Parker Breiz v2 — Noise-Budget Study of a Cavity-Squeezed Zero-Field Spin Clock in 4H-SiC

AbstractVersion 1 of this note claimed sub-attosecond precision from 10^12 entangled V_Si spinsin 4H-SiC at 77 K. This version withdraws that claim. A corrected stability model,numerical tests and a limited literature check give a more modest picture: for a 10^12-spinzero-field clock transition near 1.3 GHz, white-noise stability at 1 s lies between about 4 ×10^-12 (measured ensemble coherence, no squeezing) and about 10^-15 (single-spin-classcoherence, Markovian noise). An idealised lower bound near 10^-17 exists, but we do notregard it as credible without new measurements.We report negative results for fractal, Fibonacci-stacked honeycomb, Penrose andicosahedral spin arrangements: none beats a compact lattice for one-axis-twistingsqueezing. Cavity-mediated all-to-all twisting is the only lever that yields deep squeezing(xi^2 between 10^-5 and 10^-3 for 10^7 spins per cavity, uniform coupling), but it dependson T2*, coupling uniformity and a near-quantum-limited amplifier for dispersive readout.The key unknown is the fraction f of fast, independent frequency noise: 1 % of it costs afactor of about 24 in the optimistic case. We propose a double-branch (D ± E) scheme atthe zero-field avoided crossing of the basal divacancy, which cancels transverse-strainand electric-field noise at no sensitivity cost, and a three-stage experimental plan (77 K, 4K, millikelvin) with falsifiable predictions. The temperature coefficient of the zero-fieldsplitting, not squeezing, is the likely floor.This is a noise budget and a plan, not an experimental result. All numbers come from themodels in sections 2–7 unless a source is cited.