IBM Quantum has shown a way past the single-chip ceiling: on October 27, it connected two complete 120-qubit Nighthawk R1 superconducting processors into one system using four meter-scale cryogenic interconnects (L couplers). Quantum states crossed the links with an average fidelity of 99%, and cross-chip two-qubit operations averaged 94.75% fidelity. This is the first superconducting demonstration combining multi-channel links, meter-scale distance, and large full-size chips.
Why It's Hard
Classical chips exchange data trivially over PCIe. Qubits carry superposition and entanglement, destroyed by any heat, vibration, or electromagnetic noise. Superconducting chips sit in dilution refrigerators near absolute zero, and interconnects must carry quantum states intact through that environment. Past demos mostly ran a single cable between two small chips.
IBM's demo differs on three axes:
- Meter-scale distance. 80 cm superconducting aluminum cables, over an order of magnitude longer than previous centimeter-scale links — the scale modular machines will need.
- Multi-channel. Four links operating simultaneously; future machines will need dozens.
- Large chips. Each Nighthawk R1 is itself a complete 120-qubit processor.
- Meter-scale quantum state transfer itself was done in 2021 (Zhong et al., Nature, two three-qubit nodes). IBM's genuine first is combining meter-scale distance, four simultaneous links, and complete 120-qubit chips.
- The 99% is state-transfer fidelity only, excluding state preparation and measurement errors; cross-chip gates remain below on-chip levels.
- Full 240-qubit cooperative computation was not demonstrated; projected fidelity gains are model-based upper bounds.
How the Links Work
Each link: a qubit on chip A hands off to a dedicated bus qubit, a tunable coupler injects the state into the 80 cm superconducting aluminum cable, and a mirrored assembly on the other end delivers it into chip B.
A clever wiring choice: cables enter vertically from directly above the chip rather than from the side, where space inside the fridge is scarce. Links can be installed one at a time, each tested before the next — a factory-friendly workflow.
The numbers: average state-transfer fidelity 99% across four links, best link 99.29%; average cross-chip two-qubit gate fidelity 94.75%. Using these operations, the team created shared entangled states between the two chips — 4-qubit and 6-qubit states, plus two entangled pairs generated simultaneously over adjacent links.
The Harder Test: Thermal Cycling
All four links survived three full warm-up and cool-down cycles. Any modular quantum computer must tolerate maintenance that returns hardware to room temperature. Three rounds is far from the dozens of cycles real service will demand, but the approach is validated.
Honest Caveats
A fact-check against arXiv:2610.29986 (Sundaresan et al., 42 authors) confirms the figures. Context to keep in mind:
Roadmap Context
IBM's roadmap targets the Starling fault-tolerant quantum computer by 2029. Fault tolerance requires more physical qubits than any single chip can hold, making modularity near-mandatory. In August, IBM demonstrated connecting modular cryogenic systems; this result pushes both the precision and scale of that connectivity.
As qubit-count competition hits the physical walls of cooling and manufacturing, linking large chips will shift from an alternative route to a mainstream one — and interconnect quality will join single-chip gate fidelity as a standard spec-sheet metric.
Sources & caveats: IBM Quantum official blog (2026-10-27) on L couplers; arXiv preprint "Meter-scale quantum interconnects between large quantum processors" (Sundaresan et al., arXiv:2610.29986, not peer-reviewed); Phys.org report (Oct 31). All figures (99%, 99.29%, 94.75%, 80 cm, 120 qubits) are IBM self-reported; thermal cycling tested only three rounds; the "first" claim is scoped to superconducting multi-channel meter-scale large-chip interconnect demos.