Mechanically-intermixed indium superconducting connections for microwave quantum interconnects

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 ($Q_i=1.55\pm 0.37x10^6$) 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 $Q_i=1.40x10^6$ and $Q_i=9.39x10^5$ for even and odd-modes respectively and use an ABCD matrix model of the system to extract $R_{cont}=6x10^{-4}\hat{I}\text{c◯}$ 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 $Q_i=1.24x10^6$ and $Q_i=1.07x10^6$ respectively to extract $R_{cont}=8.5x10^{-4}\hat{I}\text{c◯}$ and $R_{shunt}=1.3x10^7\hat{I}\text{c◯}$ for the interface. With these techniques, we demonstrate a set of low-loss methods to join superconducting cables for future quantum

6 pages, 5 figures