Liquid metal printing for superconducting circuits
Alexander Kreiner, Navid Hussain, Ritika Dhundhwal, Haoran Duan, Nicolas Zapata, Gabriel Cadilha Marques, Tino Cubaynes, Torsten Scherer, Wolfgang Wernsdorfer, Michael Hirtz, Ioan Mihai Pop, Jasmin Aghassi-Hagmann, Thomas Reisinger
quant-phSubmitted Nov 12, 2025• Announced Nov 14, 2025• 0 citations
Superconducting circuits are a promising platform for implementing fault-tolerant quantum computers, quantum limited amplifiers, ultra-low power superconducting electronics, and sensors with ultimate sensitivity. Typically, circuit fabrication is realized by standard nanolithography, generally associated with a high level of control over defects and contaminants. Additive approaches have not been used so far since they are expected to be inferior in terms of superconducting properties or quantum coherence. This work shows that liquid-metal based micro-pipette printing is suited for fabricating superconducting lumped-element resonators with high internal quality factors. The applicability of our technique for low-loss superconducting device fabrication and the possibility to locally add metal structures, without affecting any preexisting circuit elements, is a further advantage. Our results open up new avenues in the hardware implementation of scaled-up superconducting quantum computers.
Superconducting circuits are a promising platform for implementing fault-tolerant quantum computers, quantum limited amplifiers, ultra-low power superconducting electronics, and sensors with ultimate sensitivity. Typically, circuit fabrication is realized by standard nanolithography, generally associated with a high level of control over defects and contaminants. Additive approaches have not been used so far since they are expected to be inferior in terms of superconducting properties or quantum coherence. This work shows that liquid-metal based micro-pipette printing is suited for fabricating superconducting lumped-element resonators with high internal quality factors. The applicability of our technique for low-loss superconducting device fabrication and the possibility to locally add metal structures, without affecting any preexisting circuit elements, is a further advantage. Our results open up new avenues in the hardware implementation of scaled-up superconducting quantum computers.
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 β-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.
Quantum computing is rapidly evolving from theoretical frameworks to functional hardware, with physical qubits now being realized across multiple experimental platforms. Precision nanofabrication has enabled the development of low-loss, high-coherence quantum devices. Among the various approaches, superconducting circuits and trapped-ion systems have emerged as the most advanced and experimentally successful platforms for realizing controllable and scalable qubits. This review presents an overview of nanoscale fabrication techniques employed in these two architectures. We first outline the operating principles that define qubit behavior, followed by a discussion of representative device designs and fabrication workflows. In superconducting circuits, we examine processes such as thin-film deposition, lithographic patterning, and Josephson-junction fabrication that determine key parameters including qubit frequency, anharmonicity, and coherence. For trapped-ion systems, we describe the transition from bulk electrode assemblies to microfabricated surface traps produced using photolithography, metal deposition, and dielectric layering, where electrode geometry and surface treatment directly influence ion confinement and motional heating. We further discuss fabrication-related challenges, including dielectric loss, surface contamination, and material defects that limit coherence and reproducibility, along with emerging mitigation strategies such as advanced cleaning, low-loss materials, and improved surface engineering. By drawing parallels between these two leading platforms, this review highlights that continued progress in nanofabrication remains central to realizing scalable, high-fidelity quantum processors.
Superconducting circuits incorporating Josephson elements represent a promising hardware platform for quantum technologies. Potential applications include scalable quantum computing, microwave quantum networks, and quantum-limited amplifiers. However, progress in Josephson junction–based quantum technologies is facing the ongoing challenge of minimizing loss channels. This is also true for parametric superconducting devices based on nonlinear Josephson resonators. In this work, we report on the fabrication and characterization of low-loss Josephson parametric devices operated in the GHz frequency range, showing record internal quality factors. Specifically, we achieve internal quality factors Q int significantly exceeding 10 5 for both Josephson parametric converters and Josephson parametric amplifiers in the single-photon regime by fitting the scattering data. We confirm the extracted Q int values by analyzing purity of squeezed vacuum states generated by these devices. These low-loss devices mark a significant step forward in realizing high-performance quantum circuits, enabling further advancements in superconducting quantum technologies.
The perhaps most promising platform for quantum information processing is the circuit-QED architecture based on superconducting circuits representing quantum bits. These circuits must be made with low losses so that the quantum information is retained for as long as possible. We developed fabrication processes achieving state-of-the-art coherence times of over 100 µs. We identified the primary source of loss to be parasitic two-level systems by studying fluctuations of qubit relaxation times.Using our high-coherence circuits, we implemented a quantum processor built on fixed-frequency qubits and frequency-tunable couplers. The tunable couplers were lumped-element LC resonators, where the inductance came from a superconducting quantum interference device (SQUID). We achieved a controlled-phase gate with a fidelity of 99% by parametric modulation of the coupler frequency. Using this device, and another similar to it, we demonstrated two different quantum algorithms, the quantum approximate optimization algorithm, and density matrix exponentiation. We achieved high algorithmic fidelities, aided by our carefully calibrated gates.Additionally, we researched parametric oscillations using frequency-tunable resonators. Previously, degenerate parametric oscillations have been demonstrated by modulation of the resonant frequency at twice that frequency. We use this phenomenon to implement a readout method for a superconducting qubit with a fidelity of 98.7%. We demonstrated correlated radiation in nondegenerate parametric oscillations by modulating at the sum of two resonant frequencies of a multimode resonator. We showed an excellent quantitative agreement between the classical properties of the oscillations with a theoretical model. Moreover, we studied higher-order modulation at up to five times their resonant frequencies. These types of parametric oscillation states might be used as a quantum resource for continuous-variable quantum computing.
For practical superconducting quantum processors, orders of magnitude improvement in coherence is required, motivating efforts to optimize hardware design and explore new materials. Among the latter, the coherence of superconducting transmon qubits has been shown to improve by forming the qubit capacitor pads from α-tantalum, avoiding the metastable β-phase that forms when depositing tantalum at room temperature, and has been previously identified to be a source of microwave losses. In this work, we show lumped element resonators containing β-phase tantalum in the form of inclusions near the metal–substrate interface with internal quality factors (Qi) up to 5.0 ± 2.5×106 in the single photon regime. They perform at least as good as resonators with no sign of the β-phase in x-ray diffraction and thermal quasi-particle loss. Our results indicate that small concentrations of β-phase can be beneficial, enhancing critical magnetic fields and potentially, for improving coherence in tantalum-based superconducting circuits.
Low-loss lumped-element capacitors for superconductive circuits were fabricated using sputter-deposited Nb electrodes. The dielectric layer was formed by partially anodizing the Nb base electrode. The deposition technique for the counterelectrode strongly affected the parasitic shunt conductance of the capacitors. It was found that this conductance could be reduced by depositing the Nb counterelectrode films by DC magnetron sputtering at a low rate and at a high Ar pressure. By optimizing these process parameters, capacitors with breakdown voltages greater than 85% of the anodic oxide formation voltage and loss tangents less than 0.003 at 10 MHz were fabricated. These capacitors were integrated with Nb thin-film inductors to produce L-C resonators with quality factors greater than 400.>