Increasing the Number of Control Lines in Mulitple Dilution Unit Fridges

Superconducting quantum computers require microwave control lines running from room temperature to the mixing chamber of a dilution refrigerator. The thermal load imposed by these cables is frequently a limiting factor in scaling up the system as the available cooling power is exceeded. Thermal models of a Bluefors XLD1000-SL dilution fridge using semi-rigid coaxial cables for the control lines estimate a practical limit of approximately 140 qubits with engineering margin [1]. 

Larger dilution fridges with multiple dilution units are now available, which are commonly operated with all dilution units thermally in parallel.

Here, we consider a wider thermal design space where the mixing chambers are allowed to operate at different temperatures, and in particular, we operate one or more mixing chambers at a significantly higher temperature of 120 mK, while reserving at least one dilution unit to cool the quantum processor to 20 mK. Since the cooling power of a dilution unit scales approximately as T^2 in this range, the available cooling power increases dramatically. Using the higher temperature dilution units to thermalize control lines to 120 mK before routing them to the 20 mK processor allows a significant increase in the number of control lines. We will present thermal simulation results showing we can accommodate up to 1200 qubits using this design.

[1] Raicu et al. arXiv:2502.01945