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
04
Aug
2026
Few-photon degenerate parametric resonance in a two-tone driven microwave resonator
Multi-tone external driving offers a route to parametric physics without directly modulating the device. However, the validity of the parametric response in the few-photon regime remains
underexplored. Here, we apply two coherent microwave tones to a Josephson-junction Kerr oscillator and stimulate degenerate parametric downconversion via four-wave mixing. Using transmission spectroscopy, we observe that the response retains the qualitative semiclassical Kerr parametric oscillator structure, including its instability lobe and bistable phase-space topology. Interestingly, we demonstrate that a conventional single-mode reduction fails to capture the system quantitatively: the predicted AC Stark shift is severely underestimated, and the reported distributions might not be fully physical when the single-photon Kerr shift K exceeds the cavity linewidth κ. Instead, we show that a full three-tone quantum description accurately reproduces the experimental observables. There, quantum fluctuations of the drive tones become dynamically dominant over dissipation, and all three interacting tones operate in a deep few-photon limit where the expected semiclassical macroscopic lobes undergo fundamental renormalization due to profound mixing with quantum variance. Our results establish two-tone-driven Kerr oscillators as potential parametric amplifiers and open new horizons to explore the quantum-to-classical crossover in driven-dissipative circuits.
03
Aug
2026
High-Frequency Gravitational Wave Detection with Superconducting Qubits
High-frequency gravitational waves (HFGWs) provide a unique window into high-energy and early-universe physics, yet they evade traditional macroscopic interferometry. To bridge this
detection gap, we propose a novel quantum-sensing paradigm utilizing superconducting transmon qubits embedded in resonant microwave cavities. Through the inverse Gertsenshtein effect, HFGWs propagating in a static magnetic field resonantly excite a cavity mode. By leveraging the characteristic spin-2 quadrupolar pattern of the induced electromagnetic field, we position qubits directly at the electric-field hot spots of the TE212 mode to act as localized sensors. Crucially, configuring this array as an entangled quantum register via symmetric Dicke states unlocks a fundamental scaling advantage: the signal probability scales quadratically with the qubit number, translating to a hmin∝n−3/4q strain sensitivity scaling. We demonstrate that an idealized global register of 800 qubits reaches a strain sensitivity that surpasses standard macroscopic cavity-power limits by five orders of magnitude. Benchmarked against representative axion-haloscope parameters, this collective quantum enhancement decisively mitigates the profound Planck-scale suppression inherent to gravitational interactions, establishing a transformative framework for next-generation HFGW searches in the GHz band.
Adaptive Reconstruction of Bosonic Quantum States
Bosonic quantum systems provide a hardware-efficient platform for quantum information processing but remain challenging to characterise due to their large Hilbert space and the high
measurement cost of state tomography. Existing approaches estimate the fidelity with respect to a single target state, making them unsuitable for applications in which physically equivalent states differ by phase space translations, rotations, or other transformations. Here, we introduce an adaptive reconstruction technique that estimates the fidelity with respect to a family of bosonic states while reconstructing the underlying Wigner function from a small number of measurements. The method combines a physics-informed parametric model with Bayesian inference, bootstrap, and active learning to iteratively select the most informative phase space sampling points. We implement the approach on a circuit quantum electrodynamics platform and benchmark it on Schrödinger cat states with amplitudes α∈[1,3]. The reconstruction yields reproducible fidelity estimates within a few minutes, remains robust to substantial displacements and rotations in phase space despite using a mismatched prior, and is sensitive to subtle state imperfections. We further compare the adaptive strategy with existing Wigner function sampling protocols experimentally, demonstrating the advantage of adaptive sampling for measurement-efficient fidelity estimation with respect to a family of cat states. Finally, we incorporate the reconstructed fidelity into the figure of merit used in a proof-of-principle closed-loop quantum optimal control experiment, demonstrating the applicability of the method to autonomous optimisation of bosonic quantum states.
