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
10
Jul
2025
Emergent Harmonics in Josephson Tunnel Junctions Due to Series Inductance
Josephson tunnel junctions are essential elements of superconducting quantum circuits. The operability of these circuits presumes a 2π-periodic sinusoidal potential of a tunnel junction,
but higher-order corrections to this Josephson potential, often referred to as „harmonics,“ cause deviations from the expected circuit behavior. Two potential sources for these harmonics are the intrinsic current-phase relationship of the Josephson junction and the inductance of the metallic traces connecting the junction to other circuit elements. Here, we introduce a method to distinguish the origin of the observed harmonics using nearly-symmetric superconducting quantum interference devices (SQUIDs). Spectroscopic measurements of level transitions in multiple devices reveal features that cannot be explained by a standard cosine potential, but are accurately reproduced when accounting for a second-harmonic contribution to the model. The observed scaling of the second harmonic with Josephson-junction size indicates that it is due almost entirely to the trace inductance. These results inform the design of next-generation superconducting circuits for quantum information processing and the investigation of the supercurrent diode effect.
09
Jul
2025
Flexible Readout and Unconditional Reset for Superconducting Multi-Qubit Processors with Tunable Purcell Filters
Qubit readout and reset are critical components for the practical realization of quantum computing systems, as outlined by the DiVincenzo criteria. Here, we present a scalable architecture
employing frequency-tunable nonlinear Purcell filters designed specifically for superconducting qubits. This architecture enables flexible readout and unconditional reset functionalities. Our readout protocol dynamically adjusts the effective linewidth of the readout resonator through a tunable filter, optimizing the signal-to-noise ratio during measurement while suppressing photon noise during idle periods. Achieving a readout fidelity of 99.3% without using Josephson parametric amplifiers or traveling-wave parametric amplifiers, even with a small dispersive shift, demonstrates its effectiveness. For reset operations, our protocol utilizes the tunable coupler adjacent to the target qubit as an intermediary to channel qubit excitations into the Purcell filter, enabling rapid dissipation. We demonstrate unconditional reset of both leakage-induced |2⟩ and |1⟩ states within 200 ns (error rate ≤1%), and reset of the |1⟩ state alone in just 75 ns. Repeated reset cycles (≤600 ns) further reduce the error rate below 0.1%. Furthermore, the filter suppresses both photon noise and the Purcell effect, thereby reducing qubit decoherence. This scalable Purcell filter architecture shows exceptional performance in qubit readout, reset, and protection, marking it as a promising hardware component for advancing fault-tolerant quantum computing systems.
08
Jul
2025
Surface-Code Hardware Hamiltonian
We present a scalable framework for accurately modeling many-body interactions in surface-code quantum processor units (QPUs). Combining a concise diagrammatic formalism with high-precision
numerical methods, our approach efficiently evaluates high-order, long-range Pauli string couplings and maps complete chip layouts onto exact effective Hamiltonians. Applying this method to surface-code architectures, such as Google’s Sycamore lattice, we identify three distinct operational regimes: computationally stable, error-dominated, and hierarchy-inverted. Our analysis reveals that even modest increases in residual qubit-qubit crosstalk can invert the interaction hierarchy, driving the system from a computationally favorable phase into a topologically ordered regime. This framework thus serves as a powerful guide for optimizing next-generation high-fidelity surface-code hardware and provides a pathway to investigate emergent quantum many-body phenomena.
How the Kerr-Cat Qubit Dies-And How to Rescue It
Kerr-cat qubits have been experimentally shown to exhibit a large noise bias, with one decay channel suppressed by several orders of magnitude. In superconducting implementations, increasing
the microwave drive on the nonlinear oscillator that hosts the Kerr-cat qubit should, in principle, further enhance this bias. Instead, experiments reveal that above a critical drive amplitude the tunneling time – the less dominant decay channel – ceases to increase and even decreases. Here, we show that this breakdown arises from the multimode nature of the circuit implementation: specifically, the buffer mode used to control the Kerr-cat qubit can induce multiphoton resonances that sharply degrade Kerr-cat coherence if its frequency is not carefully chosen. We uncover this mechanism by retaining the full circuit nonlinearities and treating the strong drive exactly within a Floquet-Markov framework that incorporates quasidegeneracies in the Kerr-cat spectrum. Our results not only provide an explanation for the sudden reduction of the tunneling time but also demonstrate the robustness of the Kerr-cat qubit when its surrounding electromagnetic environment is carefully engineered.
07
Jul
2025
Engineering giant transmon molecules as mediators of conditional two-photon gates
Artificial atoms non-locally coupled to waveguides — the so-called giant atoms — offer new opportunities for the control of light and matter. In this work, we show how to
use an array of non-locally coupled transmon „molecules“ to engineer a passive photonic controlled gate for waveguide photons. In particular, we show that a conditional elastic phase shift between counter-propagating photons arises from the interplay between direction-dependent couplings, engineered through an interplay of non local interactions and molecular binding strength; and the nonlinearity of the transmon array. We analyze the conditions under which a maximal π-phase shift — and hence a CZ gate — is obtained, and characterize the gate fidelity as a function of key experimental parameters, including finite transmon nonlinearities, emitter spectral inhomogeneities, and limited cooperativity. Our work opens the use of giant atoms as key elements of microwave photonic quantum computing devices.
