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Applied Physics Letters

Superconducting qubits in a flip-chip architecture

Christopher R. Conner, Audrey Bienfait, Hung-Shen Chang, M.-H. Chou, Étienne Dumur, Joel Grebel, G. A. Peairs, Rhys G. Povey, Haoxiong Yan, Youpeng Zhong, Andrew N. Cleland

Published Jun 7, 2021• 53 citations
Publisher

Abstract

Flip-chip architectures have recently enabled significant scaling-up of multi-qubit circuits and have been used to assemble hybrid quantum systems that combine different substrates, for example, for quantum acoustics experiments. The standard flip-chip method uses superconducting galvanic connections between two substrates, typically implemented using sophisticated indium wafer-bonding systems, which give highly reliable and temperature-cyclable assemblies, but are expensive, somewhat inflexible in design, and require robust substrates that can sustain the large compressive forces required to cold-weld the indium bonds. A much simpler method is to assemble dies using very low-force contacts and air-dried adhesives, although this does not provide a galvanic contact between the dies. Here, we demonstrate that the latter technique can be used to reliably couple superconducting qubit circuits, in which the qubits are on separate dies, without the need for a galvanic connection. We demonstrate full vector qubit control of each qubit on each of the two dies, with high-fidelity single-shot readout, and further demonstrate entanglement-generating excitation swaps as well as benchmark a controlled-Z entangling gate between the two qubits on the two dies. This exemplifies a simple and inexpensive assembly method for two-plus-one-dimensional quantum circuit integration that supports the use of delicate or unusually shaped substrates.