Adaptive Spectroscopy of Fast Two-Level-System Dynamics in Superconducting Qubits
Parasitic two-level-system (TLS) defects are a major source of energy relaxation and temporal instability in superconducting quantum processors. Our sub-second adaptive spectroscopy
reveals telegraphic switching of TLSs with a characteristic timescale of a few seconds and spectral diffusion with diffusivity D≈0.9 MHz2/s. These timescales are about 3×102 times faster than what is observed in conventional nonadaptive spectroscopy, which typically requires hours of measurement time. We resolve such fast dynamics on a field-programmable gate array (FPGA)-based controller that enables measurement of frequency- and time-resolved relaxations with sub-second temporal resolution in flux-tunable superconducting qubits. We observe similar defect dynamics across multiple qubits in independently fabricated devices measured in different laboratories. We correlate TLS-induced fluctuations with gate-level errors using randomized benchmarking. Our results reveal a previously inaccessible regime of frequency-resolved TLS dynamics and redefine the timescales relevant to TLS-aware characterization and calibration of superconducting quantum processors.
02
Aug
2026
Revisiting Thermal Scalability for Large-Scale Superconducting Quantum Systems
The readout amplification chain imposes a critical thermal scalability bottleneck in large-scale superconducting quantum systems. This happens through three mechanisms: amplifier dissipation,
passive conduction through bias wiring and Joule heating within that same wiring. These terms are absent or only partially represented in several prior system-level thermal-scalability models, leading to bottleneck misidentification and scalability overestimation. In this work, we improve upon previous system-level heat estimation models by fully accounting for the major heat sources in modern cryogenic quantum systems including the active dissipation, passive conduction, and Joule heating in the readout amplifier module. Our analysis demonstrates that amplifier-associated heat emerges as the dominant thermal bottleneck that fundamentally alters the thermal landscape of modern large-scale cryogenic systems. We explore various technology options and their tradeoffs to identify configurations that reduce this critical heat load and improve scalability. Finally, we evaluate forward-looking system configurations, including larger refrigeration platforms and optical approaches, and analyze forward-looking pathways toward single-fridge 10k-qubit cryogenic systems.
31
Jul
2026
Origins of microwave losses in superconducting circuits made with silicon-on-insulator substrates
Silicon-on-insulator technology is widely used to fabricate silicon based devices, from advanced transistors to photonic circuits or nanomechanical systems. Integrating low loss superconducting
quantum circuits with silicon-on-insulator substrates enables to couple the advantages offered by the mature silicon technology to the exquisite sensitivity of superconducting circuits. The natural approach, inherited from research in superconducting microwave devices, is to use a substrate made with highly resistive silicon, known for its low level of microwave losses. In this work, using superconducting microwave resonators, we show that counterintuitively, standard resistivity silicon-on-insulator substrates perform better than high resistivity silicon-on-insulator substrates at cryogenic temperatures. In the latter case, the presence of a parasitic sheet conduction at the interface between bulk silicon and silicon oxide acts as the dominant loss mechanism. This parasitic sheet can be suppressed using substrates with intentionally induced traps. In such substrates, losses are ultimately limited by the dielectric losses of the silicon oxide layer. These substrates offer interesting perspectives for the development of superconducting nanoelectromechanical systems. First, the release, i.e. the removal of the silicon oxide, could be limited to the moving parts, thereby maintaining the mechanical integrity of the rest of the device. Additionally, such structure would enhance heat evacuation into the bulk of the substrate which is an issue in current devices such as microwave-to-optics converters.
