What is Q Day?

• A. A cryptographic standard
• B. The predicted point when a powerful quantum computer can break today's encryption
• C. A measurement of coherence
• D. A benchmark for random circuits

Answer: (B)

Q Day is the predicted moment when a powerful quantum computer becomes able to break today’s public‑key encryption. This idea comes from Shor’s algorithm, which could factor large numbers and solve discrete logarithms efficiently on a quantum computer. Those problems underpin widely used systems like RSA and ECC, so once a big enough quantum computer exists, encrypted messages could be decrypted and digital signatures forged. That potential threat means data secured now could be exposed later, especially for highly sensitive information, so researchers push for post‑quantum cryptography and flexible, updatable security so systems can switch to quantum‑resistant methods. It’s not a cryptographic standard, a measure of coherence, or a circuit benchmark.