A trapped ytterbium ion at room temperature holds quantum coherence six million times longer than a superconducting transmon at 15 millikelvin. The field has no unifying explanation for this. This paper provides one. A cross-platform analysis of ten qubit implementations shows that coherence time correlates with the qubit’s internal structural depth (r = −0.91, R2 = 83%) and does not correlate with operating temperature (r = +0.33). Internal depth — the number of nested levels at which processes inside the qubit can independently reach definite outcomes — is the dominant variable. Each additional depth level costs approximately 77× in coherence time. The paper traces how this principle was discovered, presents the evidence, and derives six engineering directives that follow from it. The directives are concrete, actionable, and falsifiable. They reframe the engineering problem: the primary target is not the environment around the qubit but the structure within it.
Raimo van der Klein (Fri,) studied this question.