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High-density microwave interconnects for quantum computers


With 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 they 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 tools for their installation and maintenance. This is further exacerbated by the use of dilution refrigerators which cool these high-power processors, as they operate at temperatures near absolute zero.

CryoCoax is a division of Intelliconnect and has developed high-density multiway connectors based on the SMPM interface (a micro-miniature interface with a frequency range of DC to 65GHz) that not only allows to fit many more coaxial lines within a given space, but simplify installation and customisation. 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, only requiring 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. Each cooling stage is separated by a bulkhead/heatshield to minimise heat transfer, with cables fitted between each stage. The cables connect via feedthrough connectors and must provide good thermalisation – 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 1×10-8 CC He/s at 1Atm. These can be incorporated 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 ones many 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 thermalisation and close off any radiation path. CryoCoax can supply top to bottom assemblies ‘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.

The connectors are compatible with .047” (1.19mm) size coax, either in semi-rigid, flexible or conformable versions. Semi-rigid cables are available in niobium titanium, stainless steel, copper, cupronickel and beryllium copper, as well as combinations of these with silver-plated conductors, offering a large choice in terms of thermal conductivity and attenuation. To minimise passive heat load, cable materials with low thermal conductivity are preferred, however, with the exception of superconductors this typically goes hand in hand with poor electrical conductivity. Flexible cables having silver-plated copper conductors are typically used for connections from the top of the DR to connect the control electronics and can be terminated with conventional SMA connectors. Conformable copper coax options are also available and are often used at the lowest stage.

Material match

Superconducting niobium-titanium cables are often specified in quantum applications, yet this material can be challenging, if not impossible to solder. To overcome this, CryoCoax developed solderless connections for these high-density connectors as well as the traditional SMA, 2.92 and SMP connectors. Standard configurations are 8-way, 16-way and 24-way, with either smooth bore or full detent options.

Connector blocks can be ganged together, to increase the number of coax lines. Two connector blocks form a mated pair, one with a full-detent male SMPM interface and the other a smooth-bore SMPM male interface are used in conjunction with ‘bullet’ style female-to-female SMPM adaptors fitted between a pair of connector blocks. The bullet adaptors are retained in the full detent connector block. These can be combined with 8-way, 16-way and 24-way attenuator blocks available in 0dB, 3dB, 6dB, 10dB and 20dB. CryoCoax worked with a leading attenuator manufacturer in developing these attenuator blocks to complement the high-density connectors. Attenuators are used to suppress thermal noise and provide thermalisation of the centre conductors.

With assistance from the National Physical Laboratory in the UK, CryoCoax proved the design’s reliability over low temperatures for a number of thermal cycles. This was followed by in-house durability testing, to confirm that the RF performance and DC resistance remained stable after multiple mating cycles.


By Martin Spooner, Engineering Director, Intelliconnect (Europe)


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