iSWAP maximises the second-moment spectral gap in random quantum circuits
Yanying Liang, Haoran Zhu
Abstract
We prove that the iSWAP gate maximises the spectral gap of the Hermitian second-moment operator on every connected graph with at least three vertices, among all two-local unitary circuit ensembles. We further prove that the polyhedral cone defined by asymmetric four-point inequalities is invariant under the transpose of the iSWAP semigroup, yielding a componentwise comparison certificate for its positive Perron--Frobenius eigenvector. These results resolve a conjecture of Kong, Li, and Liu.
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