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
27
Jul
2025
Modeling Charge Noise in Superconducting Qubits Using Memory Multi-Fractional Brownian Motion
We introduce a novel stochastic model for charge noise in superconducting charge qubits based on memory multi-fractional Brownian motion (mmfBm), capable of capturing non- stationary
and long-memory effects. This framework reproduces key experimental fea- tures of decoherence and offers new insights into environmental interactions with supercon- ducting quantum devices.
Circuit simulation of readout process toward large-scale superconducting quantum circuits
The rapid scaling of superconducting quantum computers has highlighted the impact of device-level variability on overall circuit fidelity. In particular, fabrication-induced fluctuations
in device parameters such as capacitance and Josephson critical current pose significant challenges to large-scale integration. We propose a simulation methodology for estimating qubit fidelity based on classical circuit simulation, using a conventional Simulation Program with Integrated Circuit Emphasis (SPICE) simulator. This approach enables the evaluation of the performance of superconducting quantum circuits with 10000 qubits on standard laptop computers. The proposed method provides an accessible tool for the early stage assessment of large-scale superconducting quantum circuit performance.
25
Jul
2025
Exponentially robust non-Clifford gate in a driven-dissipative circuit
Recent work (Nathan et al, arXiv:2405.05671) proposed an architecture for a dissipatively stabilized GKP qubit, and protocols for protected Clifford gates. Here we propose a protocol
for a protected non-Clifford T‾‾√ gate at the physical qubit level, based on the inclusion of a quartic flux potential generated by ancillary Josephson junctions. We show that such a gate is topologically robust with exponentially suppressed infidelity from control or device imperfections, and operates on microsecond timescales for GHz resonators. We analyze the resilience of the protocol to noise, imperfect control, and imperfect targeting of circuit parameters.
24
Jul
2025
Analysis of RF Surface Loss in a Planar 2D Qubit
The Josephson junction and shunt capacitor form a transmon qubit, which is the cornerstone of modern quantum computing platforms. For reliable quantum computing, it is important how
long a qubit can remain in a superposition of quantum states, which is determined by the coherence time (T1). The coherence time of a qubit effectively sets the „lifetime“ of usable quantum information, determining how long quantum computations can be performed before errors occur and information is lost. There are several sources of decoherence in transmon qubits, but the predominant one is generally considered to be dielectric losses in the natural oxide layer formed on the surface of the superconductor. In this paper, we present a numerical study of microwave surface losses in planar superconducting antennas of different transmon qubit designs. An asymptotic method for estimating the energy participation ratio in ultrathin films of nanometer scales is proposed, and estimates are given for the limits of achievable minimum RF losses depending on the electrical properties of the surface oxide and the interface of the qubit with the substrate material.
22
Jul
2025
On-chip stencil lithography for superconducting qubits
Improvements in circuit design and more recently in materials and surface cleaning have contributed to a rapid development of coherent superconducting qubits. However, organic resists
commonly used for shadow evaporation of Josephson junctions (JJs) pose limitations due to residual contamination, poor thermal stability and compatibility under typical surface-cleaning conditions. To provide an alternative, we developed an inorganic SiO2/Si3N4 on-chip stencil lithography mask for JJ fabrication. The stencil mask is resilient to aggressive cleaning agents and it withstands high temperatures up to 1200\textdegree{}C, thereby opening new avenues for JJ material exploration and interface optimization. To validate the concept, we performed shadow evaporation of Al-based transmon qubits followed by stencil mask lift-off using vapor hydrofluoric acid, which selectively etches SiO2. We demonstrate average $T_1 \approx 75 \pm 11~\SI{}{\micro\second}$ over a 200 MHz frequency range in multiple cool-downs for one device, and $T_1 \approx 44\pm 8~\SI{}{\micro\second}$ for a second device. These results confirm the compatibility of stencil lithography with state-of-the-art superconducting quantum devices and motivate further investigations into materials engineering, film deposition and surface cleaning techniques.
Pulse-Level Simulation of Crosstalk Attacks on Superconducting Quantum Hardware
Hardware crosstalk in multi-tenant superconducting quantum computers poses a severe security threat, allowing adversaries to induce targeted errors across tenant boundaries by injecting
carefully engineered pulses. We present a simulation-based study of active crosstalk attacks at the pulse level, analyzing how adversarial control of pulse timing, shape, amplitude, and coupling can disrupt a victim’s computation. Our framework models the time-dependent dynamics of a three-qubit system in the rotating frame, capturing both always-on couplings and injected drive pulses. We examine two attack strategies: attacker-first (pulse before victim operation) and victim-first (pulse after), and systematically identify the pulse and coupling configurations that cause the largest logical errors. Protocol-level experiments on quantum coin flip and XOR classification circuits show that some protocols are highly vulnerable to these attacks, while others remain robust. Based on these findings, we discuss practical methods for detection and mitigation to improve security in quantum cloud platforms.
21
Jul
2025
Fast Recovery of Niobium-based Superconducting Resonators after Laser Illumination
Interfacing superconducting microwave resonators with optical systems enables sensitive photon detectors, quantum transducers, and related quantum technologies. Achieving high optical
pulse repetition is crucial for maximizing the device throughput. However, light-induced deterioration, such as quasiparticle poisoning, pair-breaking-phonon generation, and elevated temperature, hinders the rapid recovery of superconducting circuits, limiting their ability to sustain high optical pulse repetition rates. Understanding these loss mechanisms and enabling fast circuit recovery are therefore critical. In this work, we investigate the impact of optical illumination on niobium nitride and niobium microwave resonators by immersing them in superfluid helium-4 and demonstrate a three-order-of-magnitude faster resonance recovery compared to vacuum. By analyzing transient resonance responses, we provide insights into light-induced dynamics in these superconductors, highlighting the advantages of niobium-based superconductors and superfluid helium for rapid circuit recovery in superconducting quantum systems integrated with optical fields.
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.