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
21
Jul
2025
Resource-Efficient Cross-Platform Verification with Modular Superconducting Devices
Large-scale quantum computers are expected to benefit from modular architectures. Validating the capabilities of modular devices requires benchmarking strategies that assess performance
within and between modules. In this work, we evaluate cross-platform verification protocols, which are critical for quantifying how accurately different modules prepare the same quantum state — a key requirement for modular scalability and system-wide consistency. We demonstrate these algorithms using a six-qubit flip-chip superconducting quantum device consisting of two three-qubit modules on a single carrier chip, with connectivity for intra- and inter-module entanglement. We examine how the resource requirements of protocols relying solely on classical communication between modules scale exponentially with qubit number, and demonstrate that introducing an inter-module two-qubit gate enables sub-exponential scaling in cross-platform verification. This approach reduces the number of repetitions required by a factor of four for three-qubit states, with greater reductions projected for larger and higher-fidelity devices.
19
Jul
2025
Spectator Leakage Elimination in CZ Gates via Tunable Coupler Interference on a Superconducting Quantum Processor
Spectator-induced leakage poses a fundamental challenge to scalable quantum computing, particularly as frequency collisions become unavoidable in multi-qubit processors. We introduce
a leakage mitigation strategy based on dynamically reshaping the system Hamiltonian. Our technique utilizes a tunable coupler to enforce a block-diagonal structure on the effective Hamiltonian governing near-resonant spectator interactions, confining the gate dynamics to a two-dimensional invariant subspace and thus preventing leakage by construction. On a multi-qubit superconducting processor, we experimentally demonstrate that this dynamic control scheme suppresses leakage rates to the order of 10−4 across a wide near-resonant detuning range. The method also scales effectively with the number of spectators. With three simultaneous spectators, the total leakage remains below the threshold relevant for surface code error correction. This approach eases the tension between dense frequency packing and high-fidelity gate operation, establishing dynamic Hamiltonian engineering as an essential tool for advancing fault-tolerant quantum computing.
18
Jul
2025
An Effective Reflection Mode Measurement for Hanger-Coupled Microwave Resonators
Superconducting microwave resonators are used to study two-level system (TLS) loss in superconducting quantum devices. Fano asymmetry, characterized by a nonzero asymmetry angle ϕ
in the diameter correction method (DCM), results from the coupling schemes used to measure these devices, including the commonly used hanger method. ϕ is an additional fitting parameter which contains no physically interesting information and can obscure device parameters of interest. The tee-junction symmetry nominally present in these resonator devices provides an avenue for the elimination of Fano asymmetry using calibrated measurement. We show that the eigenvalue associated with the common mode excitation of the resonator is an effective reflection mode (ERM) which has no Fano asymmetry. Our analysis reveals the cause of Fano asymmetry as interference between common and differential modes. Practically, we obtain the ERM from a linear combination of calibrated reflection and transmission measurements. We utilize a 3D aluminum cavity to experimentally demonstrate the validity and flexibility of this model. To extend the usefulness of this symmetry analysis, we apply perturbation theory to recover the ERM in a multiplexed coplanar waveguide resonator device and experimentally demonstrate quantitative agreement in the extracted Q−1i between hanger mode and ERM measurements. We observe a five-fold reduction in uncertainty from the ERM compared to the standard hanger mode at the lowest measured power, -160 dBm delivered to the device. This method could facilitate an increase in throughput of low-power superconducting resonator measurements by up to a factor of 25, as well as allow the extraction of critical parameters from otherwise unfittable device data.
17
Jul
2025
Technical Review on RF-Amplifiers for Quantum Computer Circuits: New Architectures of Josephson Parametric Amplifier
Josephson Parametric Amplifiers (JPAs) are key components in quantum information processing due to their ability to amplify weak quantum signals with near-quantum-limited noise performance.
This is essential for applications such as qubit readout, quantum sensing, and communication, where signal fidelity and coherence preservation are critical. Unlike CMOS and HEMT amplifiers used in conventional RF systems, JPAs are specifically optimized for millikelvin (mK) cryogenic environments. CMOS amplifiers offer good integration but perform poorly at ultra-low temperatures due to high noise. HEMT amplifiers provide better noise performance but are power-intensive and less suited for mK operation. JPAs, by contrast, combine low power consumption with ultra-low noise and excellent cryogenic compatibility, making them ideal for quantum systems. The first part of this study compares these RF amplifier types and explains why JPAs are preferred in cryogenic quantum applications. The second part focuses on the design and analysis of JPAs based on both single Josephson junctions and junction arrays. While single-junction JPAs utilize nonlinear inductance for amplification, they suffer from gain compression, limited dynamic range, and sensitivity to fabrication variations. To overcome these challenges, this work explores JPA designs using Josephson junction arrays. Arrays distribute the nonlinear response, enhancing power handling, linearity, impedance tunability, and coherence while reducing phase noise. Several advanced JPA architectures are proposed, simulated, and compared using quantum theory and CAD tools to assess performance trade-offs and improvements over conventional designs.
A superinductor in a deep sub-micron integrated circuit
Superinductors are circuit elements characterised by an intrinsic impedance in excess of the superconducting resistance quantum (RQ≈6.45 kΩ), with applications from metrology and
sensing to quantum computing. However, they are typically obtained using exotic materials with high density inductance such as Josephson junctions, superconducting nanowires or twisted two-dimensional materials. Here, we present a superinductor realised within a silicon integrated circuit (IC), exploiting the high kinetic inductance (∼1~nH/◻) of TiN thin films native to the manufacturing process (22-nm FDSOI). By interfacing the superinductor to a silicon quantum dot formed within the same IC, we demonstrate a radio-frequency single-electron transistor (rfSET), the most widely used sensor in semiconductor-based quantum computers. The integrated nature of the rfSET reduces its parasitics which, together with the high impedance, yields a sensitivity improvement of more than two orders of magnitude over the state-of-the-art, combined with a 10,000-fold area reduction. Beyond providing the basis for dense arrays of integrated and high-performance qubit sensors, the realization of high-kinetic-inductance superconducting devices integrated within modern silicon ICs opens many opportunities, including kinetic-inductance detector arrays for astronomy and the study of metamaterials and quantum simulators based on 1D and 2D resonator arrays.
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.