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
22
Sep
2025
Noise Protected Logical Qubit in a Open Chain of Superconducting Qubits with Ultrastrong Interactions
To achieve a fault-tolerant quantum computer, it is crucial to increase the coherence time of quantum bits. In this work, we theoretically investigate a system consisting of a series
of superconducting qubits that alternate between XX and YY ultrastrong interactions. By considering the two-lowest energy eigenstates of this system as a {\it logical} qubit, we demonstrate that its coherence is significantly enhanced: both its pure dephasing and relaxation times are extended beyond those of individual {\it physical} qubits.
Specifically, we show that by increasing either the interaction strength or the number of physical qubits in the chain, the logical qubit’s pure dephasing rate is suppressed to zero, and its relaxation rate is reduced to half the relaxation rate of a single physical qubit. Single qubit and two-qubit gates can be performed with a high fidelity.
Analysis of polymerized superconducting circuits
We apply polymer quantization, a quantization technique sometimes used in high energy physics, to several superconducting circuits including: transmons, transmission line resonators,
and LC circuits. In the case of transmon qubits and transmission line resonators, experimental predictions are very close to what is found with canonical quantization, though in this approach constant charge offsets can be interpreted as quantization ambiguities. In the case of LC circuits, polymer quantization predicts nonlinearities which are not present in the canonical approach. Based on this analysis we design and analyze a qubit which uses a meander inductor instead of a Josephson junction. Implications for qubit performance and fabrication are discussed. Given a choice for an effective phase operator, relevant parameters such as anharmonicity, frequency, and dispersive shifts are calculated for this meander inductor based qubit.
18
Sep
2025
The superconducting grid-states qubit
Decoherence errors arising from noisy environments remain a central obstacle to progress in quantum computation and information processing. Quantum error correction (QEC) based on the
Gottesman-Kitaev-Preskill (GKP) protocol offers a powerful strategy to overcome this challenge, with successful demonstrations in trapped ions, superconducting circuits, and photonics. Beyond active QEC, a compelling alternative is to engineer Hamiltonians that intrinsically enforce stabilizers, offering passive protection akin to topological models. Inspired by the GKP encoding scheme, we implement a superconducting qubit whose eigenstates form protected grid states – long envisioned but not previously realized – by integrating an effective Cooper-quartet junction with a quantum phase-slip element embedded in a high-impedance circuit. Spectroscopic measurements reveal pairs of degenerate states separated by large energy gaps, in excellent agreement with theoretical predictions. Remarkably, our observations indicate that the circuit tolerates small disorders and gains robustness against environmental noise as its parameters approach the ideal regime, establishing a new framework for exploring superconducting hardware. These findings also showcase the versatility of the superconducting circuit toolbox, setting the stage for future exploration of advanced solid-state devices with emergent properties.
Magnetic-Field and Temperature Limits of a Kinetic-Inductance Traveling-Wave Parametric Amplifier
Kinetic-inductance traveling-wave parametric amplifiers (KI-TWPAs) offer broadband near-quantum-limited amplification with high saturation power. Due to the high critical magnetic fields
of high-kinetic-inductance materials, KI-TWPAs should be resilient to magnetic fields. In this work, we study how magnetic field and temperature affect the performance of a KI-TWPA based on a thin-NbTiN inverse microstrip with a Nb ground plane. This KI-TWPA can provide substantial signal-to-noise ratio improvement (ΔSNR) up to in-plane magnetic fields of 0.35T and out-of-plane fields of 50mT, considerably higher than what has been demonstrated with TWPAs based on Josephson junctions. The field compatibility can be further improved by incorporating vortex traps and by using materials with higher critical fields. We also find that the gain does not degrade when the temperature is raised to 3K (limited by the Nb ground plane) while ΔSNR decreases with temperature consistently with expectation. This demonstrates that KI-TWPAs can be used in experiments that need to be performed at relatively high temperatures. The operability of KI-TWPAs in high magnetic field opens the door to a wide range of applications in spin qubits, spin ensembles, topological qubits, low-power NMR, and the search for axion dark matter.
15
Sep
2025
High-performance multiplexed readout of superconducting qubits with a tunable broadband Purcell filter
Fast, high-fidelity, and low back-action readout plays a crucial role in the advancement of quantum error correction (QEC). Here, we demonstrate high-performance multiplexed readout
of superconducting qubits using a tunable broadband Purcell filter, effectively resolving the fundamental trade-off between measurement speed and photon-noise-induced dephasing. By dynamically tuning the filter parameters, we suppress photon-noise-induced dephasing by a factor of 7 in idle status, while enabling rapid, high-fidelity readout in measurement status. We achieve 99.6\% single-shot readout fidelity with 100~ns readout pulse, limited primarily by relaxation errors during readout. Using a multilevel readout protocol, we further attain 99.9\% fidelity in 50~ns. Simultaneous readout of three qubits using 100~ns pulses achieves an average fidelity of 99.5\% with low crosstalk. Additionally, the readout exhibits high quantum-nondemolition (QND) performance: 99.4\% fidelity over repeated measurements and a low leakage rate below 0.1\%. Building on the tunable broadband filter, we further propose a scalable readout scheme for surface code QEC with enhanced multiplexing capability, offering a promising solution for fast and scalable QEC.
