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
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.
27
Jun
2025
A scanning resonator for probing quantum coherent devices
Superconducting resonators with high quality factors are extremely sensitive detectors of the complex impedance of materials and devices coupled to them. This capability has been used
to measure losses in multiple different materials and, in the case of circuit quantum electrodynamics (circuit QED), has been used to measure the coherent evolution of multiple different types of qubits. Here, we report on the implementation of a scanning resonator for probing quantum coherent devices. Our scanning setup enables tunable coherent coupling to systems of interest without the need for fabricating on-chip superconducting resonators. We measure the internal quality factor of our resonator sensor in the single-photon regime to be > 10000 and demonstrate capacitive imaging using our sensor with zeptoFarad sensitivity and micron spatial resolution at milliKelvin temperatures. We then use our setup to characterize the energy spectrum and coherence times of multiple transmon qubits with no on-chip readout circuitry. Our work introduces a new tool for using circuit QED to measure existing and proposed qubit platforms.
25
Jun
2025
Computed tomography of propagating microwave photons
Propagating photons serve as essential links for distributing quantum information and entanglement across distant nodes. Knowledge of their Wigner functions not only enables their deployment
as active information carriers but also provides error diagnostics when photons passively leak from a quantum processing unit. While well-established for standing waves, characterizing propagating microwave photons requires post-processing of room-temperature signals with excessive amplification noise. Here, we demonstrate cryogenic and amplification-free Wigner function tomography of propagating microwave photons using a superconductor-normal metal-superconductor bolometer based on the resistive heating effect of absorbed radiation. By introducing two-field interference in power detection, the bolometer acts as a sensitive and broadband quadrature detector that samples the input field at selected angles at millikelvin with no added noise. Adapting the principles of computed tomography (CT) in medical imaging, we implement Wigner function CT by combining quadrature histograms across different projection angles and demonstrate it for Gaussian states at the single-photon level with different symmetries. Compressed sensing and neural networks further reduce the projections to three without compromising the reconstruction quality. These results address the long-standing challenge of characterizing propagating microwave photons in a superconducting quantum network and establish a new avenue for real-time quantum error diagnostics and correction.
Reflection-less filter for superconducting quantum circuits
Protecting superconducting quantum circuits from non-ideal return loss, including out-of-band circulator behavior and enhancing the performance of broadband quantum-limited amplifiers
can be accomplished using a superconducting version of a special class of microwave filters known as reflection-less filters. These filters can simultaneously permit low pass band loss to preserve quantum efficiency and broad band reflection-less characteristics in the stop and pass bands. The filter also suppresses thermal photons emitted in its pass band from the termination resistors by the nature of the dual network topology. This work will review the application, theory, design, and modeling of a superconducting reflection-less filter, followed by fabrication details and the presentation of cryogenic performance measurements of a monolithic device. The filter was fabricated using Al on Si, incorporating NiCr resistors, which allows for simple integration with other superconducting quantum devices. The filter with an area of 0.6 mm2 achieves insertion loss below 1 dB, including its connectorized package over a 80\% fractional bandwidth centered at 8 GHz, and 10 dB packaged return loss from DC to above 14.5 GHz.
24
Jun
2025
Realization of pure gyration in an on-chip superconducting microwave device
Synthetic materials that emulate tight-binding Hamiltonians have enabled a wide range of advances in topological and non-Hermitian physics. A crucial requirement in such systems is
the engineering of non-reciprocal couplings and synthetic magnetic fields. More broadly, the development of these capabilities in a manner compatible with quantum-coherent degrees of freedom remains an outstanding challenge, particularly for superconducting circuits, which are highly sensitive to magnetic fields. Here we demonstrate that pure gyration — a non-reciprocal coupling with exactly matched magnitude but non-reciprocal π phase contrast — can be realized between degenerate states using only spatio-temporal modulation. Our experiments are performed using microwave superconducting resonators that are modulated using dc-SQUID arrays. We first show the existence of continuous exceptional surfaces in modulation parameter space where coupling with arbitrarily-large magnitude contrast can be achieved, with robust volumes of π phase contrast contained within. We then demonstrate that intersection of these volumes necessarily gives rise to new continuous surfaces in parameter space where pure gyration is achieved. With this we experimentally demonstrate >58 dB isolation and the first on-chip gyrator with only superconducting circuit elements. Our method is fully agnostic to physical implementation (classical or quantum) or frequency range and paves the way to large-scale non-reciprocal metamaterials.
