Quantum Memory and the Dual Clocks of Quantum Readiness: Specifications, Trends, Theories and Hypotheses in Hardware Scaling and Cryptographic Migration

Abstract

This article synthesises the state of quantum memory specifications and the recurring narrative trends, theoretical frameworks and working hypotheses that shape contemporary discourse on quantum hardware scaling, modular cryogenic architectures and post-quantum cryptographic (PQC) migration. Quantum memory — the capacity to store and retrieve a quantum state while preserving coherence — is examined through quantitative figures of merit including storage time, fidelity, efficiency, multimode capacity and operating conditions. Recent experimental advances, including 15-second spin coherence in stoichiometric europium crystals and kilohertz-rate photonic interconnects between trapped-ion and solid-state memories, are placed in context. Three interlocking narrative shifts are identified: the elevation of memory and interconnects as the practical scaling bottleneck; the institutionalisation of the "two clocks" framing that separates the quantum-advantage timeline from the already-running cryptographic-migration timeline; and the compression of fault-tolerance roadmaps toward the late 2020s alongside explicit modularity requirements. Theoretical contributions concerning the separation of processing and memory units, autonomous quantum memories, width–reliability trade-offs in spacetime overhead, and cost-of-delay models that place classical and harvest-now-decrypt-later risks on a common metric are reviewed. The analysis concludes that quantum readiness is presently governed more by architectural lead times, crypto-agility and data-longevity considerations than by any single predicted date for a cryptographically relevant quantum computer.