Wiring Density Limits Qubit Control, Bluefors Research Shows

Bluefors Research has determined a fundamental physics limit to scaling up quantum computers, revealing how densely microwave control lines can be packed together without corrupting qubit operations. A paper published in Physical Review Letters frames the question of wiring density not as an engineering problem, but as a question of physics; the team integrated control signal crosstalk directly into a two-qubit Hamiltonian.

Researchers found a direct relationship between control line crosstalk and quantum gate fidelity, with benchmark average gate fidelity of 99.99% requiring an ELFEXT of roughly −43 dB for single-qubit operations and −73 dB for two-qubit entangling gates. This analysis clarifies where crosstalk originates and establishes critical wiring requirements for high-fidelity quantum systems with increasing qubit counts. For a 30 mm square processor, achieving 99.9% two-qubit gate fidelity bounds the device to just under 300 physical qubits.

ELFEXT Measurements Define Quantum Gate Fidelity Limits

The research team quantified ELFEXT in twelve coupled-stripline pairs, mapping the measurements to realistic cable geometry and revealing how tightly packed control lines impact qubit operations. This approach moves beyond treating wiring density as a purely engineering challenge, instead framing it as a constraint dictated by the underlying physics of quantum systems.