What is the microwave package design for a multi-qubit processor with hundreds of signal connections?

The microwave package design for a multi-qubit processor with hundreds of signal connections creates the physical and electromagnetic interface between the qubit chip and the external cryogenic wiring. The package must provide: high-density signal routing (hundreds of microwave connections (one or more per qubit for control and readout) must be routed from the chip to coaxial connectors or cable interfaces; current state-of-the-art: 100-500 connections per package; the package uses: through-silicon vias (TSVs) or through-substrate vias for vertical signal routing, re-distribution layers (RDL) for fan-out from the dense chip bond pads to the wider-spaced connectors, and multi-layer laminated substrates (similar to advanced PCB technology) with microstrip or stripline transmission lines), low microwave loss (the package's transmission lines and interconnects must introduce minimal loss at the qubit frequencies (4-8 GHz); the package substrate materials must have low dielectric loss tangent: silicon (tan_delta approximately 10^-4), alumina (approximately 10^-4), LTCC (Low Temperature Co-fired Ceramic, approximately 10^-3); losses in the package transitions and wire bonds must be minimized (total package loss less than 0.5 dB per connection)), electromagnetic isolation (each signal channel must be isolated from its neighbors to prevent crosstalk; target: greater than 60 dB isolation between adjacent channels; achieved by: ground via fences between signal traces, shielded transmission lines, and absorber material in the package cavity), and thermal anchoring (the package must be thermally connected to the mixing chamber to maintain the qubit chip at base temperature; the package body material should have good thermal conductivity at millikelvin temperatures (copper, gold-plated copper, or aluminum)).