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
Aug
2025
Transmon qubit using Sn as a junction superconductor
Superconductor qubits typically use aluminum-aluminum oxide tunnel junctions to provide the non-linear inductance. Junctions with semiconductor barriers make it possible to vary the
superconductor material and explore beyond aluminum. We use InAs semiconductor nanowires coated with thin superconducting shells of beta-Sn to realize transmon qubits. By tuning the Josephson energy with a gate voltage, we adjust the qubit frequency over a range of 3 GHz. The longest energy relaxation time, T1 = 27 microseconds, is obtained at the lowest qubit frequencies, while the longest echo dephasing time, T2 = 1.8 microseconds, is achieved at higher frequencies. We assess the possible factors limiting coherence times in these devices and discuss steps to enhance performance through improvements in materials fabrication and circuit design.
05
Aug
2025
Charge sensitivity in the transmon regime
Transmons are widely adopted in quantum computing architectures for their engineered insensitivity to charge noise and correspondingly long relaxation times. Despite this advantage,
transmons often exhibit large fluctuations in dephasing times across different devices and also within qubits on the same device. Existing transmon qubits are assumed to be insensitive to charge noise. However, very little recent attention has been paid to the dependence of dephasing on the local charge environment. In this study, we see fluctuations in the dephasing time, Tϕ, which correlate to charge offset. While charge offset fluctuations are slow, parity switches are fast processes tied to the charge offset and can affect Tϕ in Ramsey experiments. We implement a protocol to detect parity switching events using single-shot methods, which are interleaved within a Ramsey measurement. We find that events that remain in the same parity state have a higher T2 than measurements averaged over both parities. Our results show that transmons can be limited by charge-noise, even with EJ/EC≈50. Consequently, parity flip rates must be considered as a device characterization metric.
Probing strongly driven and strongly coupled superconducting qubit-resonator system
We investigated a strongly driven qubit strongly connected to a quantum resonator. The measured system was a superconducting flux qubit coupled to a coplanar-waveguide resonator which
is weakly coupled to a probing feedline. This hybrid qubit-resonator system was driven by a magnetic flux and probed with a weak probe signal through the feedline. We observed and theoretically described the quantum interference effects, deviating from the usual single-qubit Landau-Zener-Stückelberg-Majorana interferometry, because the strong coupling distorts the qubit energy levels.
Characterizing and Mitigating Flux Crosstalk in Superconducting Qubits-Couplers System
Superconducting qubits have achieved exceptional gate fidelities, exceeding the error-correction threshold in recent years. One key ingredient of such improvement is the introduction
of tunable couplers to control the qubit-to-qubit coupling through frequency tuning. Moving toward fault-tolerant quantum computation, increasing the number of physical qubits is another step toward effective error correction codes. Under a multiqubit architecture, flux control (Z) lines are crucial in tuning the frequency of the qubits and couplers. However, dense flux lines result in magnetic flux crosstalk, wherein magnetic flux applied to one element inadvertently affects neighboring qubits or couplers. This crosstalk obscures the idle frequency of the qubit when flux bias is applied, which degrades gate performance and calibration accuracy. In this study, we characterize flux crosstalk and suppress it in a multiqubit-coupler chip with multi-Z lines without adding additional readout for couplers. By quantifying the mutual flux-induced frequency shifts of qubits and couplers, we construct a cancellation matrix that enables precise compensation of non-local flux, demonstrating a substantial reduction in Z-line crosstalk from 56.5permilleto 0.13permille which is close to statistical error. Flux compensation corrects the CZ SWAP measurement, leading to a symmetric map with respect to flux bias. Compared with a crosstalk-free calculated CZ SWAP map, the measured map indicates that our approach provides a near-zero crosstalk for the coupler-transmon system. These results highlight the effectiveness of our approach in enhancing flux crosstalk-free control and supporting its potential for scaling superconducting quantum processors.
01
Aug
2025
Nonclassical microwave radiation from the parametric dynamical Casimir effect in the reversed-dissipation regime of circuit optomechanics
We propose an experimentally feasible optomechanical system (OMS) that is dispersively driven and operates in the reversed dissipation regime (RDR), where the mechanical damping rate
far exceeds the cavity decay rate. We demonstrate that coherent, fast-time modulation of the driving laser frequency-on time scales longer than the mechanical decoherence time-allows for adiabatic elimination of the mechanical mode, resulting in strong parametric amplification of quantum vacuum fluctuations of the intracavity field. This mechanism, known as the parametric dynamical Casimir effect (parametric-DCE), leads to the generation of Casimir photons. In the dispersive RDR, we find that the total system Hamiltonian-including the DCE term-is intrinsically modified by a generalized optomechanical Kerr-type nonlinearity. This nonlinearity not only saturates the mean number of radiated Casimir photons on short time scales, even without dissipation, but also induces oscillatory behavior in their dynamics and quantum characteristics. Remarkably, the presence of the Kerr nonlinearity causes the generated DCE photons to exhibit nonclassical features, including sub-Poissonian statistics, negative Wigner function and quadrature squeezing which can be controlled by adjusting the system parameters. The proposed nonclassical microwave radiation source possesses the potential to be applied in quantum information processing, quantum computing as well as microwave quantum sensing.
30
Jul
2025
Placing and Routing Non-Local Quantum Error Correcting Codes in Multi-Layer Superconducting Qubit Hardware
Quantum error correcting codes (QECCs) with asymptotically lower overheads than the surface code require non-local connectivity. Leveraging multi-layer routing and long-range coupling
capabilities in superconducting qubit hardware, we develop Hardware-Aware Layout, HAL: a robust, runtime-efficient heuristic algorithm that automates and optimizes the placement and routing of arbitrary QECCs. Using HAL, we perform a comparative study of hardware cost across various families of QECCs, including the bivariate bicycle codes, the open-boundary tile codes, and the constant-depth-decodable radial codes. The layouts produced by HAL confirm that open boundaries significantly reduce the hardware cost, while incurring reductions in logical efficiency. Among the best-performing codes were low-weight radial codes, despite lacking topological structure. Overall, HAL provides a valuable framework for evaluating the hardware feasibility of existing QECCs and guiding the discovery of new codes compatible with realistic hardware constraints.
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.