Leveraging Machine Learning Force Fields (MLFFs) to Simulate Large Atomistic Systems for Fidelity Improvement of Superconducting Qubits and Sensors
Materials engineering using atomistic modeling is an essential tool for the development of qubits and quantum sensors. Traditional density-functional theory (DFT) does however not adequately
capture the complete physics involved, including key aspects and dynamics of superconductivity, surface states, etc. There are also significant challenges regarding the system sizes that can be simulated, not least for thermal properties which are key in quantum-computing applications. The QuantumATK tool combines DFT, based on LCAO basis sets, with non-equilibrium Green’s functions, to compute the characteristics of interfaces between superconductors and insulators, as well as the surface states of topological insulators. Additionally, the software leverages machine-learned force-fields to simulate thermal properties and to generate realistic amorphous geometries in large-scale systems. Finally, the description of superconducting qubits and sensors as two-level systems modeled with a double-well potential requires many-body physics, and this paper demonstrates how electron-electron interaction can be added to the single-particle energy levels from an atomistic tight-binding model to describe a realistic double-quantum dot system.
25
Aug
2025
Time Domain Design of a Josephson Parametric Amplifier and Comparison with Input Output Theory
Quantum-limited amplifiers, such as Josephson Traveling Wave Parametric Amplifiers (JTWPAs) and Joseph- son Parametric Amplifiers (JPAs), are essential components in quantum computers.
They amplify low-power microwave signals from qubits at the 10 mK stage before further amplification at the 4 K stage using HEMT amplifiers. In JPAs, parametric amplification is based on the nonlinear properties of Josephson Junctions. While JPAs are typically designed and analyzed using input-output theory based on quantum physics, we propose an alternative approach based on an equivalent circuit model of JPAs, implemented using open-source Josephson circuit simula- tors. We compare the results with those obtained from input- output theory. This method enables the use of circuit optimizers for various objective functions and significantly reduces design time compared to quantum theory-based approaches.
21
Aug
2025
Identity and Quantify Various Dissipation Mechanisms of Josephson Junction in Superconducting Circuits
Pinpointing the dissipation mechanisms and evaluating their impacts to the performance of Josephson junction (JJ) are crucial for its application in superconducting circuits. In this
work, we demonstrate the junction-embedded resonator (JER) as a platform which enables us to identify and quantify various dissipation mechanisms of JJ. JER is constructed by embedding JJ in the middle of an open-circuit, 1/2 {\lambda} transmission-line resonator. When the 1st and 2nd harmonics of JER are excited, JJ experiences different boundary conditions, and is dominated by internal and external dissipations, respectively. We systematically study these 2 dissipation mechanisms of JJ by varying the JJ area and number. Our results unveil the completely different behaviors of these 2 dissipation mechanisms, and quantitatively characterize their contributions, shedding a light on the direction of JJ optimization in various applications.
07
Aug
2025
Material-Driven Optimization of Transmon Qubits for Scalable and Efficient Quantum Architectures
One of the most crucial steps in creating practical quantum computers is designing scalable and efficient superconducting qubits. Coherence times, connections between individual qubits,
and reduction of environmental noise are critical factors in the success of these qubits. Because they can be lithographically fabricated and are less sensitive to charge noise, superconducting qubits, especially those based on the Transmon architecture, have emerged as top contenders for scalable platforms. In this work, we use a combination of design iteration, material analysis, and simulation to tackle the superconducting qubit optimization challenge. We created transmon-based layouts for 4 qubits and 8 qubits using Qiskit Metal and conducted an individual analysis for each qubit. We investigated anharmonicity and extracted eigenfrequencies, computing participation ratios across several design passes, and identifying the top five energy eigenstates using Ansys HFSS. We then created a 2D cross section of a single qubit design in COMSOL Multiphysics to evaluate how different materials affect performance. This enables us to assign various superconducting materials and substrates and investigate their effects on energy loss and electromagnetic properties. Qubit coherence and overall device quality are significantly influenced by the materials chosen. This integrated framework of material based simulation and circuit design offers a workable way to create reliable superconducting qubit systems and supports continued attempts to create scalable, fault-tolerant quantum computing.
Third harmonic-mediated amplification in TWPA
In Josephson Traveling-Wave Parametric Amplifiers, higher-order harmonics of the pump tone and its sidebands are commonly present and typically regarded as parasitic. Consequently,
most design efforts have focused on suppressing these harmonics. In spite of that, motivated by transient simulations, we extend the coupled-mode theory and demonstrate that, contrary to conventional belief, the third harmonic can enhance amplifier performance, improving both gain and bandwidth. We show that the recently developed plasma oscillation-based amplifier is particularly well-suited for exploiting this effect. Their dispersion relation enables us to observe the phenomenon in transient numerical simulations using JoSIM and WRspice. These simulations reveal improvement of the amplifier’s performance, specifically the doubling of the bandwidth and an increase in the gain.