This paper describes the Coherence Signature — a set of six structural criteria whose joint occurrence in a physical system indicates stable resonance with the quantum vacuum: (C1) Topological Closure, (C2) Scalar Recursion, (C3) Phase Criticality, (C4) Coherent Medium, (C5) Geometrically Intrinsic Temporal Resonance, and (C6) Chiral Alignment. The criteria are material-agnostic, scale-invariant, and applicable across scientific disciplines. C1 defines the depth of topological nesting (supertoroidal order N); C2 defines the scale-invariant proportions of that nesting (phi ratios). Maximum resonance requires both: depth structured by harmonic proportions. The physical foundation rests on three levels. The Casimir effect confirms that geometry acts on the vacuum field. Keren et al. (Nature 2026) provided the first direct experimental proof that vacuum fluctuations alone — without photons or external energy — can modify a superconducting material when cavity geometry (C1+C2) resonantly matches the target's eigenfrequencies (C5). C5 is described at three levels of realisation: externally imposed rhythm (incomplete), geometrically intrinsic resonance (confirmed by Keren 2026 as the operative coupling condition), and spontaneous time crystal phase (complete). C6 is bidirectional: same-handedness relative to the vacuum's chiral asymmetry strengthens coupling; opposite handedness enables decoupling or neutralisation. The paper includes a recognition table applying the six criteria to fourteen physical systems, an explicit limits-and-caveats section distinguishing confirmed, theoretical, and speculative claims, and an outlook on technical resonator design. Version: v601 · Language: English