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
29
Okt
2025
Decoherence Estimation of Superconducting Qubit
Decoherence of quantum bits arises primarily from the parasitic resistance within the qubit. This study presents the analysis of the decoherence process due to physical interactions
between the qubit photons and parasitic resistance atoms, utilizing exclusively the Caldeira-Leggett electrical model, without relying on external Hamiltonians. The analysis shows a good agreement between the model of the electrical noise and the Johnson-Nyquist noise. The emission and absorption rates of the qubit’s coherent loss, required for the Lindblad master equation that approximates the decoherence, are obtained. A numerical substitution in the analysis result yields a strong correlation with previous measurements. The present analysis enables also the derivation of the appropriate circuit characteristics for future simulations.
28
Okt
2025
Exploring the Fidelity of Flux Qubit Measurement in Different Bases via Quantum Flux Parametron
High-fidelity qubit readout is a fundamental requirement for practical quantum computing systems. In this work, we investigate methods to enhance the measurement fidelity of flux qubits
via a quantum flux parametron-mediated readout scheme. Through theoretical modeling and numerical simulations, we analyze the impact of different measurement bases on fidelity in single-qubit and coupled two-qubit systems. For single-qubit systems, we show that energy bases consistently outperform flux bases in achieving higher fidelity. In coupled two-qubit systems, we explore two measurement models: sequential and simultaneous measurements, both aimed at reading out a single target qubit. Our results indicate that the highest fidelity can be achieved either by performing sequential measurement in a dressed basis over a longer duration or by conducting simultaneous measurement in a bare basis over a shorter duration. Importantly, the sequential measurement model consistently yields more robust and higher fidelity readouts compared to the simultaneous approach. These findings quantify achievable fidelities and provide valuable guidance for optimizing measurement protocols in emerging quantum computing architectures.
27
Okt
2025
Heat measurement of quantum interference
Quantum coherence plays a key role in the operation and performance of quantum heat engines and refrigerators. Despite its importance for the fundamental understanding in quantum thermodynamics
and its technological implications, coherence effects in heat transport have not been observed previously. Here, we measure quantum features in the heat transfer between a qubit and a thermal bath in a system formed of a driven flux qubit galvanically coupled to a λ/4 coplanar-waveguide resonator that is coupled to a heat reservoir. This thermal bath is a normal-metal mesoscopic resistor, whose temperature can be measured and controlled. We detect interference patterns in the heat current due to driving-induced coherence. In particular, resonance peaks in the heat transferred to the bath are found at driving frequencies which are integer fractions of the resonator frequency. A selection rule on the even/odd parity of the peaks holds at the qubit symmetry point. We present a theoretical model based on Floquet theory that captures the experimental results. The studied system provides a platform for studying the role of coherence in quantum thermodynamics. Our work opens the possibility to demonstrate a true quantum thermal machine where heat is measured directly.
A Scalable Superconducting Circuit Framework for Emulating Physics in Hyperbolic Space
Theoretical studies and experiments in the last six years have revealed the potential for novel behaviours and functionalities in device physics through the synthetic engineering of
negatively-curved spaces. For instance, recent developments in hyperbolic band theory have unveiled the emergence of higher-dimensional eigenstates — features fundamentally absent in conventional Euclidean systems. At the same time, superconducting quantum circuits have emerged as a leading platform for quantum analogue emulations and digital simulations in scalable architectures. Here, we introduce a scalable superconducting circuit framework for the analogue quantum emulation of tight-binding models on hyperbolic and kagome-like lattices. Using this approach, we experimentally realize three distinct lattices, including, for the first time to our knowledge, a hyperbolic lattice whose unit cell resides on a genus-3 Riemann surface. Our method encodes the hyperbolic metric directly into capacitive couplings between high-quality superconducting resonators, enabling tenable reproduction of spectral and localization properties while overcoming major scalability and spectral resolution limitations of previous designs. These results set the stage for large-scale experimental studies of hyperbolic materials in condensed matter physics and lay the groundwork for realizing hyperbolic quantum processors, with potential implications for both fundamental physics and quantum computing
24
Okt
2025
Probing Sensitivity near a Quantum Exceptional Point using Waveguide Quantum Electrodynamics
Non-Hermitian Hamiltonians with complex eigenenergies are useful tools for describing the dynamics of open quantum systems. In particular, parity and time (PT) symmetric Hamiltonians
have generated interest due to the emergence of exceptional-point degeneracies, where both eigenenergies and eigenvectors coalesce as the energy spectrum transitions from real- to complex-valued. Because of the abrupt spectral response near exceptional points, such systems have been proposed as candidates for precision quantum sensing. In this work, we emulate a passive \PT~dimer using a two-mode, non-Hermitian system of superconducting qubits comprising one high-coherence qubit coupled to an intentionally lossy qubit via a tunable coupler. The loss is introduced by strongly coupling the qubit to a continuum of photonic modes in an open waveguide environment. Using both pulsed and continuous-wave measurements, we characterize the system dynamics near the exceptional point. We observe a behavior broadly consistent with an ideal passive \PT~dimer with some corrections due to the tunable coupler element. We extract the complex eigenenergies associated with the two modes and calculate the sensitivity as a function of the coupling strength. Confirming theoretical predictions, we observe no sensitivity enhancement near the quantum exceptional point. This study elucidates the limitations of exceptional-point systems as candidates for quantum-enhanced sensing.
