TBNQuantum
How wide must an eavesdropper be? Leakage of a toric-code logical bit to a strip and the Nishimori point
Publication: quant-ph updates on arXiv.org
Author: Konstantinos Karagiannis
Source published: 11/11/2026, 05:00:00 UTC
Briefing created: 11/11/2026, 07:14:07 UTC
Based on a feed excerpt; full article content has not been acquired. Source evidence retrieved 11/11/2026, 05:13:26 UTC.
Briefings
A new study proves that in fault-tolerant quantum computing, the width of an eavesdropper's observation strip determines its advantage. The research establishes that the optimal advantage is exactly 1 minus twice the failure probability of maximum-likelihood decoding within that region. Key findings include that geometric coverage matters more than noise type, and the operational threshold for such strips matches the Nishimori point of a random-bond Ising model. The study was conducted on two IBM Heron processors with simulated strip adversaries showing a predicted jump in advantage at a width of three.
Key points
- The optimal advantage for an observer learning a logical bit is exactly 1 - 2 P_fail, where P_fail is the failure probability of maximum-likelihood decoding confined to the specific region observed.
- The operational threshold for a strip adversary is the Nishimori point of the random-bond Ising model, which was located to within a few thousandths using a Walsh-Hadamard transfer matrix up to width 16.
Context
This research provides design rules for partitioned decoding and multiplexed readout in fault-tolerant quantum computers. It clarifies that geometry, not noise, provides the protection against eavesdropping, offering a critical insight for quantum hardware security.
Limitations
The study was conducted on two specific IBM Heron processors with simulated strip adversaries. The results are based on emulated conditions and do not account for non-uniform qubit errors or real-world hardware imperfections beyond the simulated scenarios.
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