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
Feb
2025
Stress Accommodation in Nanoscale Dolan Bridges Designed for Superconducting Qubits
Josephson junctions are the principal circuit element in numerous superconducting quantum information devices and can be readily integrated into large-scale electronics. However, device
integration at the wafer scale necessarily depends on having a reliable, high-fidelity, and high-yield fabrication method for creating Josephson junctions. When creating Al/AlOx based superconducting qubits, the standard Josephson junction fabrication method relies on a sub-micron suspended resist bridge, known as a Dolan bridge, which tends to be particularly fragile and can often times fracture during the resist development process, ultimately resulting in device failure. In this work, we demonstrate a unique Josephson junction lithography mask design that incorporates stress-relief channels. Our simulation results show that the addition of stress-relief channels reduces the lateral stress in the Dolan bridge by more than 70% for all the bridge geometries investigated. In practice, our novel mask design significantly increased the survivability of the bridge during device processing, resulting in 100% yield for over 100 Josephson junctions fabricated.
30
Jan
2025
Direct Implementation of High-Fidelity Three-Qubit Gates for Superconducting Processor with Tunable Couplers
Three-qubit gates can be constructed using combinations of single-qubit and two-qubit gates, making their independent realization unnecessary. However, direct implementation of three-qubit
gates reduces the depth of quantum circuits, streamlines quantum programming, and facilitates efficient circuit optimization, thereby enhancing overall performance in quantum computation. In this work, we propose and experimentally demonstrate a high-fidelity scheme for implementing a three-qubit controlled-controlled-Z (CCZ) gate in a flip-chip superconducting quantum processor with tunable couplers. This direct CCZ gate is implemented by simultaneously leveraging two tunable couplers interspersed between three qubits to enable three-qubit interactions, achieving an average final state fidelity of 97.94% and a process fidelity of 93.54%. This high fidelity cannot be achieved through a simple combination of single- and two-qubit gate sequences from processors with similar performance levels. Our experiments also verify that multi-layer direct implementation of the CCZ gate exhibits lower leakage compared to decomposed gate approaches. To further showcase the versatility of our approach, we construct a Toffoli gate by combining the CCZ gate with Hadamard gates. As a showcase, we utilize the CCZ gate as an oracle to implement the Grover search algorithm on three qubits, demonstrating high performance with the target probability amplitude significantly enhanced after two iterations. These results highlight the advantage of our approach, and facilitate the implementation of complex quantum circuits.
29
Jan
2025
The quantromon: A qubit-resonator system with orthogonal qubit and readout modes
The measurement of a superconducting qubit is implemented by coupling it to a resonator. The common choice is transverse coupling, which, in the dispersive approximation, introduces
an interaction term which enables the measurement. This cross-Kerr term provides a qubit-state dependent dispersive shift in the resonator frequency with the device parameters chosen carefully to get sufficient signal while minimizing Purcell decay of the qubit. We introduce a two-mode circuit, nicknamed quantromon, with two orthogonal modes implementing a qubit and a resonator. Unlike before, where the coupling term emerges as a perturbative expansion, the quantromon has intrinsic cross-Kerr coupling by design. Our experiments implemented in a hybrid 2D-3D cQED architecture demonstrate some unique features of the quantromon like weak dependence of the dispersive shift on the qubit-resonator detuning and intrinsic Purcell protection. In a tunable qubit-frequency device, we show that the dispersive shift (2χ/2π) changes by only 0.8 MHz while the qubit-resonator detuning (Δ/2π) is varied between 0.398 GHz – 3.288 GHz. We also demonstrate Purcell protection in a second device where we tune the orthogonality between the two modes. Finally, we demonstrate a single-shot readout fidelity of 98.3% without using a parametric amplifier which is comparable to the state-of-the-art and suggests a potential simplification of the measurement circuitry for scaling up quantum processors.
Readout-induced leakage of the fluxonium qubit
Dispersive readout is widely used to perform high-fidelity measurement of superconducting qubits. Much work has been focused on the qubit readout fidelity, which depends on the achievable
signal-to-noise ratio and the qubit relaxation time. As groups have pushed to increase readout fidelity by increasing readout photon number, dispersive readout has been shown to strongly affect the post-measurement qubit state. Such effects hinder the effectiveness of quantum error correction, which requires measurements that both have high readout fidelity and are quantum non-demolition (QND). Here, we experimentally investigate non-QND effects in the fluxonium. We map out the state evolution of fluxonium qubits in the presence of resonator photons and observe that these photons induce transitions in the fluxonium both within and outside the qubit subspace. We numerically model our system and find that coupling the fluxonium-resonator system to an external spurious mode is necessary to explain observed non-QND effects.
Optimizing Superconducting Qubit Performance: A Theoretical Framework for Design, Analysis, and Calibration
Designing a qubit architecture is one of the most critical challenges in achieving scalable and fault-tolerant quantum computing as the performance of a quantum computer is heavily
dependent on the coherence times, connectivity and low noise environments. Superconducting qubits have emerged as a frontrunner among many competing technologies, primarily because of their speed of operations, relatively well-developed and offer a promising path toward scalability. Here, we address the challenges of optimizing superconducting qubit hardware through the development of a comprehensive theoretical framework that spans the entire process – from design to the calibration of hardware through quantum gate execution. We develop this framework in four key steps: circuit design, electromagnetic analysis, spectral analysis, and pulse sequencing with calibration. We first refine the qubit’s core components – such as capacitance, Josephson junctions, and resonators – to set the foundation for strong performance. The electromagnetic analysis, using the Lumped Oscillator model, allows us to map out the capacitance matrix, ensuring that we minimize spectral dispersion while maximizing coherence times. Following this, we conduct spectral analysis to fine-tune the qubit’s frequency spectrum and coherence properties, ensuring that the qubit parameters are optimized. Finally, we focus on pulse sequencing, including pulse-width optimization, DRAG optimization, and randomized benchmarking, to achieve high gate fidelity. We applied this framework to both Transmon and Fluxonium qubits, obtaining results that closely match those found in experimental studies. This work provides a detailed and practical approach to the design, optimization, and calibration of superconducting qubits, contributing to the broader effort to develop scalable quantum computing technologies.