DOI
10.1063/5.0050173

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Flip-chip architectures have recently enabled significant scaling-up of multi-qubit circuits and have been used to assemble hybrid quantum systems that combine different substrates, for example, for quantum acoustics experiments. The standard flip-chip method uses superconducting galvanic connections between two substrates, typically implemented using sophisticated indium wafer-bonding systems, which give highly reliable and temperature-cyclable assemblies, but are expensive, somewhat inflexible in design, and require robust substrates that can sustain the large compressive forces required to cold-weld the indium bonds. A much simpler method is to assemble dies using very low-force contacts and air-dried adhesives, although this does not provide a galvanic contact between the dies. Here, we demonstrate that the latter technique can be used to reliably couple superconducting qubit circuits, in which the qubits are on separate dies, without the need for a galvanic connection. We demonstrate full vector qubit control of each qubit on each of the two dies, with high-fidelity single-shot readout, and further demonstrate entanglement-generating excitation swaps as well as benchmark a controlled-Z entangling gate between the two qubits on the two dies. This exemplifies a simple and inexpensive assembly method for two-plus-one-dimensional quantum circuit integration that supports the use of delicate or unusually shaped substrates.
Abstract As quantum computing evolves from the multi‐qubit stage to large multimodularprocessors, addressability and scalability of quantum hardware will require 3D integration and packaging. The advancement of flip‐chip technology is critical to enabling progress in this area. However, various types of coupling, such as capacitive, inductive, and galvanic, still require systematic research into the principles, quantification, and loss mechanisms. In this article, different types of 3D architecture are examined using flip‐chip technology. The and superconducting resonators with qubit circuits are designed in a modular configuration consisting of two silicon substrates. This arrangement allows numerical analysis and systematic experimental investigation of capacitive, inductive, and galvanic coupling mechanisms. The transmission of short control pulses for fast qubit manipulation is demonstrated and the feasibility of quantum state transfer is theoretically explored, highlighting the potential of this approach for scalable quantum processors. Particular focus is also on the fabrication technology of superconducting indium bumps between silicon substrates.
Flip-chip integration offers a promising route toward scalable superconducting quantum processors and hybrid semiconductor-superconductor quantum devices. We develop a three-dimensional transmon architecture using electroplated indium in which the qubit electric field is shared nearly equally between two bump-bonded substrates while maintaining low participation at the indium-bump interface. The resulting geometry is well suited for future hybrid qubits, enabling the integration of distinct material platforms while minimizing sensitivity to bump-interface loss. Using this platform, we evaluate electroplated indium interconnects for superconducting quantum circuits. Flip-chip transmons incorporating electroplated indium bumps exhibit qubit quality factors around 10610^6106. In addition, a systematic study of coplanar-waveguide resonators is used to identify losses associated with the electroplating process. In particular, we find that surface losses associated with the gold-layer, used to enable good electric contact with the indium, is likely the primary contributor to the qubit decay rate. These results demonstrate the compatibility of electroplated indium technology with high-coherence superconducting circuits and establish a promising platform for three-dimensional hybrid quantum integration.
A challenge faced by experimenters exploring hybrid quantum systems is how to integrate and interconnect different materials and different substrates in a quantum-coherent fashion. Here, we present a simple and inexpensive flip-chip bonding process, suitable for integrating hybrid quantum devices on chips from different substrates. The process only requires equipment and materials used routinely for contact photolithography, and it is possible to undo the bonding and reuse the chips. The technique requires minimal compressive force, so it is compatible with a wide range of different substrates. Unlike indium-based bonding, this process does not establish a galvanic connection between the two chips, but as we show, in some situations this is not necessary. We demonstrate the technique using lithographically patterned quarter-wave coplanar waveguide resonators, fabricated on one chip, and couple these inductively to a transmission line patterned lithographically on a separate chip. The two chips have a vertical interchip gap of about 7 μm, and we can repeatedly achieve lateral alignments of better than 2 μm. We measure electromagnetic resonances with low-power (∼1 photon) internal quality factors Qi around 5 × 105, comparable to single-chip performances, with as-designed coupling quality factors Qc ranging from 2 × 102 to 5 × 105.
Flip-chip 3D integration is a key enabling technology for overcoming the routing and layout constraints of planar superconducting quantum processors, but it requires accurate and uniform control of the inter-chip spacing while preserving qubit coherence. In conventional processes, thick indium bumps are deposited symmetrically on both chips, while dedicated spacers are introduced to limit the residual tilt. Here, we present a flip-chip bonding process based on an asymmetric indium architecture, combining 20 μm20\,μ\mathrm{m}20μm thick indium bumps on the control chip with 200 nm200\,\mathrm{nm}200nm thin indium pads on the qubit chip. Using electrical and microwave measurements, we demonstrate low resistance and low loss indium interconnects, and show that transmon qubits integrated with this process reach relaxation and coherence times of T1=(159±14) μsT_1=(159\pm14)\,μ\mathrm{s}T1​=(159±14)μs and T2,echo=(294±42) μsT_{\mathrm{2,echo}}=(294\pm42)\,μ\mathrm{s}T2,echo​=(294±42)μs, respectively, comparable to planar devices. Combined with a multi angle compression procedure that suppresses systematic tilt, this approach reduces the average post bonding tilt to (20±12) μrad(20\pm12)\,μ\mathrm{rad}(20±12)μrad. These results establish an asymmetric indium architecture combined with multi angle compression as a robust, coherence preserving route toward 3D-integrated superconducting quantum processors.
We present design results of scalable superconducting quantum circuits using the flip-chip assembly. In order to achieve the longest relaxation time (T1T_{1}T1​) of a qubit possible, gap between two chips in flip-chip assembly is determined to be 12 μm where low surface dielectric loss from two-level system (TLS) defects can be achieved while maintaining large readout coupling strength between two chips in the flip-chip assembly. Superconducting transmon qubit design is then optimized to have a target anharmonicity of 200 MHz and to have the longerT1T_{1}T1​in consideration of surface energy participation ratios of TLS defects within the compact footprint. To achieve strong coupling strength but low qubit to qubit crosstalk (ZZ) for fast and high fidelity multi-qubit gates, direct coupler and compact multi-path coupler designs for coupling qubits are compared by numerically solving Hamiltonian of the coupled two qubits. Monte-Carlo simulation results on the yield rate of the extended circuits indicate that low manufacturing errors are required.
The strong anharmonicity and high coherence times inherent to fluxonium superconducting circuits are beneficial for quantum information processing. In addition to requiring high-quality physical qubits, a quantum processor needs to be assembled in a manner that minimizes crosstalk and decoherence. In this paper, we report on fluxonium qubits packaged in a flip-chip architecture, where a classical control and readout chip is bump bonded to the quantum chip, forming a multichip module. The modular approach allows for improved connectivity between the qubits and control and readout elements, and separate fabrication processes. We characterize the coherence properties of the individual fluxonium qubits, demonstrate high-fidelity single-qubit gates with 6-ns microwave pulses (without DRAG), and identify the main decoherence mechanisms to improve on the reported results.