With the accelerated roll-out of quantum computing and the development of large-scale high qubit machines, a growing number of microwave cables are now necessary for control and readout lines to the quantum processor. These have traditionally been provided by individual coaxial cables fitted with SMA connectors which offer limited packaging due to their 8mm hex coupling, which also requires space for access for tooling during installation.
CryoCoax, a division of Intelliconnect, have now developed high-density multiway connectors based around the SMPM interface which not only provides more coaxial lines in a given space but also simplifies the installation and customisation within a dilution refrigerator.
Typical spacing using SMA connectors is limited to approximately 16mm, whereas the new Q-CON 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 two fasteners.
The connector components are machined from high purity copper and beryllium copper and are gold plated to provide best thermal conductivity. Non-magnetic versions can also be specified if required. The connectors are compatible with .047” (1.19mm) size coax, either semi-rigid, flexible or conformable versions are available. 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, providing the user with a large choice in terms of thermal conductivity and attenuation.
To minimise passive heat load, cable materials with low thermal conductivity are normally favoured, 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 dilution refrigerator to connect the control electronics at room temperature and can be terminated with conventional SMA connectors. Conformable copper coax options are also available and are often used at the lowest stage.
Superconducting Niobium-Titanium cables are often specified in quantum applications 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 of the new Q-CON high-density connectors are 8-way, 16-way and 24-way with either smooth bore or full detent options, connector blocks can be ganged together to potentially give hundreds of coax lines in a dilution refrigerator. Two connector blocks form a mated pair, one having a full detent male SMPM interface, and the other a smooth bore SMPM male interface, these 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 compliment the high-density connectors. Attenuators are employed to suppress thermal noise and provide thermalisation of the centre conductors.
