There is an increasing demand for low temperature electronic components to support the development of quantum computers based on cryogenic platforms (temperatures less than 1 kelvin). The prospect of a step change in computational ability via quantum computing is exciting, but there are many problems to overcome to create and control a quantum system.
One technical problem is of bridging the boundary between a deep cryogenic quantum system and its room temperature control circuitry. The reliability of the bridging interconnects is critical but the environment in which they operate is challenging. Cables must bridge large thermal gradients and can therefore only be made from a limited range of materials. Joints can be stressed by multiple thermal cycles but are inaccessible in the event of a failure, without warming the entire system up. Finally, there is a conflict between the scaling up of the number of lines and the space constraints of a typical dilution fridge.
The conventional solution has been to install many individual coaxial cable segments into a radio frequency (RF) assembly, using standard SMA connectors (sub-miniature version A). The segmentation enables cable materials to be varied between stages and for cables to be thermalised at intermediate bulkheads with additional components (including attenuators, filters and amplifiers) inserted at each stage. This approach can be bulky and difficult to install when scaling to a large number of channels. Improved interconnect solutions that can increase scalability without sacrificing performance are required.
Intelliconnect are a leading manufacturer of RF connectors, adaptors and cable assemblies. Their specialist cryogenic division CryoCoax have created a high-density RF connector for use in cryogenic systems. Their approach to increase the scalability of cryogenic RF interconnects is to use 'ganged' rather than individual connectors. The tiny size of each connection with a shared mechanical anchor reduces the footprint of the assembly, simplifying the installation of large numbers of lines between stages.
Intelliconnect asked NPL to validate the reliable performance of their connector, including tests at low temperature. There are stringent requirements for ganged solutions to show they are an acceptable substitute for more conventional solutions. A key requirement is that the ganged connector must maintain its RF performance under cryogenic conditions and repeated thermal cycling. NPL has several cryogenic systems for developing quantum electrical standards and testing components, in which RF measurements can be performed. Working within NPL’s Measurement for Quantum programme, NPL performed tests on Intelliconnect’s prototype 8-way high density connector using a top-loading dry dilution fridge.
After a round of cryogenic testing on a prototype connector at low temperature, the performance of the high-density connector was compared with existing SMA connectors. Our work showed that the mechanical properties and dimensions of the Intelliconnect connector were suitable for use in compact cryogenic environments with high bandwidth requirements.
NPL’s tests have helped Intelliconnect establish the quality of their component and demonstrate that these connectors, with their smaller footprint per channel, can support the scaling up of quantum device technology in the space-constrained environments of dilution refrigerators.