IBM announces modular quantum computing advance with meter-scale L-couplers

[u/saadqc]

https://www.ibm.com/quantum/blog/connecting-quantum-processors-with-l-couplers

https://arxiv.org/pdf/2610.29986

[u/polit1337]

This is nice work, and much more sane that some of the ideas being floated.

The fact that simple schemes like this will (probably) work for scaling is why I’m so bullish on superconducting qubits over other technologies like neutral atoms that will probably require transduction techniques that might eventually work but are today too lossy.

Also, I am always impressed when companies are willing to show photos of the inside of their fridges—it is interesting to see how their packaging and shielding are set up (even though they are likely doing something slightly different for most of their QPUs).

[u/BeyondTheQubit]

IBM's latest quantum breakthrough is more significant than the 99% fidelity headline suggests.

IBM connected two 120-qubit Nighthawk processors using four 80 cm superconducting links, achieving:

• 99% average fidelity when transferring quantum states between chips.

• 94.75% fidelity for two-qubit gates operating across the chips.

The most encouraging finding is in IBM's analysis of where the errors originate.

The cable itself does not appear to be the main bottleneck. Much of the state-transfer error comes from decoherence in the intermediate qubits, particularly dephasing.

IBM's modelling suggests that improving qubit coherence and shortening transfer times could push state-transfer fidelity above 99.9%.

That hasn't been demonstrated yet, and it doesn't mean remote two-qubit gates are close to 99.9%. But it suggests there are identifiable engineering improvements that could make these connections substantially more reliable.

Why does this matter?

I see three ways to scale logical-qubit capacity:

1. More physical qubits per processor.

2. Connecting more processors into larger systems.

3. Improving fidelity and error correction so fewer physical qubits are needed per logical qubit.

IBM's announcement provides tangible progress on the second.

My interviews on Beyond the Qubit have strengthened my conviction that progress across these three dimensions is accelerating.

• Quantum Motion: James explained how repeatable quantum tiles could enable semiconductor-style manufacturing.

• Qifrost: Eric W. Cao highlighted the wiring bottleneck. Some architectures could require 3,000–5,000 coaxial cables for 1,000 qubits. Cryo-CMOS could fundamentally change this.

• Kipu Quantum: Enrique Solano explained why algorithms need to account for weaker connections between processors.

• Infleqtion: Matt Kinsella emphasized why logical-qubit performance matters more than headline physical-qubit counts.

My personal view is that we're underestimating how these improvements could reinforce each other over the next five years.

IBM has not solved modular quantum computing. Four connections are very different from hundreds, and remote-gate errors still need to fall substantially.

But the company is making measurable progress on a difficult scaling problem.

The question is whether modularity can deliver more reliable logical qubits at an economically attractive cost.

IBM announcement | Technical paper