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
21
Okt
2026
High-fidelity and robust controlled-Z gates for superconducting transmon qubits. I. Intrinsic errors and decoherence
Fault-tolerant quantum computing requires quantum gates that combine short duration, high fidelity, and robustness to noise. As quantum processors scale, robustness becomes increasingly
important because exhaustive characterization and frequent recalibration of individual gates become impractical. We present a comprehensive optimal-control framework for controlled-Z (CZ) gates between directly coupled, frequency-tunable transmons that systematically accounts for intrinsic Hamiltonian errors and realistic decoherence mechanisms, including energy relaxation and dephasing. Using smooth, single-shot flux pulses, we obtain 12-ns CZ gates—close to the theoretical speed limit of 11.6 ns—with intrinsic errors suppressed to the 2×10−5 level. We construct dephasing noise models, calibrated against published experimental measurements, that capture fast dephasing from room-temperature control electronics and slow dephasing from 1/f flux noise. Using these models, we identify dephasing sweet spots that suppress sensitivity to dephasing noise. We further exploit transmon asymmetry as a design parameter to access distinct noise-resilient operating regimes. These results establish an experimentally applicable framework for balancing gate speed, fidelity, and noise robustness while reducing characterization and recalibration overhead.
20
Okt
2026
Bias-Preserving Entanglement of Kerr-Cat Qubits with an ATS Coupler
Kerr-cat qubits offer a hardware-efficient route to fault-tolerant quantum computing by suppressing bit-flip errors exponentially, while phase-flip errors increase only linearly with
the mean photon number. To maintain this protection, entangling operations must be bias-preserving, ensuring that phase-flip errors occurring during the gate do not propagate into bit-flip errors. In this paper, we propose an asymmetrically threaded SQUID (ATS) parametric coupler to implement high-fidelity, bias-preserving entangling gates between Kerr-cat qubits. By engineering a photon-number asymmetry between the control and target modes, we enable fast gate operations that suppress non-adiabatic leakage without complex pulse shaping. Through numerical simulations of the full circuit Hamiltonian, we demonstrate a bias-preserving CX gate achieving 99.69% fidelity in the absence of photon loss for a gate duration of 50 ns. Under a single-photon loss rate of 100 kHz, our protocol yields a noise bias of 9922, exceeding the noise bias of a naive non-bias-preserving CX implementation by approximately a factor of 570. These results support the ATS coupler as a valuable building block for concatenated cat-qubit architectures.
Engineering \(XY\) and \(ZZ\) interactions between two remote superconducting qubits via a multimode waveguide
Waveguides provide a flexible platform for long-range interactions between superconducting qubits. Here, we study how to engineer the effective (XY) and (ZZ) interactions between
two qubits via qubit frequency assignments and multimode interference determined by the mode configuration and standing-wave parities of a finite multimode waveguide. For a single-mode waveguide, we find that \(XY\) and \(ZZ\) interactions cannot be simultaneously suppressed to zero and the residual qubit-qubit coupling always exists. For example, the \(XY\) interaction may be suppressed to a negligible level when two qubits and the waveguide mode satisfy the two-photon resonant condition, but the \(ZZ\) interaction cannot be switched off. For multimode configurations within the central dispersive window, the engineered interaction types are very different for odd and even numbers of modes. Even-mode configurations can readily produce strong \(XY\) interactions, whereas odd-mode configurations can strongly suppress the \(XY\) interaction while retaining a finite \(ZZ\) interaction. Full-Hamiltonian simulations of minimal two- and three-mode configurations confirm \(XY\)-dominated SWAP dynamics and \(ZZ\)-dominated CZ-type dynamics, respectively. These results establish a frequency assignment criterion of qubits and waveguide modes for selectively engineering long-range qubit-qubit interaction types induced by the waveguide with finite multimode.
Multimode Parametric Amplifier for K-Band Quantum Measurements
We demonstrate K-band nondegenerate three-wave-mixing amplification in a multimode resonator comprising an array of 500 SNAILs. Pairing K-band signal modes with lower-frequency internal
idler modes enables 20 dB of gain continuously from 19.6 to 25.6 GHz with pump frequencies below 32.2 GHz. At 20 dB gain, the 3-dB bandwidth ranges from 2.6 to 11.7 MHz, and the input-referred 1-dB compression power reaches -100.9 dBm. In the 20-22 GHz band, the average added noise is 0.73 quanta. A two-mode nondegenerate parametric amplification model describes the measured gain response and gain-bandwidth behavior, while a multimode Kerr model captures the overall compression scale. Our amplifier provides a practical solution for implementing a state-of-the-art readout of superconducting qubits in the K-band.
Controlled-Z Gates via Floquet Coupling to Two Auxiliary States in Transmon Circuits
High-fidelity entangling gates are essential for quantum information processing. In transmon circuits, higher excited states provide additional pathways for engineering interactions
within the computational subspace. Here we demonstrate controlled-Z (CZ) gates that exploit two noncomputational states, $\ket{02}$ and $\ket{20}$, through selective Floquet coupling. The nontrivial parametric modulation can activate both auxiliary transitions from $\ket{11}$ while suppressing the leading exchange between $\ket{01}$ and $\ket{10}$. Based on this approach, we develop two gate protocols: a nonadiabatic round trip through a bright auxiliary superposition and adiabatic evolution along a zero-energy dark state, both achieving average gate fidelities above 99.99% in numerical simulations. Control of the modulation phase further extends the bright-state protocol to conventional and unconventional geometric CZ gates. Effective-model simulations show that unconventional control improves tolerance to residual ZZ coupling and static qubit-frequency offsets relative to conventional control over the parameter ranges examined. Simulations including neighboring spectator qubits show that the original nonadiabatic bright-state and adiabatic dark-state CZ protocols retain high gate fidelities. This framework offers new routes toward the experimental realization of CZ gates and their geometric implementations.
