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
16
Okt
2026
Probing wetting and solidification dynamics of neon thin films with a high-Tc superconducting circuit
Integrating condensed noble gases with superconducting circuits offers new opportunities for operating hybrid quantum devices. In particular, electrons on solid neon have recently emerged
as exceptionally coherent charge qubits, however the growth of the underlying neon films on patterned devices remains poorly controlled. We monitor neon films near the triple point using a multimode resonator fabricated from high-Tc YBCO, which remains superconducting at temperatures where the liquid phase exists and whose geometry sets a thickness-sensing window near 1 μm. In the liquid state we observe rich film dynamics: uniform wetting that saturates near 20 nm, drainage of thicker films, and spontaneous thickness oscillations, reflecting the extreme sensitivity of thick films in this regime. A galvanic connection to our sensing volume enables in-situ electrostatic tuning of the liquid film thickness. Statistics of freezing the neon across various liquid levels reveal variations of the solid film thickness by approximately an order of magnitude. We also observe a two-fold increase in the quality factor of the resonators at the level of 103 with added neon and rule out dielectric loss as the limiting factor. Our results demonstrate that hermetic packaging facilitates the investigation of both rich non-equilibrium fluid dynamics and the impact of condensed noble gas films on microwave loss in superconducting circuits, but may not be the best modality for growing solid neon films with precisely controlled thickness.
Experimental Demonstration of a Balanced Travelling Wave Parametric Amplifier
Superconducting travelling-wave parametric amplifiers (TWPAs) provide broadband gain, low added noise, and high dynamic range, making them attractive for applications requiring wide
instantaneous bandwidth. This is crucial for amplifying multiplexed narrow band signals spread over a wide frequency range, such as qubit resonator arrays, or amplifying inherently wide band signals, such as in spectrometers or radiometers. In many cases, however, the useful bandwidth of a TWPA is limited by the production of the idler tone at an image frequency. For three-wave-mixing (3WM) TWPAs, for example, an input signal at fs is indistinguishable from one at fp−fs, which can produce spectral ambiguity when the input spectrum is broadband or not known in advance. We experimentally demonstrate a balanced architecture that doubles the instantaneous image-rejected bandwidth available to a 3WM TWPA by spatially separating the amplified signal from the output pump and generated idler, which resolves the sideband degeneracy. The architecture consists of two amplifiers connected between a pair of 180∘ hybrid couplers, with the signal and pump injected and output through the sum and difference ports, respectively. Using two kinetic-inductance TWPAs, we demonstrate approximately 20 dB of signal gain across nearly an octave of bandwidth, together with pump rejection below –20 dB and an average image rejection of approximately –15 dB over the operating band. These results establish balanced TWPAs as a practical approach to broadband parametric amplification, enabling improved contamination free bandwidth and eliminating the need for complex filtering.
15
Okt
2026
Gyrators for superconducting circuit design
Assuming only the solubility of Kirchhoff’s laws, the conservation of energy, and the form of the constitutive relations, we present a flux-charge symmetric Hamiltonian formalism
that always admits a symplectic reduction for nonsingular, well-posed circuits involving gyrators, inductors, and capacitors (LCG circuits) with arbitrary non-linearity. The formalism introduces a notion of circuit duality on non-linear non-reciprocal circuits and a family of point transformations that inserts, removes, and moves reciprocal elements across gyrators while preserving the Hamiltonian spectrum. With these transformations, we extend Tellegen’s classic result that gyrators make reciprocal elements behave as their dual to non-linear reciprocal elements such as the Josephson junction and quantum phase slip. Furthermore, we present conditions under which non-reciprocal circuits can be transformed to dynamically equivalent reciprocal circuits.
ZZ-Free bSWAP Gates under Strong Driving in Far-Detuned Fixed-Frequency Transmons
Far-detuned frequency allocations in fixed-frequency superconducting processors mitigate spectral crowding but constrain cross-resonance interactions at practical drive amplitudes.
Here, we demonstrate that a single strong drive induces a large XX−YY interaction in the far-detuned regime, while a weak second tone tunes the accompanying ZZ phase. The interaction realizes a 150-ns bSWAP‾‾‾‾‾‾‾‾√ gate, which can directly prepare a maximally entangled state from the ground state. For the implementations on the same qubit pair at matched drive strengths, cross resonance gives a shorter CNOT operation, while bSWAP‾‾‾‾‾‾‾‾√ gives a 33% shorter SWAP operation with a higher IRB fidelity. The multilevel theory further predicts a regime where stronger driving can yield a speed advantage for CNOT as well, a possibility absent in the two-level model. As an application of the XX−YY interaction, we also demonstrate two-spin squeezing beyond the standard quantum limit through two-axis countertwisting. The interaction realized here complements cross resonance and offers a route to efficient gate synthesis and quantum simulation of interacting spin systems.
Superconducting circuits host two-level systems with lifetimes exceeding one second
Understanding the microscopic environment of superconducting qubits is a central challenge for quantum technologies. Parasitic two-level systems (TLSs) of various microscopic origins
are ubiquitous and limit qubit performance through decoherence, frequency drifts, and leakage. Recently, long-lived TLSs with lifetimes exceeding those of state-of-the-art qubits have been shown to induce correlated qubit dynamics, with direct implications for quantum error correction. Here, we perform wide-band quantum-jump correlation spectroscopy on flux-tunable transmon qubits and identify microscopic degrees of freedom that outlive the qubit by orders of magnitude. Pump-probe measurements at varying detunings from these objects reveal intrinsic lifetimes exceeding one second. Their weak coupling to the qubit, in the kilohertz range, is consistent with positional defects described by the standard tunneling model, without excluding other microscopic origins such as magnetic spin impurities or trapped quasiparticles. More broadly, these results establish superconducting qubits as a sensitive probe of exceptionally long-lived microscopic degrees of freedom and open a route toward their controlled access and functionalization.
