The paper "Suppressing quantum errors by scaling a surface code logical qubit" by the Google Quantum AI team was published yesterday in Nature. I previously wrote about this work when the preprint was posted to arXiv last year.
The peer review file accompanying the paper is an interesting read. The authors mention challenges involved in making superconducting quantum computers that are robust to catastrophic noise induced by cosmic rays - radiation shielding will not be sufficient:
"Given the current error scale of an impact event is essentially unsurvivable, the event rate needs to be at least on the order of the computation time. Reasonable estimates for fault tolerant computations are generally measured in hours (c.f. [5] which proposes 8 hours for Shor’s algorithm), so current event rates should need to improve by around 3000x from current event rates. Additionally, if chip areas grow approximately proportional to number of qubits, given that the event rate is directly proportional to chip area, the event rate will also increase by around 100,000x from this effect, which will need to be overcome as well. The 10x reduction offered by the use of lead shielding and a low-radiation laboratory is nowhere near sufficient to solve this problem at scale."
Solving this problem will require new superconducting qubit and quantum processor designs that can be more robust to cosmic rays, e.g. localizing their disruptive effects to a small fraction of the qubits so that quantum error correction can still be applied.
The authors also write in their reply to referees that the performance fine-tuning required for the distance-5 code to match the performance of the distance-3 code took place over 6 weeks.
As I wrote before - this is an impressive achievement, but in the race to build a working, fault-tolerant quantum computer it should be seen of the end of the beginning, not the beginning of the end!
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