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arXiv:2409.04634

Mechanically-intermixed indium superconducting connections for microwave quantum interconnects

Y. Martin, Neereja Sundaresan, Jae-Woong Nah, Rachel Steiner, Marco Turchetti, Kevin G. Stawiasz, Chi Xiong, Jason S. Orcutt

Published Sep 6, 2024• 0 citations
arXivHTMLPDF

Abstract

Superconducting coaxial cables represent critical communication channels for interconnecting superconducting quantum processors. Here, we report mechanically-intermixed indium joins to aluminum coaxial cables for low loss quantum interconnects. We describe an ABCD matrix formalism to characterize the total resonator internal quality factor ($Q_i$) and any contact ($R_{cont}$) or shunt resistance ($R_{shunt}$) associated with the mechanically-intermixed indium joins. We present four resonator test systems incorporating three indium join methods over the typical frequency range of interest (3-5.5GHz) at temperatures below $20mK$. We measure high internal quality factor aluminum cables ($Q_i = 1.55 \pm 0.37 x 10^6$) through a push-to-connect indium join of the outer conductor that capacitively couples the inner conductor for reflection measurements. We then characterize the total internal quality factors of modes of a cable resonator with a push-to-connect superconducting cable-splice at the midpoint to find mean $Q_i = 1.40 x 10^6$ and $Q_i = 9.39 x 10^5$ for even and odd-modes respectively and use an ABCD matrix model of the system to extract $R_{cont} = 6x10^{-4} Ω$ for the indium join of the inner conductor. Finally, we demonstrate indium press-mold cable-to-chip connections where the cable-to-chip join is placed at a current node and voltage node through varying on-chip waveguide lengths with mean $Q_i = 1.24 x 10^6$ and $Q_i = 1.07 x 10^6$ respectively to extract $R_{cont} = 8.5x10^{-4} Ω$ and $R_{shunt} = 1.3x10^7 Ω$ for the interface. With these techniques, we demonstrate a set of low-loss methods to join superconducting cables for future quantum