Accurate gap control of 3D-integrated superconducting quantum circuits using asymmetric indium architecture and multi-angle flip-chip bonding
Flip-chip 3D integration is a key enabling technology for overcoming the routing and layout constraints of planar superconducting quantum processors, but it requires accurate and uniform
control of the inter-chip spacing while preserving qubit coherence. In conventional processes, thick indium bumps are deposited symmetrically on both chips, while dedicated spacers are introduced to limit the residual tilt. Here, we present a flip-chip bonding process based on an asymmetric indium architecture, combining 20μm thick indium bumps on the control chip with 200nm thin indium pads on the qubit chip. Using electrical and microwave measurements, we demonstrate low resistance and low loss indium interconnects, and show that transmon qubits integrated with this process reach relaxation and coherence times of T1=(159±14)μs and T2,echo=(294±42)μs, respectively, comparable to planar devices. Combined with a multi angle compression procedure that suppresses systematic tilt, this approach reduces the average post bonding tilt to (20±12)μrad. These results establish an asymmetric indium architecture combined with multi angle compression as a robust, coherence preserving route toward 3D integrated superconducting quantum processors.
14
Okt
2026
Mechanical trisqueezing in circuit quantum acoustodynamics with SNAIL devices
Continuous-variable quantum information processing with mechanical oscillators requires a uni- versal gate set, including a genuine non-Gaussian evolution. While circuit quantum acoustodynamics
(cQAD) experiments have realized bilinear and quartic (four-wave mixing) operations by coupling mechanical modes to transmon qubits, the intrinsic Kerr nonlinearity of these devices precludes access to odd-order effective Hamiltonians such as trisqueezing. Here we propose a cQAD architec- ture in which a mechanical mode, hosted in a high-overtone bulk acoustic-wave resonator (HBAR), is coupled to a superconducting nonlinear asymmetric inductive element (SNAIL). By combining the cubic nonlinearity of the SNAIL with parametric flux driving, we engineer a three-wave mixing interaction that generates trisqueezed mechanical states, producing the characteristic triangular, star-shaped Wigner function with negative regions indicative of non-Gaussianity. We derive the effective trisqueezing Hamiltonian from the full circuit model, identify the drive frequencies and flux modulation required to isolate the desired interaction from competing terms, and quantify the achievable coupling strengths for parameters compatible with current fabrication capabilities. Our results establish a hardware-efficient route to non-Gaussian resource generation in mechanical quantum systems, extending the toolbox of cQAD toward universal continuous-variable quantum computation and enhanced quantum-limited sensing.
Syndrome-Resolved Decoding of Qubit Readout Errors
Readout errors in superconducting qubits include incorrect state assignment, bit flips within the computational subspace, and leakage to non-computational states. These errors require
different mitigation strategies, yet conventional measurement fidelity and repeatability metrics do not resolve their individual contributions. Previous repeated-readout studies have shown that temporal correlations reveal additional information about readout errors. Here we introduce the Readout Error Decoder (RED), which converts repeated thresholded binary records into a channel-resolved error budget. Controlled error injections validate the channel-specific response of the decoder. We demonstrate RED on both fluxonium and transmon qubits and across distinct leakage-lifetime regimes. RED identifies the error channels limiting readout performance without requiring dedicated discrimination of non-computational states, providing a basis for targeted readout optimization across different qubit modalities.
Cryogenic Multiplexing Control Chip for Superconducting Quantum Processor
This paper proposes a scheme and design of cryogenic multiplexing control chip (MCC) which enables one microwave cable to control many superconducting qubits. Designed based on adjustable
inductor, MCC is a non-reciprocal device which generates microwave signals between 4 and 8 GHz and constructs XY control circuit to operate qubits. The effects of filters, isolators, power splitters, qubits and load impedance on MCC are analyzed in the frequency-domain simulation. And in time-domain simulation, the response time, Gaussian pulses, single-sideband, amplitude and frequency compensation of MCC are examined. The MCC can be integrated with the superconducting qubits on the same package at 20 mK temperature stage, and output specific microwave pulses used to drive the superconducting qubits without crosstalks. The MCC can greatly reduce the number of microwave control lines and is beneficial to the development of large-scale superconducting quantum processor.
13
Okt
2026
Development and experimental implementation of noise characterisation techniques on a superconducting quantum processor
Noise characterisation not only enables error mitigation but also provides valuable information about the intrinsic performance of a quantum device, identifying dominant error contributions
and bounding noise stability well beyond the aggregated calibration data. Deploying such methods on a specific hardware topology must combine efficiency and accuracy, assess noise stability, and validate the reconstructed noise model. Here we develop and implement these methods on a superconducting processor with square-lattice topology. First, we revise the cycle-benchmarking protocol and reduce the bias arising from unlearnable degrees of freedom via a regularised cost function. Second, we extend a previously developed tensor-network tomography method to match the device layout and show how its accuracy can be improved by repeated layer execution. Both methods allow approximate sublayer noise models to be extracted from parent ones. Using noise characterisation as a tool, we map the noise structure, track drift, and validate the adequacy of the learnt noise models against experimentally observed noisy estimations in benchmark physical and chemistry circuits. These results inform further improvements in the underlying technology and provide estimates of the size of noise-mitigated circuits that the device can support.