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arXiv:2404.00770 · Applied Physics Letters

Low-loss liquid metal interconnects for superconducting quantum circuits

Zhancheng Yao, Martin Sandberg, David W. Abraham, D. J. Bishop

Published Jun 24, 2024• 3 citations
arXivHTMLPDF

Abstract

Building a modular architecture with superconducting quantum computing chips is one of the means to achieve qubit scalability, allowing the screening, selection, replacement, and integration of individual qubit modules into large quantum systems. However, the nondestructive replacement of modules within a compact architecture remains a challenge. Liquid metals, specifically gallium alloys, can be alternatives to solid-state galvanic interconnects. This is motivated by their self-healing, self-aligning, and other desirable fluidic properties, potentially enabling the nondestructive replacement of modules at room temperatures, even after operating the entire system at millikelvin regimes. In this study, we present coplanar waveguide resonators (CPWRs) interconnected by gallium alloy droplets, achieving high internal quality factors up to nearly one million and demonstrating performance on par with the continuous solid-state CPWRs. Leveraging the desirable fluidic properties of gallium alloys at room temperature and their compact design, we envision a modular quantum system enabled by liquid metals.

DOI
10.1063/5.0211244

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Building a modular architecture with superconducting quantum computing chips is one of the means to achieve qubit scalability, allowing the screening, selection, replacement, and integration of individual qubit modules into large quantum systems. However, the nondestructive replacement of modules within a compact architecture remains a challenge. Liquid metals, specifically gallium alloys, can be alternatives to solid-state galvanic interconnects. This is motivated by their self-healing, self-aligning, and other desirable fluidic properties, potentially enabling the nondestructive replacement of modules at room temperatures, even after operating the entire system at millikelvin regimes. In this study, we present coplanar waveguide resonators (CPWRs) interconnected by gallium alloy droplets, achieving high internal quality factors up to nearly one million and demonstrating performance on par with the continuous solid-state CPWRs. Leveraging the desirable fluidic properties of gallium alloys at room temperature and their compact design, we envision a modular quantum system enabled by liquid metals.
Modular architectures are a promising route toward scalable superconducting quantum processors, but finite fabrication yield and the lack of high quality temporary interconnects impose fundamental limitations on system size. Here, we demonstrate chip-scale liquid-metal interconnects that show promise for plug-and-play superconducting quantum circuits by enabling non-destructive module replacement while maintaining high microwave performance. Using gallium-based liquid metals, we realize high-quality inter-module signal and ground interconnects, comparable in performance to conventional coplanar waveguide resonators. We illustrate consistent device characteristics across three thermal cycles between room temperature and 15 mK, as well as the ability to reform superconducting connections following module replacement. A width-dependent resonance frequency shift reveals a significant kinetic inductance fraction, which we attribute to the presence of βββ-phase tantalum as confirmed by X-ray characterization. Finally, we investigate power-dependent loss mechanisms and observe high-power dissipative nonlinearities qualitatively consistent with a readout-power heating model. These results establish liquid metals as viable chip-scale interconnects for reconfigurable, modular superconducting quantum systems.
This paper proposes a design of a coplanar waveguide resonator (CPWR) to integrate two distant superconducting qubits which is a function required for a quantum information system. In this CPWR, the states of one of the super conducting qubits can be moved to and recovered from the other qubit by microwave resonator photons. The superconducting qubits are relatively straightforward to fabricate from materials such as aluminium using electron beam lithography. The superconducting qubits, which are typically a few hundred micrometres in size, are well suited to perform fast quantum logic gate operations. The CPWR highly couples the two qubits through the tightly confined oscillating magnetic field of the resonator microwave photons. It is shown that, in the proposed CPW resonator structure, the microwave photons can mediate coupling between two distant superconducting qubits (21 mm apart) to realize quantum logic gates.
The path toward practical superconducting quantum processors requires the integration of a large number of high-performance qubits. Modular architectures could offer a way to address the scaling limitations of monolithic designs by partitioning a large quantum processor into physically separated modules, or chiplets, linked through long-range interconnects. In this context, although fluxonium qubits have emerged as a compelling platform for quantum computing due to their long coherence times and high-fidelity gates, existing coupling schemes remain restricted to qubits in close proximity on a single chip. This limitation inherently precludes the long-range interconnects essential for modular integration. In this work, we propose a long-range tunable coupler designed to interconnect fluxonium qubits separated by more than one centimeter, thereby supporting the realization of modular fluxonium quantum processors. Under realistic assumptions, the proposed coupler has the potential to achieve inter-module two-qubit gate performance, specifically sub-100-ns gates with intrinsic errors below 10−410^{-4}10−4, comparable to that of intra-module (intra-chiplet) gates, while enabling modular integration with low quantum crosstalk, a key requirement for scalable systems. We further discuss the integration of this coupler into modular fluxonium lattices and explore its feasibility for achieving the higher connectivity and longer coupling range required for complex quantum error correction codes. This work could contribute to the development of large-scale fluxonium quantum processors by bridging their demonstrated potential with modular scalability.
Superconducting coplanar waveguide (SCPW) resonators, key components for quantum computing and sensing applications, require a high internal quality factor ( Q i ) for effective qubit readout and quantum sensing applications. Minimizing two-level system (TLS) losses, particularly at material interfaces, is critical for gatemon and topological qubits operating at low temperatures and in high magnetic fields. NbTi, a superconducting alloy with a high upper critical field, enables SCPW resonators resilient to such conditions. We fabricated NbTi SCPW resonators on Si/SiO 2 substrates and systematically characterized their TLS-limited quality factors as functions of temperature and microwave photon number. Our results demonstrate that NbTi-based SCPWs on Si/SiO 2 substrates provide a promising platform for developing next-generation quantum circuits. • Successfully fabricated NbTi SCPW resonators on Si/SiO 2 substrates. • Characterized NbTi SCPW across temperature and photon ranges for quantum apps. • Achieved record TLS-limited quality factor Q TLS,0 = 9.21 × 10 4 for SCPW on Si/SiO 2 . • NbTi SCPW on Si/SiO 2 is promising for gatemon and topological qubits.
Superconducting qubits are a leading platform for scalable quantum computation. This platform offers high gate speeds and can make use of readily scalable complementary metal-oxide semiconductor (CMOS) fabrication techniques. However, superconducting qubits are susceptible to a variety of losses that arise in systems with superconductors, such as due to ubiquitous two-level systems, quasiparticles, and more. Designing fabrication processes to mitigate and manage these loss sources is critical to achieving scalable quantum computation. However, many of these loss sources are set during fabrication processing, of which there can be many different steps, each with its own parameter space. Exploring this space with qubits would be prohibitively time-consuming. Coplanar waveguide resonators provide a suitable proxy system for minimizing loss while costing less time and money to fabricate and measure. In this thesis, we explore the results of such an optimization performed on niobium resonators. Niobium is a widely used material platform for superconducting qubit circuits. We report highly consistent state-of-the-art resonator quality factors, with clear improvements over the previous fabrication recipe, and correlate the observed improvements to the internal quality factors with microstructural changes induced by hydrofluoric acid treatment, photoresist stripping, and choice of sputter deposition conditions. Additionally, we provide a detailed description of our fab process to ensure reproducibility in other lab facilities. Finally, we address some intriguing anomalies uncovered in temperature-dependent quality factors of the niobium resonators and show a correlation of the quality factors with compressive film stress.