DOI
10.48550/arxiv.2409.04634

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Superconducting coaxial cables represent critical communication channels for interconnecting superconducting quantum processors. Here, we report mechanically-intermixed indium joins to aluminum coaxial cables for low loss quantum interconnects. We describe an ABCD matrix formalism to characterize the total resonator internal quality factor (QiQ_iQi​) and any contact (RcontR_{cont}Rcont​) or shunt resistance (RshuntR_{shunt}Rshunt​) associated with the mechanically-intermixed indium joins. We present four resonator test systems incorporating three indium join methods over the typical frequency range of interest (3-5.5GHz) at temperatures below 20mK20mK20mK. We measure high internal quality factor aluminum cables (Qi=1.55±0.37x106Q_i = 1.55 \pm 0.37 x 10^6Qi​=1.55±0.37x106) through a push-to-connect indium join of the outer conductor that capacitively couples the inner conductor for reflection measurements. We then characterize the total internal quality factors of modes of a cable resonator with a push-to-connect superconducting cable-splice at the midpoint to find mean Qi=1.40x106Q_i = 1.40 x 10^6Qi​=1.40x106 and Qi=9.39x105Q_i = 9.39 x 10^5Qi​=9.39x105 for even and odd-modes respectively and use an ABCD matrix model of the system to extract Rcont=6x10−4ΩR_{cont} = 6x10^{-4} ΩRcont​=6x10−4Ω for the indium join of the inner conductor. Finally, we demonstrate indium press-mold cable-to-chip connections where the cable-to-chip join is placed at a current node and voltage node through varying on-chip waveguide lengths with mean Qi=1.24x106Q_i = 1.24 x 10^6Qi​=1.24x106 and Qi=1.07x106Q_i = 1.07 x 10^6Qi​=1.07x106 respectively to extract Rcont=8.5x10−4ΩR_{cont} = 8.5x10^{-4} ΩRcont​=8.5x10−4Ω and Rshunt=1.3x107ΩR_{shunt} = 1.3x10^7 ΩRshunt​=1.3x107Ω for the interface. With these techniques, we demonstrate a set of low-loss methods to join superconducting cables for future quantum
Abstract Reproducible low-loss microwave interconnects are essential for scaling modular superconducting quantum processors and networks. Wire bonds provide a simple and flexible route to galvanic connections in such systems, but their microwave loss and reproducibility at millikelvin temperatures remain insufficiently quantified. Here we demonstrate reproducible low-loss aluminum wire-bond interconnects in both superconducting cable resonators and planar tantalum resonators. In the cable devices, a wire-bond connection is placed at the midpoint of a high-quality resonator, where the different current distributions of odd and even modes separate connection-induced loss from intrinsic cable dissipation within the same device. Across devices fabricated with different bond numbers and ultrasonic powers, a stable bonding window yields no resolvable connection resistance above the practical resolution of order 0.1 m Ω . Measurements of planar resonators on two chips independently show no systematic degradation of the internal quality factor from wire-bond ground terminations within the device-to-device variation. These results establish aluminum wire bonding as a reproducible low-loss interconnect for superconducting microwave circuits, supporting the scalable assembly of modular superconducting quantum processors and networks.
Abstract We describe a superconducting (SC) cable-to-cable connection method for thin-film flexible (Nb/polyimide) parallel transmission line cables that exhibits low loss from DC to microwave frequencies. Design, fabrication procedures, assembly methods and characterization results are reported. The connector assembly incorporates a monolithic array of short superconductive bridging traces that function as a ganged electrical splice between two ribbon cables. The assembly is micro-mechanically self-aligned and uses pressure to make simultaneous electrical contacts between multiple 50 μ m wide microstrip signal traces, as well as between ground planes. Each contacting surface comprises thin SC Nb overcoated with ultra-thin Au, providing nearly zero DC resistance and very good RF performance. A simulation model of the microwave performance of microstrip-to-microstrip interconnections at 4.2 K was successfully matched to measurement results. This new cable-to-cable connection approach exhibited an insertion loss <0.12 dB and a return loss >15 dB up to 14 GHz. The connections performed reproducibly throughout multiple thermal cycles and multiple disassembly/reassembly cycles.
Distributed quantum computing architectures require high-performance quantum interconnects between quantum information processing units, while previous implementations have been fundamentally limited by transmission line losses. Here, we demonstrate a low-loss interconnect between two superconducting modules using an aluminum coaxial cable, achieving a bus mode quality factor of 1.7e6. By employing SNAIL as couplers, we realize inter-modular state transfer in 0.8 μs via a three-wave mixing process. The state transfer fidelity reaches 98.2% for quantum states encoded in the first two energy levels, achieving a Bell state fidelity of 92.5%. Furthermore, we show the capability to transfer high-dimensional states by successfully transmitting binomially encoded logical states. Systematic characterization reveals that performance constraints have shifted from transmission line losses (contributing merely 0.2% infidelity) to module-channel interface effects and local Kerr nonlinearities. Our work advances the realization of quantum interconnects approaching fundamental capacity limits, paving the way for scalable distributed quantum computing and efficient quantum communications.
Modular quantum architecture requires long-distance quantum interconnects to stitch together individual processor modules. Such interconnects, long-distance couplers or L-couplers, must support coherent quantum operation over meter-scale distances and scale to multiple connections between any two modules. Here we demonstrate four L-couplers operating between two 120-qubit IBM Nighthawk R1 superconducting qubit processors. Each L-coupler joins two Nighthawk lattice qubits through two frequency-tunable L-qubits, two RF-SQUID based "i2m" couplers, and 80 cm of superconducting aluminum coax. Each cable installs perpendicular to the processor surface, to enable denser connections, and is joined to the chip by an indium-to-indium room temperature pressure bond made after both modules were mounted. L-coupler modes near 5 GHz, with internal quality factors of 0.86 to 1.09e6, carry relay-based state transfer at a mean network benchmarking fidelity of 99.00%, with a best link of 99.29 +/- 0.07%. We then realize distant CZ gates between lattice qubits in separate modules at a mean fidelity of 94.75%, with a best link of 96.42 +/- 0.28%, and use them to generate entangled states across the two processors, including a demonstration of simultaneous Bell pair generation using two adjacent L-couplers. These four high-fidelity L-couplers establish a practical path to scalable quantum interconnects for modular quantum computing.
Superconducting enclosures will be key components of scalable quantum computing devices based on circuit quantum electrodynamics. Within a densely integrated device, they can protect qubits from noise and serve as quantum memory units. Whether constructed by machining bulk pieces of metal or microfabricating wafers, 3D enclosures are typically assembled from two or more parts. The resulting seams potentially dissipate crossing currents and limit performance. In this letter, we present measured quality factors of superconducting cavity resonators of several materials, dimensions, and seam locations. We observe that superconducting indium can be a low-loss RF conductor and form low-loss seams. Leveraging this, we create a superconducting micromachined resonator with indium that has a quality factor of two million, despite a greatly reduced mode volume. Inter-layer coupling to this type of resonator is achieved by an aperture located under a planar transmission line. The described techniques demonstrate a proof-of-principle for multilayer microwave integrated quantum circuits for scalable quantum computing.
Scalable architectures characterized by quantum bits (qubits) with low error rates are essential to the development of a practical quantum computer. In the superconducting quantum computing implementation, understanding and minimizing material losses are crucial to the improvement of qubit performance. A new material that has recently received particular attention is indium, a low-temperature superconductor that can be used to bond pairs of chips containing standard aluminum-based qubit circuitry. In this work, we characterize microwave loss in indium and aluminum/indium thin films on silicon substrates by measuring superconducting coplanar waveguide resonators and estimating the main loss parameters at powers down to the sub-photon regime and at temperatures between 10 and 450 mK. We compare films deposited by thermal evaporation, sputtering, and molecular beam epitaxy. We study the effects of heating in a vacuum and ambient atmospheric pressure as well as the effects of pre-deposition wafer cleaning using hydrofluoric acid. The microwave measurements are supported by thin film metrology including secondary-ion mass spectrometry. For thermally evaporated and sputtered films, we find that two-level state are the dominant loss mechanism at low photon number and temperature, with a loss tangent due to native indium oxide of ∼5×10−5. The molecular beam epitaxial films show evidence of the formation of a substantial indium-silicon eutectic layer, which leads to a drastic degradation in resonator performance.