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
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.
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.