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Author: Martin Spooner, Intelliconnect (Europe)
These are traditionally provided by separate coaxial cables equipped with SMA connectors, due to their 8mm hex coupling nuts offering limited packaging-which also requires space for tools during installation.
CryoCoax, a division of Intelliconnect, has developed a high-density multi-way connector based on the SMPM interface, which not only allows more coaxial cables in a given space, but also simplifies the installation and customization in the dilution refrigerator. The typical spacing using SMA connectors is limited to approximately 16 mm, while the spacing of these high-density connectors is designed to be 4.75 mm. The SMPM interface provides a simple push-to-mate connection, requiring only a hex wrench to tighten the two fasteners.
Quantum processors need to operate at temperatures close to absolute zero, which can be achieved using dilution refrigerators (DR). A typical dilution refrigerator has a vacuum-tight top plate and some cooling stage below it, reaching a temperature below 20mK in the lowest stage.
Each cooling stage is separated by baffles/insulation panels to minimize heat transfer. The cables are installed between each stage and connected by feedthrough connectors and must provide good heating. Thorough heating of cables, attenuators and microwave components at different temperature stages is not only important for reducing the thermal load of the dilution refrigerator, but also for reducing thermal noise in the system.
The glass hermetic seal provides a vacuum seal on the top of the DR, and has a hermeticity of 1x10^(-8) CC He/Sec at 1 Atm. These can be incorporated into custom-designed top plates as well as standard ISO or KF style plates. A typical ISO 100 board can support up to 120 coaxial cables, depending on the connector configuration. Custom boards can support more coaxial cables.
Standard non-sealed feedthrough plates are required between the various cooling stages. These plates are simply machined with mounting holes to accept connectors. The connector body contains a flange that abuts the top or bottom surface of the board to provide heating and close any radiation paths.
CryoCoax can provide components "coaxial rods" from top to bottom, attenuator blocks and a full set of cable types and materials can be fully customized to provide connections from room temperature to all cooling stages to low temperature levels. These RF tests performed at room temperature can provide test data of DC resistance, return loss, and insertion loss up to 40GHz. Most applications run at frequencies far below this.
The connector components are processed from high-purity copper and beryllium copper, and are plated with gold to provide the best thermal conductivity. If required, a non-magnetic version can also be specified.
These connectors are compatible with 0.047 inch (1.19 mm) size coaxial cables and are available in semi-rigid, flexible or conformable versions. Semi-rigid cables include niobium-titanium, stainless steel, copper, cupronickel and beryllium copper, and combinations of these with silver-plated conductors, providing users with a large number of thermal conductivity and attenuation options.
In order to minimize the passive thermal load, cable materials with low thermal conductivity are usually preferred; however, with the exception of superconductors, this is usually closely related to poor electrical conductivity.
Flexible cables with silver-plated copper conductors are usually used to connect from the top of the DR to connect control electronics at room temperature and can be terminated with traditional SMA connectors. A copper coaxial cable option that meets the requirements is also available, usually for the lowest level.
Superconducting niobium-titanium cables are commonly used for quantum applications. This material can be challenging, if not impossible to weld. To overcome this problem, CryoCoax has developed solderless connections for these high-density connectors as well as traditional SMA, 2.92 and SMP connectors.
The standard configuration is 8-way, 16-way and 24-way, with smooth hole or full pawl options. The connector blocks can be combined to provide hundreds of coaxial cables in DR. Two connector blocks form a pair, one with a complete pawl male SMPM interface, and the other with a smooth hole SMPM male interface.
They are used in conjunction with "bullet" female-to-female SMPM adapters mounted between a pair of connector blocks. The "bullet" adapter remains in the complete pawl connector block. They can be used in combination with 0dB, 3dB, 6dB, 10dB and 20dB 8-way, 16-way and 24-way attenuator modules.
CryoCoax worked with a leading attenuator manufacturer to develop these attenuator modules to match high-density connectors. The attenuator is used to suppress thermal noise and provide heating of the center conductor.
With the assistance of NPL (National Physical Laboratory), CryoCoax was able to prove the low temperature reliability of the design in multiple thermal cycles, and then conduct internal durability tests to confirm that the RF performance and DC resistance remained stable after multiple mating cycles.
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