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arXiv:2608.29707

Tight Global Bound on Pairwise Entanglement of Formation in Three-Qubit Systems

Wei Song, Xiao-Lan Zong, Ming Yang

quant-phSubmitted Aug 30, 2026• Announced Sep 1, 2026• 0 citations• Revised Sep 2, 2026 (v2)
v1 · Aug 30, 2026v2 · Sep 2, 2026v3 · Sep 20, 2026
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Abstract

We derive a global bound on the sum of pairwise squared entanglement of formation in three-qubit systems. The bound is tight and can be saturated by states containing a maximally entangled bipartite pair with an uncorrelated third qubit. Moreover, using this relation, we can map the three bipartite entanglements to three coordinates, such that any three-qubit state corresponds to a point in three-dimensional space, thereby endowing this global inequality with a very natural geometric picture. Accordingly, the dynamical evolution can be interpreted as trajectories in this geometric space; we select representative initial states and investigate the dynamical behavior of the three pairwise entanglements under various Markovian noise models.

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11 pages, 6 figures, comments and collaborations are welcome

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We derive a global bound on the sum of pairwise squared entanglement of formation in three-qubit systems. The bound is tight and can be saturated by states containing a maximally entangled bipartite pair with an uncorrelated third qubit. Moreover, using this relation, we can map the three bipartite entanglements to three coordinates, such that any three-qubit state corresponds to a point in three-dimensional space, thereby endowing this global inequality with a very natural geometric picture. Accordingly, the dynamical evolution can be interpreted as trajectories in this geometric space; we select representative initial states and investigate the dynamical behavior of the three pairwise entanglements under various Markovian noise models.
For pure symmetric three-qubit states there are only three algebraically independent entanglement measures; one choice is the pairwise concurrence C\mathcal{C}C, the three-tangle $\ensuremath{\tau}$, and the Kempe invariant $\ensuremath{\kappa}$. Using a canonical form for symmetric nnn-qubit states derived from their Majorana representation, we derive the explicit achievable region of triples $(\mathcal{C},\ensuremath{\tau},\ensuremath{\kappa})$.
We investigates the joint compatibility problem of the three pairwise concurrences, the three-tangle, and the Kempe invariant in arbitrary pure three-qubit states, and gives necessary and sufficient conditions for these five quantities to be simultaneously realizable by the same pure state. This five-dimensional compatibility region can be viewed as a fiber structure over the previously derived four-dimensional reachable region. We further find that the residual continuous local-unitary freedom depends on the position of the state within this region. In the generic interior, one continuous degree of freedom remains. It is fixed if the three-tangle vanishes or a pairwise concurrence becomes zero, and also at the boundary of the four-dimensional tangle region. The Kempe invariant gives the simplest algebraic parametrization of the fifth coordinate, but the same degree of freedom may be described by a known pure-state entanglement monotone. The compatibility conditions can also be expressed in terms of concurrence-based bipartite measures and multipartite quantities fixed by the pairwise concurrences and the three-tangle.
Consider three qubits A, B, and C which may be entangled with each other. We show that there is a trade-off between A's entanglement with B and its entanglement with C. This relation is expressed in terms of a measure of entanglement called the concurrence, which is related to the entanglement of formation. Specifically, we show that the squared concurrence between A and B, plus the squared concurrence between A and C, cannot be greater than the squared concurrence between A and the pair BC. This inequality is as strong as it could be, in the sense that for any values of the concurrences satisfying the corresponding equality, one can find a quantum state consistent with those values. Further exploration of this result leads to a definition of an essential three-way entanglement of the system, which is invariant under permutations of the qubits.
Various parametrizations for the orbits under local unitary transformations of three-qubit pure states are analyzed. The interconvertibility, symmetry properties, parameter ranges, calculability, and behavior under measurement are looked at. It is shown that the entanglement monotones of any multipartite pure state uniquely determine the orbit of that state under local unitary transformations. It follows that there must be an entanglement monotone for three-qubit pure states which depends on the Kempe invariant defined in Phys. Rev. A 60, 910 (1999). A form for such an entanglement monotone is proposed. A theorem is proved that significantly reduces the number of entanglement monotones that must be looked at to find the maximal probability of transforming one multipartite state to another.
We experimentally test the recently predicted anisotropic invariance properties of pure three-qubit states, via generation and measurement of polarization-path entangled three-qubit states. These properties do not require aligned reference frames, and can be determined from measurements on any two of the qubits. They have several applications, such as a universal ordering of pairwise quantum correlations, strong monogamy relations for Bell inequalities and quantum steering, and a complementarity relation for Bell nonlocality versus 3-tangle, some of which we also test. The results indicate that anisotropic invariance, together with the three-qubit Bloch vector lengths, provide a robust and complete set of invariants for such states under local unitary transformations.
We report the discovery of two new invariants for three-qubit states which, similarly to the three-tangle, are invariant under local unitary transformations and permutations of the parties. These quantities have a direct interpretation in terms of the anisotropy of pairwise spin correlations. Applications include a universal ordering of pairwise quantum correlation measures for pure three-qubit states; trade-off relations for anisotropy, three-tangle and Bell nonlocality; strong monogamy relations for Bell inequalities, Einstein-Podolsky-Rosen steering inequalities, geometric discord and fidelity of remote state preparation (including results for arbitrary three-party states); and a statistical and reference-frame-independent form of quantum secret sharing.