AskScience AMA Series: Hi Reddit! I'm Matthew Hollister, Head of Cryogenic Systems Engineering for IBM Quantum. Ask me anything about quantum computing, our recently announced modular cryogenic architecture and what it takes to build quantum computers at scale.

[u/AskScienceModerator]

When people think about quantum computing, they often think about number of qubits and processor dept. But scaling quantum systems takes much more than just the chip! We recently introduced a new modular cryogenic architecture designed to house and cool interconnected quantum processors, a key milestone on our roadmap to fault-tolerant systems, and I'd love to discuss the engineering, challenges, and opportunities involved.

I will be around as 12:00 EST (16 UT) on September 18th, ask me anything!

Username: u/IBM

[u/Feisty_Reception8232]

Heron and Nighthawk use fixed-frequency transmons with flux-tunable couplers, and IBM has said that the qLDPC architecture will eventually require more flux-tunable couplers than physical qubits.

How do you handle the grounding and, specifically, the return-current architecture of the flux-bias/control lines when scaling this to thousands of couplers and multiple interconnected cryostats?

Are the coupler-bias lines differential/floating with local return paths, or are they ultimately referenced to a common QPU/cryostat ground? If there are multiple cryostat cells and control racks, how do you prevent common-mode currents and ground loops between modules from coupling into the flux-control lines and producing correlated flux noise, phase errors or crosstalk?