The most entangled states of four and seven qubits

Abstract

Pure states of four and seven qubits cannot be maximally entangled across every balanced bipartition; we determine how entangled they can be on average. For four qubits we prove the conjecture of Higuchi and Sudbery: the mean von Neumann entropy of the three $2|2$ cuts is at most $1+\tfrac12\log_2 3$ bits, with equality exactly on the local-unitary orbits of their state $M_4$ and its complex conjugate. For seven qubits the mean purity of the 35 balanced $3|4$ cuts is at least $121/917$ and their mean entropy is at most $2.965356029\ldots$ bits, each with a unique optimiser up to local unitaries and qubit permutations. The two optimisers are distinct members of one icosahedral family, so minimum purity and maximum entropy select different states. The proofs bound moments of the marginal spectra by positive identities on several copies of the state, some of them spin flipped, and bound the entropy by a polynomial in those moments. The four-qubit maximum has an analytic proof; the seven-qubit proofs, and some of the four-qubit refinements below, are computer-assisted, with exact arithmetic and rigorous error bounds. We also prove the Gour--Wallach conjecture for Rényi entropies of all orders, with the maximiser changing from $M_4$ to cluster states at $q_*\approx1.588$, and obtain sharp Tsallis bounds, mixed-state bounds and quantitative stability.