Ramya ., Rashmi .
The maturation of quantum computing hardware and algorithms has transformed what was once a theoretical concern into an active planning priority for governments, enterprises, and standards bodies. This paper assesses the current and projected threat that quantum computation poses to classical public-key cryptographic standards, namely RSA, elliptic curve cryptography (ECC), and Diffie-Hellman key exchange, all of which derive their security from mathematical problems that Shor's algorithm renders tractable on a sufficiently large fault-tolerant quantum computer. We synthesize recent hardware progress, algorithmic refinements that have substantially reduced estimated qubit requirements, and the finalization of NIST's post-quantum cryptography (PQC) standards (FIPS 203, 204, and 205) to provide an evidence-based assessment of where the field stands as of mid-2026. Particular attention is given to the widening gap between optimistic resource estimates, including a contested March 2026 claim of a thousand-fold reduction in factoring resources, and the practical reality that no existing quantum computer approaches the logical qubit counts required to threaten deployed encryption. We further examine migration challenges, including performance overhead, cryptographic agility, constraints on embedded and IoT systems, and the harvest-now-decrypt-later threat model that makes migration urgency independent of exact timeline predictions. Drawing on NIST transition guidance and recent G7 coordination targets, we argue that organizations should treat cryptographic migration as a present-tense operational requirement rather than a future contingency, regardless of unresolved disagreement among experts about when a cryptographically relevant quantum computer will exist. The paper concludes with practical recommendations for risk-based migration prioritization and identifies open research questions in cryptographic agility and quantum-resistant system design.