Cryptographically relevant quantum computer
CRQC
A hypothetical quantum computer large and stable enough to break deployed public-key cryptography — meaning millions of physical qubits supporting thousands of stable logical qubits. No such machine exists publicly today.
In practice
Expert surveys such as the Global Risk Institute's Quantum Threat Timeline put a roughly 1-in-7 chance of a CRQC by 2033.
Related terms
A quantum algorithm that factors large integers and computes discrete logarithms in polynomial time. It is the specific reason RSA, Diffie-Hellman and elliptic-curve cryptography would fail against a sufficiently large quantum computer.
An error-corrected qubit assembled from many noisy physical qubits. Logical qubits are the meaningful unit for cryptographic threat estimates, and current ratios run from hundreds to thousands of physical qubits per logical one.
Informal shorthand for the day a cryptographically relevant quantum computer becomes available. It is a planning device rather than a forecast — the useful question is whether your migration finishes before it, not when it lands.
Techniques that protect quantum information from noise by spreading one logical qubit across many physical ones. It is the central engineering obstacle between today's machines and a cryptographically relevant one.