Quantum supremacy

Quantum supremacy (also called quantum computational advantage) is the milestone at which a programmable Quantum computer performs a specific task that no classical computer can complete in a feasible time. The task is usually contrived to be hard for classical machines and has no practical use by itself. The physicist John Preskill introduced the term in 2012.

#The 2019 demonstration

Google reported the first claim in 2019, using a 53-qubit superconducting processor to sample the output distribution of random quantum circuits, a problem believed to be classically intractable. Later classical algorithms and follow-up experiments narrowed and contested the exact margin, and the phrase "quantum advantage" is now often preferred.

#Not a cryptographic threat

Quantum supremacy is frequently confused with a Cryptographically relevant quantum computer. The two differ enormously in difficulty: sampling benchmarks exploit Superposition over many qubits without needing error correction, whereas breaking deployed cryptography with Shor's algorithm requires fault-tolerant logical qubits far beyond any demonstrated device. A supremacy result says nothing about when RSA or elliptic-curve keys become vulnerable.