A strong-driving toolkit for topological Floquet energy pumps with superconducting circuits
Topological Floquet energy pumps — which use periodic driving to create a topologically protected quantized energy current — have been proposed and studied theoretically,
but have never been observed directly. Previous work proposed that such a pump could be realized with a strongly-driven superconducting qubit coupled to a cavity. Here, we experimentally demonstrate that the proposed hierarchy of energy scales and drive frequencies can be realized using a transmon qubit. We develop an experimental toolkit to realize the adiabatic driving field required for energy pumping using coordinated frequency modulation of the transmon and amplitude modulation of an applied resonant microwave drive. With this toolkit, we measure adiabatic evolution of the qubit under the applied field for times comparable to T1, which far exceed the bare qubit dephasing time. This result paves the way for direct experimental observation of topological energy pumping.
A Linear Quantum Coupler for Clean Bosonic Control
Quantum computing with superconducting circuits relies on high-fidelity driven nonlinear processes. An ideal quantum nonlinearity would selectively activate desired coherent processes
at high strength, without activating parasitic mixing products or introducing additional decoherence. The wide bandwidth of the Josephson nonlinearity makes this difficult, with undesired drive-induced transitions and decoherence limiting qubit readout, gates, couplers and amplifiers. Significant strides have been recently made into building better `quantum mixers‘, with promise being shown by Kerr-free three-wave mixers that suppress driven frequency shifts, and balanced quantum mixers that explicitly forbid a significant fraction of parasitic processes. We propose a novel mixer that combines both these strengths, with engineered selection rules that make it essentially linear (not just Kerr-free) when idle, and activate clean parametric processes even when driven at high strength. Further, its ideal Hamiltonian is simple to analyze analytically, and we show that this ideal behavior is first-order insensitive to dominant experimental imperfections. We expect this mixer to allow significant advances in high-Q control, readout, and amplification.
28
Jan
2025
High-fidelity QND readout and measurement back-action in a Tantalum-based high-coherence fluxonium qubit
Implementing a precise measurement of the quantum state of a qubit is very critical for building a practical quantum processor as it plays an important role in state initialization
and quantum error correction. While the transmon qubit has been the most commonly used design in small to medium-scale processors, the fluxonium qubit is emerging as a strong alternative with the potential for high-fidelity gate operation as a result of the high anharmonicity and high coherence achievable due to its unique design. Here, we explore the measurement characteristics of a tantalum-based high-coherence fluxonium qubit and demonstrate single-shot measurement fidelity (assignment fidelity) of 96.2% and 97.8% without and with the use of a Josephson Parametric Amplifier respectively. We study the back-action of the measurement photons on the qubit and measure a QND (repeatability) fidelity of 99.6%. We find that the measurement fidelity and QND nature are limited by state-mixing errors and our results suggest that a careful study of measurement-induced transitions in the fluxonium is needed to further optimize the readout performance.
Space-Time-Coupled Qubits for Enhanced Superconducting Quantum Computing
The pursuit of scalable and robust quantum computing necessitates innovative approaches to overcome the inherent challenges of qubit connectivity, decoherence, and susceptibility to
noise and crosstalk. Conventional monochromatic qubit coupling architectures, constrained by nearest-neighbor interactions and limited algorithmic flexibility, exacerbate these issues, hindering the realization of practical large-scale quantum processors. In this work, we introduce a paradigm leveraging a space-time-modulated cryogenic-compatible Josephson metasurface to enable polychromatic qubit coupling. This metasurface facilitates frequency-selective interactions, transforming nearest-neighbor connectivity into all-to-all qubit interactions, while significantly enhancing coherence, noise robustness, and entanglement fidelity. Our proposed approach capitalizes on the unique capabilities of space-time-modulated Josephson metasurfaces, including dynamic four-dimensional wave manipulation, nonreciprocal state transmission, and state-frequency conversion, to mediate multi-frequency qubit interactions. By isolating qubit couplings into distinct spectral channels, the cryogenic-compatible metasurface mitigates crosstalk and environmental decoherence, extending coherence times and preserving quantum state fidelity. Full-wave simulations and quantum performance analyses demonstrate a significant enhancement in the operational efficiency of a superconducting qubit array, showcasing improved connectivity, robustness, and entanglement stability. This study establishes the potential of space-time-modulated cryogenic-compatible Josephson metasurfaces as a transformative platform for next-generation quantum computing, addressing critical bottlenecks and paving the way for scalable, high-performance quantum processors.
Dephasing-induced leakage in multi-level superconducting quantum circuits
Superconducting quantum circuits, such as the transmon, have multiple quantum states beyond the computational subspace. These states can be populated during quantum logic operations;
residual population in such states is known as leakage. While control methods can eliminate this error in ideal systems, leakage will arise from transient population in the presence of dephasing. This dephasing-induced leakage effect is analyzed, both analytically and numerically, for common single and two-qubit operations used in transmon-based approaches to quantum information processing.