Optimal Quantum-Circuit Synthesis and Implementation of General Unitaries with Parallelized Bitstring-Conditioned Rotations
We theoretically introduce a quantum-circuit synthesis protocol to realize any n-qubit transformation U∈U(2n), up to a diagonal. The resulting circuit is directly native to and optimal
in gate count and depth for systems where bitstring-conditioned rotations with tunable angle and axis, C(n−1)R(θ,ϕ), form the set of native operations. Subsequently, we suggest how this protocol can be implemented in a few-mode superconducting circuit, and experimentally demonstrate the method for the smallest nontrivial case, i.e., for two qubits. We report an average gate fidelity of 97.6 % for Haar-random two-qubit unitaries up to phase shifts in the computational basis. Importantly, these results suggest that multimode devices, such as those in the multimon or multiunimon families may become competitive with qubit-array networks for general-unitary implementations for qubit counts as low as three.
19
Okt
2026
Sidewall Metrology Protocol for Superconducting Qubit Devices
Sidewall morphology is a potentially important factor influencing superconducting qubit performance, yet its quantitative characterization remains challenging due to the absence of
standardized measurement protocols. In this work, we develop a scanning probe microscopy-based measurement framework for the quantitative characterization of superconducting qubit sidewalls. The framework introduces a refined root mean square lateral sidewall roughness descriptor and a new curvature-equivalent radius for quantifying vertical sidewall morphology. The descriptors and measurement procedures are applied to different components of qubit devices, forming the basis of a proposed sidewall characterization protocol. This provides a foundation for quantitative comparisons of fabrication processes across laboratories, enabling future investigations of correlations between sidewall geometry and qubit performance.
Cavity models of non-Abelian gauge theories
In quantum optics and related fields, cavity quantum electrodynamics provides a powerful framework for modeling the low-energy properties of bound charges coupled to electromagnetic
fields, using a minimal set of atomic levels and isolated photonic modes. Here, we develop a similar framework for non-Abelian gauge theories, focusing on the pure-gauge dynamics of SU(2) fields as the conceptually simplest example. Starting from the corresponding Yang-Mills Lagrangian, we derive an SU(2) cavity Hamiltonian in the Coulomb gauge, represented in terms of only three localized bosonic modes. Unlike its non-interacting electromagnetic counterpart, this model exhibits a nontrivial excitation spectrum featuring bound composite gauge particles (‚glueballs‘) as the lowest-energy configurations. We extend this framework to one-dimensional arrays of coupled SU(2) cavities and discuss the driven excitation of a glueball condensate and the clustering of multi-glueball states as two elementary interaction-induced processes in these models. These examples offer a glimpse into the multitude of novel many-body phenomena that arise within this cavity SU(2) framework and we show how this physics at the interface between particle physics and quantum optics can be experimentally explored using state-of-the-art superconducting quantum circuits.
Active isolation of transmon readout with a tunable inductive bridge
Scalable superconducting quantum computation requires fast, high-fidelity readout. Currently, this is assisted through cryogenic amplification of the readout signal. In traditional
dispersive readout schemes, the amplifiers that are used to provide high signal-to-noise ratio also generate thermal noise that can decohere the qubits, demanding additional ferrite-based isolators to suppress this noise. Here, we integrate a chip-scale microwave switch, the tunable inductive bridge, into the readout chain of a fixed-frequency transmon. Flux-pulsing the switch, such that it transmits only during the readout, did not make a discernable difference to qubit coherence, residual excited-state population, or readout fidelity metrics. By injecting broadband noise that mimics amplifier thermal photons, we show that the switch suppresses the resulting qubit dephasing by more than 30~dB, consistent with its independently measured on-off ratio, establishing it as a potential chip-based integrated replacement for readout-chain isolators.
16
Okt
2026
Optimal robust single-qubit gates in superconducting quantum processors
Fault-tolerant quantum computing requires high-fidelity quantum gates that are resilient to control errors and device imperfections. Here, we develop energy-optimal control pulses for
single-qubit gates that are simultaneously robust against frequency detuning and waveform inhomogeneities while also suppressing leakage outside the computational subspace. Combining the optimized pulse envelopes with an adaptive control scheme, we experimentally demonstrate optimal robust gates on a superconducting quantum processor. For a gate duration of 90ns, we achieve fidelities above 99.9% for the X gate and 99.6% for Hadamard-type gates. For the X gate, we maintain fidelities above 99.5% for simultaneous drive-amplitude variations up to 20% and qubit detuning up to 4 MHz. This robustness outperforms that of conventional Gaussian pulses by an order of magnitude, without much difference in peak gate fidelity. These results establish a route towards robust single-qubit control in large-scale quantum processors, advancing the development of fault-tolerant quantum computing.