Q-Day

Q-Day is the informal name for the moment a quantum computer first becomes capable of breaking the public-key cryptography in use today — principally RSA-2048 and elliptic-curve schemes. The term is used loosely: sometimes it means RSA-2048 specifically, sometimes it means the arrival of any cryptographically relevant quantum computer, and the two are not the same milestone. Reaching it depends on running Shor's algorithm on a large fault-tolerant machine, and most published resource estimates put that at millions of physical qubits once error-correction overhead is included — several orders of magnitude beyond current devices. No credible date exists; estimates vary enormously depending on assumptions about physical error rates, code choice, magic state distillation cost, and engineering progress, so any confident single year should be treated with suspicion. More importantly, the date matters less than people assume. Under "harvest now, decrypt later", an adversary can record encrypted traffic today and decrypt it whenever the capability arrives, so anything with a long confidentiality lifetime is already exposed. Migration to post-quantum cryptography is the response, and it is a present-tense task rather than a future one.