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
08
Jul
2026
Super-Logarithmic Entanglement Scaling in a Monitored Superconducting Chain
We develop a Keldysh-replica non-linear sigma model (NLSM) for the entanglement dynamics of a monitored one-dimensional spinful s-wave BCS chain in the rare-measurement regime, γ≪J,Δ.
Although the clean spinful s-wave BCS Hamiltonian belongs to symmetry class CI, spin-resolved measurements and projection to a conserved f-sector reduce the effective problem to class C. Starting from the corresponding parent symplectic saddle, we show that measurement backaction and the pairing amplitude impose complementary mass constraints that gap out different fluctuation channels. Their interplay dynamically projects the surviving massless modes onto an SO(R) target manifold in replica space. A one-loop renormalization group analysis of this SO(R) NLSM shows that, in the replica limit R→1, the beta function becomes negative, producing a weak-anti-localization flow. This flow yields a super-logarithmic steady-state entanglement scaling S(L)∼ln2L in the rare-measurement regime. Our field-theoretic result explains the numerical evidence reported in the companion Letter [arXiv:2604.04375] and shows that a topologically trivial monitored s-wave superconductor can realize an SO(R) weak-anti-localizing critical phase without relying on a Wess-Zumino-Witten term.
Observation of coherent flux-charge interaction in a gate-tunable fluxonium
Interactions that mix conjugate variables, such as the flux through a circuit element and the charge across it, lie outside the reach of the elementary couplings of superconducting
circuits. Capacitors connect charge to charge, and inductors connect flux to flux, while no two-terminal element couples flux to charge directly. A native flux-charge coupling would thus serve as a circuit primitive in its own right, opening direct routes to non-reciprocity, protected modes, and unconventional readout. In this work, we demonstrate a flux-charge coupling by harnessing a voltage-tunable Josephson junction with parametrically modulated critical current, which mediates the interaction between a classical charge variable and a quantum flux operator. Relying on parity-selection rules in a hybrid superconducting-semiconductor fluxonium, we isolate the flux-charge coupling from other parasitic capacitive contributions and perform cross-quadrature-activated coherent control of states. Critically, we realize a flux-charge coupling that scales linearly with driving amplitude while keeping the transition energy first-order-insensitive to gate voltage. Such unconventional interaction broadens the toolbox of superconducting circuits with a critical missing component that enables the coherent coupling of conjugate variables.
07
Jul
2026
Quantum error correction of a grid-state qubit with state preparation and measurement errors below 10−3
Grid state qubits offer a hardware-efficient approach to large-scale fault-tolerant quantum computing. They access the information redundancy required for quantum error correction by
exploiting the large Hilbert space naturally available in harmonic oscillators. Superconducting architectures are particularly suitable to implement grid state qubits due to their fast and high-fidelity operations. Grid states in superconducting circuits enable quantum error correction (QEC) with performance beyond break-even. However, the state preparation and measurements (SPAM) errors of grid states has been a significant limitation to computational performances. In this work, we leverage high-performance QEC to enable repeat-until-success state preparation of both cardinal and magic states of the single-mode grid-state qubit. We combine this with an improved measurement protocol that corrects for both finite-energy envelope and auxiliary qubit readout errors, and increases robustness to photon loss. Our experiments, using both techniques, achieve a combined state-preparation and measurement error below 10−3. This represents two orders-of-magnitude improvement over the state of the art, bringing this platform on par with standard SPAM error levels measured in transmon qubits.
Localized Thermometry via Dayem Bridges Integrated on Superconducting Qubit Chips
Accurate knowledge of the on-chip temperature is essential for understanding and optimizing the performance of superconducting qubits, yet direct thermometry at millikelvin temperatures
remains challenging. While qubits themselves are sensitive to the temperature of their environment, other factors may affect the qubits` effective temperature, and using them as thermometers with any accuracy requires specialized measurement protocols and qubit designs, limiting their practicality for routine diagnostics and adding complex infrastructure to any hardware testing apparatus. Here we demonstrate a complementary on-chip thermometry method based on superconducting Dayem bridges that are integrated on the same chip as transmon qubits. By extracting the critical current of the Dayem bridge from I-V measurements, we obtain a local, quantitative measure of the chip temperature without the need for microwave calibration or qubit-specific control sequences. To demonstrate the utility of the Dayem bridges as thermometers, we fabricate them in-situ with qubits on the same chip, calibrate the Dayem bridge critical current as a function of temperature, and characterize its resolution and stability at cryogenic temperatures. We additionally perform simultaneous measurements of the Dayem bridge thermometer and qubit excited-state population, and show agreement over the relevant temperature range, validating the method against established qubit thermometry. Furthermore, we correlate the independently measured chip temperature with qubit energy relaxation and dephasing times, demonstrating the utility of this approach for diagnosing temperature-dependent decoherence mechanisms. These results establish integrated Dayem bridges as a simple, non-invasive, and scalable tool for cryogenic hardware development, and on chip thermometry in superconducting quantum circuits.
