Feature: Interconnects


High-density microwave interconnects for quantum computers


By Martin Spooner, Engineering Director, Intelliconnect (Europe)


W


ith the growing adoption of quantum computing and the development


of large-scale high-qubit machines, there’s a need for microwave cables for these systems’ control and readout lines. Traditionally, these have been provided by individual coaxial cables fitted with SMA connectors. However, the SMA connectors’ 8mm hex coupling nut limits the space they can fit into and require even more space for the tools for installation and maintenance. This is further exacerbated by the use of dilution refrigerators, which cool the high-power processors as they operate at temperatures near absolute zero. CryoCoax, a division of Intelliconnect,


has developed high-density multiway connectors based on the SMPM (Sub- Miniature Push-on Micro) interface that not only allows to fit many more coaxial lines within a given space, but simplifies installation and customisation. The


typical spacing using SMA connectors is limited to approximately 16mm, whereas CryoCoax’s high-density connectors are designed with a pitch of 4.75mm. The SMPM interface provides a simple push- to-mate connection, requiring only a hex key to tighten the fasteners.


A connector fit for the space A typical dilution refrigerator has a vacuum-sealed top plate and several cooling stages below it, reaching temperatures of 20mK at the lowest stage. The cooling stages are separated by a bulkhead/heatshield to minimise heat transfer, with cables fitted between the stages. The cables connect via feedthrough connectors, which must provide good thermal isolation – very important for reducing the heat load of the dilution refrigerator (DR), but also for reducing thermal noise within the system. Glass hermitic seals provide the


vacuum-tight seal at the top of the DR, with hermeticity of 1x10-8


CC He/s at 1Atm. These can be incorporated 38 July/August 2023 www.electronicsworld.co.uk


into custom-designed top plates, also standard ISO and KF-style plates. A typical ISO 100 plate supports up to 120 coax lines, with custom plates even more. Standard non-hermetic feedthrough


plates are required between the various cooling stages, and these are simply machined with mounting holes for the connectors. The connector body incorporates a flange that tightens against the top or bottom face of the plate, to provide thermal isolation and close off any radiation path. CryoCoax can supply top-to-bottom assemblies, known as “coax sticks”, which can be fully customised with attenuator blocks, and a full complement of cable types and materials to provide connections for the entire temperature range in the setup. These undergo RF testing, with test data for DC resistance, return loss, and insertion loss to 40GHz. The connector components are


machined from high-purity copper and beryllium copper and are gold-plated for best thermal conductivity. There are non-magnetic versions, if required.


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