23
Okt
2025
Fabrication and Structural Analysis of Trilayers for Tantalum Josephson Junctions with Ta2O5 Barriers
Tantalum (Ta) has recently emerged as a promising low-loss material, enabling record coherence times in superconducting qubits. This enhanced performance is largely attributed to its
stable native oxide, which is believed to host fewer two-level system (TLS) defects key − contributors to decoherence in superconducting circuits. Nevertheless, aluminum oxide (AlOx) remains the predominant choice for Josephson junction barriers in most qubit architectures. In this study, we systematically investigate various techniques for forming high-quality oxide layers on α-phase tantalum (α-Ta) thin films, aiming to develop effective Josephson junction barriers. We explore thermal oxidation in a tube furnace, rapid thermal annealing, as well as plasma oxidation of both room-temperature and heated Ta films, and propose a mechanistic picture of the underlying oxidation mechanisms. All methods yield Ta2O5, the same compound as tantalum’s native oxide. Among these, plasma oxidation produces the smoothest and highest-quality oxide layers, making it particularly well-suited for Josephson junction fabrication. Furthermore, we demonstrate the successful epitaxial growth of α-Ta atop oxidized α-Ta films, paving the way for the realization of trilayer Ta/Ta-O/Ta Josephson junctions with clean, low-loss interfaces.
Low-temperature electron dephasing rates indicate magnetic disorder in superconducting TiN films
We investigate electron transport and phase-breaking processes in thin titanium nitride (TiN) films of epitaxial quality. Previous studies show that a minute surface magnetic disorder
significantly reduces the critical temperature (Tc) and broadens the superconducting transition as the film thickness and device size decrease. We measure electron dephasing rates via magnetoresistance from Tc to ∼4Tc in various-thickness TiN films. Electron dephasing occurs on the picosecond timescale and is nearly independent of temperature, differing from the expected inelastic scattering due to the electron-phonon and electron-electron interactions near Tc, which occur over a nanosecond timescale. We propose spin-flip scattering as a possible additional phase-breaking mechanism. The significant increase in the dephasing rate for the thinnest film indicates that magnetic disorder resides near the surface of naturally oxidized films. Our research suggests that magnetic disorder may be a significant contributor to RF dissipation in superconducting devices based on TiN.
Electronically-controlled one- and two-qubit gates for transmon quasicharge qubits
Superconducting protected qubits aim to achieve sufficiently low error rates so as to allow realization of error-corrected, utility-scale quantum computers. A recent proposal encodes
a protected qubit in the quasicharge degree of freedom of the conventional transmon device, here referred to as the `quasicharge qubit‘. Operating such a protected qubit requires implementing new strategies. Here we show that an electronically-controllable tunnel junction formed by two topological superconductors can be used to implement single- and two-qubit gates on quasicharge qubits. Schemes for both these gates are based on dynamical 4π-periodic Josephson effect and therefore have gate speeds of the same order. The simulation of the dynamics of a topological Josephson junction in a parameter regime with non-negligible charging energy is the key novelty of this work. We also characterize the robustness of such gate operations against charge noise using Fermi’s golden rule. Our results point to a compelling strategy for implementation of quasicharge qubit gates based on junctions of minimal Kitaev chains of quantum dots.
Parametric Phase Modulation in Superconducting Circuits
Parametric modulation is widely employed in superconducting circuits for quantum simulations and high-fidelity two-qubit gates, valued for its versatility. Conventionally, the qubit
coupling strength is determined by the amplitude of the parametric flux pulse, which affects qubit parameters dramatically. In this article, we propose and implement a phase modulation scheme to tune the interaction strength via adjusting the relative phase between the parametric flux pulses applied to two coupled qubits. We characterize this modulation for sideband couplings, at both sweet and offsweet spots, achieving a broad range of coupling strengths as confirmed by both population dynamics and spectroscopy methods. This approach enables phase-controlled modulation of coupling strength, providing a promising candidate for parametrically driven quantum simulations and gate operations.
Nontrivial topological phases in „Zig-Zag“ arrays of polarization transmons
In recent years, quantum simulators of topological models have been extensively studied across a variety of platforms and regimes. A new promising research direction makes use of meta-atoms
with multiple intrinsic degrees of freedom, which to date have been predominantly studied in the classical regime. Here, we propose a superconducting quantum simulator to study an extension of the well-known „Zig-Zag“ model with long-range cross-polarization couplings using polarization transmons hosting degenerate dipole orbitals. We map the phase transitions of the extended „Zig-Zag“ model both numerically and analytically using inverse participation ratios and topological invariants. We demonstrate the existence of in-gap localized trivial and Tamm edge states. With linearized meta-atoms, we show via electromagnetic modeling that the proposed arrangement closely reproduces the extended „Zig-Zag“ model. This work paves the way towards experimental investigation of the previously inaccessible topological quantum many-body phenomena.