Q-Day
Shorthand for the day a quantum computer can break today's public-key cryptography, including the elliptic-curve signatures Bitcoin uses.
Q-Day names a capability threshold, not a date: a fault-tolerant quantum computer large enough to run Shor's algorithm against RSA-2048 or secp256k1 within a useful time. Public resource estimates have fallen over the years as error-correction improved (the 2025 estimate for RSA-2048 was under a million noisy qubits for under a week, against roughly twenty million in 2019), while the largest publicly demonstrated ECC key break remains tiny. Nobody has published a credible break of a 256-bit curve. ByKaranteli does not forecast Q-Day; it publishes the quantities that would matter on that day (exposed supply, dormant P2PK movement) and a sourced timeline of standards and capability claims, updated only when a primary source publishes.