Metamaterials in Superconducting and Cryogenic Quantum Technologies
The development of fault-tolerant quantum computers based on superconducting circuits faces critical challenges in qubit coherence, connectivity, and scalability. This review establishes
metamaterials, artificial structures with on-demand electromagnetic properties, as a transformative solution. By engineering the photonic density of states, metamaterials can suppress decoherence via the Purcell effect and create multi-mode quantum buses for hardware-efficient control and long-range qubit coupling. We provide a comprehensive overview, from foundational principles and Hamiltonian engineering to the materials science of high-coherence devices. We survey state-of-the-art performance, highlighting record coherence times and coupling strengths achieved through metamaterial design. Furthermore, we explore advanced applications where engineered environments give rise to exotic excitations and topologically protected states, enabling novel error correction schemes and qubit architectures. Ultimately, we argue that metamaterials are evolving from passive components into the core architectural element of next-generation quantum technologies, paving a viable path toward scalable quantum computation.
23
Jun
2025
Towards a hybrid 3D transmon qubit with topological insulator-based Josephson junctions
Superconducting quantum circuits provide a versatile platform for studying quantum materials by leveraging precise microwave control and utilizing the tools of circuit quantum electrodynamics
(QED). Hybrid circuit devices incorporating novel quantum materials could also lead to new qubit functionalities, such as gate tunability and noise resilience. Here, we report experimental progress towards a transmon-like qubit made with a superconductor-topological insulator-superconductor (S-TI-S) Josephson junction using exfoliated BiSbTeSe2. We present a design that enables us to systematically characterize the hybrid device, from DC transport of the S-TI-S junction, to RF spectroscopy, to full circuit QED control and measurement of the hybrid qubit. In addition, we utilize a high-quality-factor superconducting cavity to characterize material and fabrication-induced losses, thereby guiding our efforts to improve device quality.
21
Jun
2025
Quasiparticle Dynamics in NbN Superconducting Microwave Resonators at Single Photon Regime
Exchanging energy below the superconducting gap introduces quasiparticle energy distributions in superconducting quantum circuits, which will be responsible for their decoherence. This
study examines the impact of quasiparticle energy on the performance of NbN superconducting microwave coplanar waveguide resonators on silicon chips. We measured the resonance frequency and internal quality factor in response to temperature sweeps to evaluate the effect of quasiparticle dynamics. Moreover, by calculating the complex conductivity of the NbN film, we identified the contribution of quasiparticle density to the experimental results.
20
Jun
2025
Experimental Extraction of Coherent Ergotropy and Its Energetic Cost in a Superconducting Qubit
Quantum coherence, encoded in the off-diagonal elements of a system’s density matrix, is a key resource in quantum thermodynamics, fundamentally limiting the maximum extractable
work, or ergotropy. While previous experiments have isolated coherence-related contributions to work extraction, it remains unclear how coherence can be harnessed in a controllable and energy-efficient manner. Here, we experimentally investigate the role of initial-state coherence in work extraction from a superconducting transmon qubit. By preparing a range of pure states and implementing three complementary extraction protocols, we reveal how coherence governs the partitioning of ergotropy. We find that the choice of initial state depends on the dominant decoherence channel-energy relaxation or dephasing. By further accounting for thermodynamic costs, we identify optimal initial states that maximize the efficiency. These results establish initial-state design as a practical and scalable approach to coherence control, offering guidance for the development of efficient quantum thermodynamic devices.