Separating transient leakage exposure from endpoint cancellation in fast transmon single-qubit gates
Fast single-qubit gates on weakly anharmonic transmons are limited by leakage to noncomputational states, and standard mitigations such as DRAG (derivative removal by adiabatic gate)
act on the leakage amplitude at the end of the gate. We show that this endpoint amplitude and the transient leakage exposure accumulated during the gate are two distinct control objectives that can be assigned to separate modules. The endpoint is a single sample of the drive spectrum, |Λ̃ (η)|2; the exposure is a band integral about η and governs leakage under dephasing, and the spectral-null condition Λ̃ (η)=0 constrains only the former. We realize this split in a path–endpoint separation pulse (PESP): a path-shaping pulse suppresses the exposure, and a two-tone endpoint-cancellation pulse cancels the residual amplitude. For a 10 ns RX(π/2) gate at η/2π=0.2 GHz, in numerical simulations the path-shaping pulse reduces the dephasing exposure by ∼21% relative to cosine DRAG and the independently simulated Lindblad excess leakage by ∼20%, consistent with Pϕexcess≃γϕTP¯dephA, whereas matched-budget endpoint-only and spectral-null controls leave it essentially unchanged. The residual endpoint floor splits exactly into a |2⟩ back-action and a |3⟩ cascade, which the two tones cancel one-to-one, driving the floor at the path-exposure knee from ∼7×10−7 to ∼3×10−8 without perturbing the path. By separating transient exposure from endpoint leakage, PESP turns leakage suppression in fast weakly anharmonic gates into a modular, interpretable control problem: dephasing-induced leakage and the coherent residual error are reduced by separate, individually verifiable modules.
02
Jul
2026
Neural-Network Inverse Design of SRF Cavities and Transmons for Bosonic Quantum Computation
Three-dimensional superconducting radio-frequency (SRF) cavities provide exceptionally long-lived electromagnetic modes and, when coupled to nonlinear elements such as transmon qubits,
become promising architectures for bosonic quantum information processing. The inverse design of such systems, i.e., recovering device geometries that produce specified electromagnetic and coupling targets, is generally a one-to-many problem. The qubit-cavity coupling strength depends sensitively on both the transmon geometry and its position within the cavity’s electromagnetic field. As these systems scale up and their design parameter spaces grow, the cost of conventional iterative simulation becomes prohibitive. We present two deep neural network (DNN) approaches that address this inverse-design problem at complementary levels of the design stack. The first proposes SRF cavity geometries that produce target cavity observables. The second proposes transmon qubit designs that produce target qubit-cavity parameters — the coupling rate, qubit frequency, and anharmonicity (g,νq,α). The recovered candidate designs match the targets to within ∼5\% (cavity) and ∼2\% (transmon), confirmed by end-to-end re-simulation. Both approaches map desired device behavior directly to candidate designs, a fast alternative to the iterative simulation studies usually required.
01
Jul
2026
High-Precision Calibration Workflow Achieves Above 99.9% CZ Gate Fidelity on a Scalable Superconducting Processor
High-fidelity universal two-qubit gates are critical for building fault-tolerant quantum computers. In scalable superconducting processors, shortened coherence times introduce more
incoherent errors in gate operations. With a constrained error budget, there is reduced tolerance for coherent errors stemming from parameter deviations. In this work, we develop a closed-loop workflow to enhance the CZ gate calibration precision. Utilizing the echoed leakage error amplification (ELEA) and the repurposed context-aware fidelity estimation (CAFE) circuits, we suppress the population leakage to non-computational states, and, for the first time, demonstrate a CZ gate fidelity exceeding 99.9% on an 84-qubit processor, with coherent error suppressed to 0.007%. Meanwhile, we obtain a median fidelity of 99.25% among 72 CZ gates, demonstrating that the workflow can be generalized to the calibration of parallel CZ gates. Finally, we realize automated calibration and observe enhanced stability of the CZ gate throughout 9-hour comparative monitoring experiments. Our results, realized on a completely domestic platform, establish an efficient and automated route to quantum computation with superconducting quantum systems.
