A Sharp Threshold Where Min-Cut Renyi Entropies Cease to Be a Spectrum
Zongming Liu
Abstract
Because Trρk+1 are the moments of a probability measure on the unit interval, Hausdorff’s theorem makes complete monotonicity a complete criterion for a set of R ́enyi entropies to be the spectrum of a density matrix. Applied to the minimal-cut prescription of random tensor networks and monitored circuits, the criterion fails at a threshold with no free parameters: the sequence is a spectrum if and only if the competing cuts cross at R ́enyi index at most two, that is, when the multiplicity ratio does not exceed the bond dimension raised to the area difference. The obstruction is exact, and at small bond dimension it lies at a difference order below one hundred, so it is exhibitable by direct computation rather than only provable.