Engineering a Multi-Mode Purcell Filter for Superconducting-Qubit Reset and Readout with Intrinsic Purcell Protection
Efficient qubit reset and leakage reduction are essential for scalable superconducting quantum computing, particularly in the context of quantum error correction. However, such operations
often require additional on-chip components. Here, we propose and experimentally demonstrate a mode-efficient approach to qubit reset and readout using a multi-mode Purcell filter in a superconducting quantum circuit. We exploit the inherent multi-mode structure of a coplanar waveguide resonator, using its fundamental and second-order modes for qubit reset and readout, respectively, thereby avoiding additional circuit elements. Implemented in a flip-chip architecture, our device achieves unconditional reset with residual excitation below 1% in 220 ns, and a leakage reduction unit that selectively resets the second excited state within 62 ns. Simulations predict Purcell-limited relaxation times exceeding 1 ms over an 800 MHz bandwidth. To our knowledge, this is the first experimental trial that exploits different-order modes of a microwave resonator for distinct qubit operations, representing a new direction toward scalable, mode-efficient quantum processor design.
04
Jul
2025
Circuit-QED simulator of the Bose-Hubbard model for quantum spin dynamics
We demonstrate an experimentally feasible circuit-QED Bose-Hubbard simulator that reproduces the complex spin dynamics of Heisenberg models. Our method relies on mapping spin-1/2 systems
onto bosonic states via the polynomially expanded Holstein-Primakoff (HP) transformation. The HP transformation translates the intricate behavior of spins into a representation that is compatible with bosonic devices like those in a circuit QED setup. For comparison, we also implement the Dyson-Maleev (DM) encoding for spin-1/2 and show that, in this limit, DM and HP are equivalent. We show the equivalence of the DM and the HP transformations for spin-1/2 systems. Rigorous numerical analyses confirm the effectiveness of our HP-based protocol. Specifically, we obtain the concurrence between the spin dynamics and the behavior of microwave photons within our circuit QED-based analog simulator that is designed for the Bose-Hubbard model. By utilizing the microwave photons inherent to circuit QED devices, our framework presents an accessible, scalable avenue for probing quantum spin dynamics in an experimentally viable setting.
High-power readout of a transmon qubit using a nonlinear coupling
The field of superconducting qubits is constantly evolving with new circuit designs. However, when it comes to qubit readout, the use of simple transverse linear coupling remains overwhelmingly
prevalent. This standard readout scheme has significant drawbacks: in addition to the Purcell effect, it suffers from a limitation on the maximal number of photons in the readout mode, which restricts the signal-to-noise ratio (SNR) and the Quantum Non-Demolition (QND) nature of the readout. Here, we explore the high-power regime by engineering a nonlinear coupling between a transmon qubit and its readout mode. Our approach builds upon previous work by Dassonneville et al. [Physical Review X 10, 011045 (2020)], on qubit readout with a non-perturbative cross-Kerr coupling in a transmon molecule. We demonstrate a readout fidelity of 99.21% with 89 photons utilizing a parametric amplifier. At this elevated photon number, the QND nature remains high at 96.7%. Even with up to 300 photons, the QNDness is only reduced by a few percent. This is qualitatively explained by deriving a critical number of photons associated to the nonlinear coupling, yielding a theoretical value of n¯critr=377 photons for our sample’s parameters. These results highlight the promising performance of the transmon molecule in the high-power regime for high-fidelity qubit readout.
01
Jul
2025
Temperature and Magnetic-Field Dependence of Energy Relaxation in a Fluxonium Qubit
Noise from material defects at device interfaces is known to limit the coherence of superconducting circuits, yet our understanding of the defect origins and noise mechanisms remains
incomplete. Here we investigate the temperature and in-plane magnetic-field dependence of energy relaxation in a low-frequency fluxonium qubit, where the sensitivity to flux noise and charge noise arising from dielectric loss can be tuned by applied flux. We observe an approximately linear scaling of flux noise with temperature T and a power-law dependence of dielectric loss T3 up to 100 mK. Additionally, we find that the dielectric-loss-limited T1 decreases with weak in-plane magnetic fields, suggesting a potential magnetic-field response of the underlying charge-coupled defects. We implement a multi-level decoherence model in our analysis, motivated by the widely tunable matrix elements and transition energies approaching the thermal energy scale in our system. These findings offer insight for fluxonium coherence modeling and should inform microscopic theories of intrinsic noise in superconducting circuits.
30
Jun
2025
Spectroscopy of drive-induced unwanted state transitions in superconducting circuits
Microwave drives are essential for implementing control and readout operations in superconducting quantum circuits. However, increasing the drive strength eventually leads to unwanted
state transitions which limit the speed and fidelity of such operations. In this work, we systematically investigate such transitions in a fixed-frequency qubit subjected to microwave drives spanning a 9 GHz frequency range. We identify the physical origins of these transitions and classify them into three categories. (1) Resonant energy exchange with parasitic two-level systems, activated by drive-induced ac-Stark shifts, (2) multi-photon transitions to non-computational states, intrinsic to the circuit Hamiltonian, and (3) inelastic scattering processes in which the drive causes a state transition in the superconducting circuit, while transferring excess energy to a spurious electromagnetic mode or two-level system (TLS) material defect. We show that the Floquet steady-state simulation, complemented by an electromagnetic simulation of the physical device, accurately predicts the observed transitions that do not involve TLS. Our results provide a comprehensive classification of these transitions and offer mitigation strategies through informed choices of drive frequency as well as improved circuit design.