Vanadium superconducting microwave resonators on silicon wafers
Understanding the correlation between material properties and microwave losses in superconducting films is a crucial subject for developing low-loss materials for quantum circuits.
We focus on vanadium (V) as a novel material for superconducting quantum devices and discuss loss in V films in relation to their structural properties. Using a sputtering method, we grow four V-film structures on (001)-oriented Si wafers, employing Nb and Ta as the buffer and capping layer materials, respectively: Nb/V/Ta, Nb/V, V/Ta, and V. X-ray diffraction and atomic force microscopy reveal that the V films grown on the Nb buffer layers have higher uniformity of lattice orientation and smaller grain size than that directly grown on the Si wafer. Coplanar waveguide resonators are fabricated from the four V-film structures, and averaged photon number (⟨nph⟩) dependences of internal quality factor (Qint) are obtained by performing microwave measurements. By analyzing the obtained Qint vs ⟨nph⟩, it is found that loss at the V surface is dominated by ⟨nph⟩-independent non-two-level-system (non-TLS) losses, which can be mitigated by introducing the Ta capping layer. Furthermore, the V films on the Nb buffer layers exhibit lower Qint in the ⟨nph⟩ range from 100 to 106 and higher non-TLS loss than that directly grown on Si wafers, even though the former has higher lattice-orientation uniformity than the latter. Origins of these trends might be relevant to V oxides, of which presence at surfaces and grain boundaries in bulk regions in the V resonators is suggested by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, and/or V hydrides.
Characterization of Hydroxyls in Surface Oxide of Superconducting Tantalum and Their Mitigation in Quantum Circuits
Recently, tantalum (Ta) has gained attention in superconducting quantum circuits due to the longer coherence times achieved when replacing niobium (Nb) in capacitor pads. Previous literature
shows that surface oxides that form upon ambient exposure on superconducting metals such as Ta, Al, and Nb host two-level system (TLS) defects, which are a leading source of microwave loss and decoherence. While the surface oxides of Nb and Al have been extensively studied, Ta oxides remain less well understood. Using secondary ion mass spectrometry of alpha-Ta films deposited at 300 mm wafer scale, we show for the first time that hydroxyls accumulate in the Ta suboxide region above the underlying Ta. Angle-resolved X-ray photoelectron spectroscopy shows that the surface region is dominated by Ta2O5, with sub-stoichiometric TaOx present in between the Ta2O5 and underlying Ta. The thickness of the tantalum oxide is confirmed by transmission electron microscopy. We demonstrate that [OH] incorporation can be suppressed by replacing the native oxide with an oxide formed during chemical mechanical planarization of alpha-Ta films. Our findings support the hypothesis that TLS defects are non-uniform within the oxide thickness and suggest hydroxyls as a probable molecular origin of these loss channels. Furthermore, we show the feasibility of plasma nitridization as a method to decrease hydroxyl loading on alpha-Ta surfaces. The modulation of hydroxyl content through surface engineering of alpha-Ta can enable the fabrication of more robust, high-coherence superconducting quantum circuits by addressing a potential TLS source.
Surface Platinum Alloying for Passivation of Oxide Interfaces on Superconducting Niobium Films
Dielectric loss arising from two-level systems (TLS) at surfaces and interfaces remains a primary limitation to coherence in superconducting transmon qubits. Niobium (Nb), a widely
used material in superconducting quantum circuits, readily forms native oxides under ambient conditions, leading to lossy dielectric interfaces that degrade device performance. Here, a robust and scalable fabrication strategy is demonstrated for chemically stabilizing Nb surfaces and mitigating further oxidation, including protection of both surface and sidewall regions. High-purity Nb films were fabricated with bulk-like superconducting transition temperatures (Tc=9.30±0.10) K. We demonstrate that a thin Pt encapsulation layer, deposited after native oxide formation, can be transformed via thermal annealing into a Nb-Pt alloy at the surface. Spectroscopic and microscopic analyses confirm the formation of a chemically stable metallic alloy layer and its ability to suppress further oxide growth. Ab initio simulations elucidate the atomic-scale rearrangement and electronic structure evolution associated with Pt incorporation on native niobium oxide, providing insight into the stabilization mechanism of the alloyed surface. This approach offers a materials pathway for engineering chemically robust Nb interfaces, including sidewalls, toward higher-coherence superconducting qubit architectures.“