The Qubit Floor for Breaking Bitcoin's Curve Drops to 835, a Third Lower in Three Months

A team spanning four Chinese research institutions posted a space-efficient quantum algorithm for elliptic-curve discrete logarithms that needs just 835 logical qubits for a 256-bit curve like secp256k1 - the lowest width yet reported, well under the previous floors of 1,098 (Chevignard et al.) and 1,175 (Google's low-qubit design). The same group's April preprint stood at 1,333; three months of optimization removed roughly a third of that. The design trades width for gates, spending far more Toffolis, so it does not lead the combined qubit-Toffoli metric - but it lowers the minimum machine size that could ever run the attack, and that floor is still falling. Fewer qubits needed means the hardware roadmaps have less distance to cover.