In the last few years, several technology companies including Google, Microsoft, and IBM, have massively invested in quantum computing systems based on microwave superconducting circuit platforms in an effort to scale them up from small research-oriented systems to commercialized computing platforms. But fulfilling the potential of quantum computers requires a significant increase in the number of qubits, the building blocks of quantum computers, which can store and manipulate quantum information.
But quantum signals can be contaminated by thermal noise generated by the movement of electrons. To prevent this, superconducting quantum systems must operate at ultra-low temperatures—less than 20 milli-Kelvin—which can be achieved with cryogenic helium-dilution refrigerators.
The output microwave signals from such systems are amplified by low-noise high-electron mobility transistors (HEMTs) at low temperatures. Signals are then routed outside the refrigerator by microwave coaxial cables, which are the easiest solutions to control and read superconducting devices but are poor heat isolators, and take up a lot of space; this becomes a problem when we need to scale up qubits in the thousands.
