Between Computability and Physical Realizability: The Computational Reducibility Theorem and the Fundamental Limit of Quantum Computation

This note connects two claims in the Quantumograph program. The Computational Reducibility Theorem shows that a finite unitary quantum graph is, in principle, exactly forecastable from one spectral slice: the state at one time together with the spectrum of the evolution operator. The Fundamental Limit of Quantum Computation (FLQC) adds a physical constraint: any realizable quantum device has a finite phase resolution, so exact spectral data and exact phase gates cannot be implemented with arbitrary precision. Realism, in this setting, is the position between these two statements. The universe is computable in principle, but nature imposes a nonzero floor on the precision with which that computability can be instantiated. The note derives the phase floor, compares it with the phase resolution required by QFT- and Shor-type circuits, and gives a concrete experimental signature on superconducting quantum processors.