30
Jul
2026
Mitigating quantum decoherence via global optimal control
We show that global optimal control can drastically suppress the impact of decoherence in globally driven superconducting quantum computing architectures, taking as a prototype a recently
proposed quasi-two-dimensional ladder geometry. Using a tensor-network-based approach, we quantify how amplitude-damping and dephasing channels degrade the flow of quantum information along the ladder and the fidelity of one- and two-qubit gate operations. We then demonstrate that shaping the global drive compresses the gate sequences by an order of magnitude in time, restoring high gate fidelities. We stress that this mitigation is far from trivial: in a globally driven processor, dissipation acts on every physical qubit—including those outside the logical register that sustain the surrounding ordered phases—so its impact cannot be suppressed by protecting an isolated subsystem, and is instead overcome purely through the temporal shaping of the global drive.
29
Jul
2026
Native CCZ Gate with Fluxonium Qubits and a Microwave-Driven Coupler
Native multi-qubit gates could reduce the overhead associated with decompositions into single- and two-qubit operations, but whether they can simultaneously provide high fidelity, simple
control and robustness against parasitic interactions in scalable architectures remains unclear. Here we experimentally realize a 65-ns native controlled-controlled-phase operation, locally equivalent to the Toffoli gate, with a fidelity of 99.39(5)% in a three-qubit processor unit based on fluxonium qubits coupled via a microwave-driven transmon coupler. The implemented operation would require CZ fidelities of approximately 99.94% if realized through a conventional decomposition. The gate is implemented with a single control pulse, that relies on a simple calibration procedure yielding coherence-limited performance. This processor unit naturally extends to scalable two-dimensional layouts with low parasitic interactions. Altogether, these results establish native multi-qubit gates as a viable hardware-efficient primitive for scalable superconducting quantum processors.
Hardware-efficient erasure-error detection with an integer fluxonium
Erasure-error detection can improve the efficiency of quantum error correction by revealing the times and locations of their error events. In this work, we demonstrate erasure conversions
and mid-circuit erasure detections in a single integer fluxonium, in which the states |g⟩,|f⟩ encode the logical states and |e⟩ encodes the erasure state. The integer fluxonium suppresses direct |f⟩→|g⟩ transitions and allows the dominant |f⟩→|e⟩ transitions to be converted into detectable erasures. Furthermore, we identified a design space that nullifies the resonant-frequency shift between the two logical states, enabling ancilla-free mid-circuit erasure checks using the same resonator employed for final readout. By discarding the detected erasure events, we achieved an 8.4-fold increase in the |f⟩ state lifetime, a 1.38-fold increase in the Hahn-echo time, and a reduction of single-qubit gate error from 0.061(2)% to 0.030(5)%. Our results establish integer fluxonium as a hardware-efficient platform for erasure-error detection and conversion, while identifying the improvements required to realize an effective erasure qubit with high erasure bias.
28
Jul
2026
Precise 2D electric field density simulations for superconducting quantum devices
Dielectric loss due to two-level systems is a limiting factor for superconducting qubit relaxation times. These losses arise mostly from nanometer-scale interfacial defect regions in
superconducting devices with planar dimensions of microns to millimeters, thus making it resource intensive to accurately simulate the electric field density in these regions with traditional electromagnetic solvers. In this work, we demonstrate a fast boundary integral equation solver that allows precise simulation of electric field density in these thin regions, showing a speedup of around two orders of magnitude over traditional solvers, with relative errors around 10−7 for a ten-minute solution runtime. By computing participation ratios through Green’s first identity without squaring the electric field, our approach is less susceptible to the field singularities near conductor corners. We apply this solver to a basic untrenched coplanar waveguide cross-section, showing that the common assumption of participation ratio linearity with dielectric constant holds well for some interfaces and not others; in particular, while the metal-air (MA) top and corner follow this linear relationship strongly, the MA sidewall does not. We then compare isotropic and anisotropic etching, showing that the MA sidewall and the metal-air-substrate triple junction are the most strongly affected. We are currently leveraging this solver to explore geometries that will uniquely isolate the participation ratios of the different dielectrics. Finally, we are working to combine this solver framework with a full 3D microwave solver to accurately calculate participation ratios for the thin dielectrics that are known sources of loss in superconducting qubits.