Superconducting qubits in the millions: the potential and limitations of modularity

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The development of fault-tolerant quantum computers (FTQCs) is receiving increasing attention within the quantum computing community.

• Like conventional digital computers, FTQCs, which utilize error correction and millions of physical qubits, have the potential to address some of humanity's grand challenges.
• However, accurate estimates of the tangible scale of future FTQCs, based on transparent assumptions, are uncommon.
• How many physical qubits are necessary to solve a practical problem intractable for classical hardware? What costs arise from distributing quantum computation across multiple machines? This paper presents an architectural model of a potential FTQC based on superconducting qubits, divided into discrete modules and interconnected via coherent links.