IBM Connects Two 120-Qubit Chips with 99% Transfer Fidelity
Unlike classical computing, quantum information can’t be copied due to the no-cloning theorem. This makes IBM’s announcement particularly noteworthy: the company successfully moved an unknown quantum state from one 120-qubit chip to another with 99% fidelity. This isn’t a duplication; the original state is consumed, and the quantum information reappears on the destination chip. It’s a true transfer, and the high fidelity achieved is a significant achievement given this constraint.
What Was Demonstrated and Its Scale
The demonstration involved two IBM Nighthawk R1 superconducting processors, each with 120 qubits, linked by four 80-centimeter aluminum coaxial cables. These superconducting microwave links operate within the ultracold environment of the quantum computing system itself, analogous to internal wiring between adjacent server cabinets.
The team reported an average state-transfer fidelity of 99%, with all four links exceeding 98.86% and the best achieving 99.29% (with an uncertainty of 0.07 percentage points). These are measured results, and the work is currently available as a preprint on arXiv, awaiting peer review.
It’s important to note the scale: though the chips have 120 qubits each, the experiment didn’t involve 240 interacting qubits. IBM focused on wiring and measuring specific qubits near the connections to expedite development. Therefore, the headline signifies a plumbing demonstration between two large chips, not a fully integrated 240-qubit computation.
The Transfer Mechanism Explained
Each cable uses dedicated qubits at both ends solely for information transfer. Adjustable couplers control when these qubits interact with the cable. The researchers benchmarked transfer in both directions using network benchmarking, a technique that separates transfer errors from the inherent noise of qubit preparation and readout.
Though state transfer is one achievement, performing a quantum gate across the gap is more complex. To execute a controlled-Z gate between computing qubits on separate chips, IBM combined two state transfers, three local two-qubit gates, and several single-qubit controls. Each additional step introduces more opportunities for error.
Key Performance Figures
All figures are measured:
- State-transfer fidelity: 99% average; best link 99.29% (±0.07 pp); all four links above 98.86%.
- Cross-chip two-qubit gate fidelity: 94.75% average; best 96.42% (±0.28 pp).
- Four-qubit entangled state across chips: 77.23% raw; 91.70% after measurement-error correction.
- Six-qubit state: 67.67% raw; 90.87% corrected.
- Durability: All four links endured three warm-cool cycles without requiring reconnection.
- Modeled (not measured): With improved qubit coherence, the error model predicts transfer fidelity between 99.68% and 99.77%.
Regarding the correction figures, it’s crucial to remember that measurement-error correction improves readout accuracy but doesn’t alter the underlying physical process. Both the four- and six-qubit states achieved genuine entanglement even without correction, which is the more significant metric.
Impact on the Quantum Roadmap
Quantum chips face physical limitations regarding qubit density, wiring complexity, and cooling requirements. Modular scaling offers a solution: build smaller, interconnected chips that operate as a single system. IBM’s demonstration, described as the first multichannel quantum interconnect between large superconducting processors, is a step in this direction. The durability of the links through thermal cycling is also vital, as modular systems require long-term maintenance.
This achievement doesn’t, however, signify a working fault-tolerant computer using these links for actual computation, so that remains a future goal. The team also identified a challenge: trapped magnetic fields during aluminum cooling negatively affected the cable’s ability to hold microwave energy. Their described shielding fix is temporary, and the underlying mechanism needs further resolution.
Implications for Post-Quantum Security
Each advance in quantum computing intensifies the urgency for post-quantum cryptographic migration for systems relying on RSA. Recent reports highlight this pressure: NowSecure claims nearly all mobile apps lack quantum-safe encryption. Zcash developers have set a January 2027 development target for hash-based signatures, as detailed in this post. Black Kite has integrated quantum readiness into its vendor risk scoring, and Bitcoin developers are working on a quantum-safe fallback, reportedly with transaction costs dropping below $70. These are company-reported claims and should be interpreted accordingly.
While significant, this four-link lab demonstration doesn’t mean Shor’s algorithm is imminent at scale. It’s a foundational component. The real challenge of numerous reproducible links supporting error-corrected computation is still ahead. Continued vigilance is warranted, but immediate alarm is not.
For additional information, refer to QNews’s coverage of quantum-safe Bitcoin economics.