several calibration protocols, including measurements of a single quadrature at multiple times and of two quadratures at a single time, for a fixed number of total measurements. Our results predict that the latter approach yields the highest precision and robustness in both cases of isolated and coupled qubits. We validate experimentally our approach using a single NV center as well as superconducting transmons. Our approach enables accurate parameter extraction with significantly fewer measurements, resulting in up to a 50\% reduction in calibration time while maintaining estimation accuracy.
13
Jul
2025
Intrinsic Multi-Mode Interference for Passive Suppression of Purcell Decay in Superconducting Circuits
Decoherence due to radiative decay remains an important consideration in scaling superconducting quantum processors. We introduce a passive, interference-based methodology for suppressing
radiative decay using only the intrinsic multi-mode structured environment of superconducting circuits. By taking into account the full electromagnetic mode-mode couplings within the device, we derive analytic conditions that enable destructive interference. These conditions are realized by introducing controlled geometric asymmetries — such as localized perturbations to the transmon capacitor — which increase mode hybridization and activate interference between multiple decay pathways. We validate this methodology using perturbation theory, full-wave electromagnetic simulations, and experimental measurements of a symmetry-broken transmon qubit with improved coherence times.
12
Jul
2025
Impedance-Engineered Josephson Parametric Amplifier with Single-Step Lithography
We present the experimental demonstration of an impedance-engineered Josephson parametric amplifier (IEJPA) fabricated in a single-step lithography process. Impedance engineering is
implemented using a lumped-element series LC circuit. We use a simpler lithography process where the entire device — impedance transformer and JPA — are patterned in a single electron beam lithography step, followed by a double-angle Dolan bridge technique for Al-AlOx-Al deposition. We observe nearly quantum-limited amplification with 18 dB gain over a wide 400 MHz bandwidth centered around 5.3 GHz, and a saturation power of -114 dBm. To accurately explain our experimental results, we extend existing theories for impedance-engineered JPAs to incorporate the full sine nonlinearity of both the JPA and the transformer. Our work shows a path to simpler realization of broadband JPAs and provides a theoretical foundation for a novel regime of JPA operation.
Superinductor-based ultrastrong coupling in a superconducting circuit
We present an ultrastrong superinductor-based coupling consisting of a flux qubit galvanically coupled to a resonator. The coupling inductor is fabricated in granular Aluminum, a superinductor
material able to provide large surface inductances. Spectroscopy measurements on the qubit-resonator system reveal a Bloch-Siegert shift of \SI{23}{\mega\hertz} and a coupling fraction of g/ωr≃0.13, entering the perturbative ultrastrong coupling (USC) regime. We estimate the inductance of the coupler independently by low-temperature resistance measurements providing Lc=(0.74±0.14)nH, which is compatible with g/ω≳0.1. Our results show that superinductors are a promising tool to study USC physics in high-coherence circuits using flux qubits with small loop areas and low persistent currents.
11
Jul
2025
Universal scaling of microwave dissipation in superconducting circuits
Improving the coherence of superconducting qubits is essential for advancing quantum technologies. While superconductors are theoretically perfect conductors, they consistently exhibit
residual energy dissipation when driven by microwave currents, limiting coherence times. Here, we report a universal scaling between microwave dissipation and the superfluid density, a bulk property of superconductors related to charge carrier density and disorder. Our analysis spans a wide range of superconducting materials and device geometries, from highly disordered amorphous films to ultra-clean systems with record-high quality factors, including resonators, 3D cavities, and transmon qubits. This scaling reveals an intrinsic bulk dissipation channel, independent of surface dielectric losses, that originates from a universal density of nonequilibrium quasiparticles trapped within disorder-induced spatial variations of the superconducting gap. Our findings define a fundamental limit to coherence set by intrinsic material properties and provide a predictive framework for selecting materials and the design of next-generation superconducting quantum circuits.