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Blekherman, Grigoriy; Madhusudhana, Bharath Hebbe (2020): Quantum entanglement, symmetric nonnegative quadratic polynomials and moment problems. In: Mathematical Programming
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Quantum states are represented by positive semidefinite Hermitian operators with unit trace, known as density matrices. An important subset of quantum states is that of separable states, the complement of which is the subset of entangled states. We show that the problem of deciding whether a quantum state is entangled can be seen as amoment problem in real analysis. Only a small number of such moments are accessible experimentally, and so in practice the question of quantum entanglement of a many-body system (e.g, a system consisting of several atoms) can be reduced to a truncated moment problem. By considering quantum entanglement of n identical atoms we arrive at the truncated moment problem defined for symmetric measures over a product of n copies of unit balls in Rd. We work with moments up to degree 2 only, since these are most readily available experimentally. We derive necessary and sufficient conditions for belonging to the moment cone, which can be expressed via a linear matrix inequality of size at most 2d+ 2, which is independent of n. The linear matrix inequalities can be converted into a set of explicit semialgebraic inequalities giving necessary and sufficient conditions formembership in themoment cone, and showthat the two conditions approach each other in the limit of large n. The inequalities are derived via considering the dual cone of nonnegative polynomials, and its sum-of-squares relaxation. We show that the sum-of-squares relaxation of the dual cone is asymptotically exact, and using symmetry reduction techniques (Blekherman and Riener: Symmetric nonnegative forms and sums of squares. arXiv:1205.3102, 2012;Gatermann and Parrilo: J Pure Appl Algebra 192(1-3):95-128. https:// doi.org/10.1016/ j.jpaa.2003.12.011, 2004), it can be written as a small linear matrix inequality of size at most 2d + 2, which is independent of n. For the cone of symmetric nonnegative polynomials with the relevant supportwe also prove an analogue of the half-degree principle for globally nonnegative symmetric polynomials (Riener: J Pure Appl Algebra 216(4): 850-856. https:// doi. org/10.1016/ j.jpaa.2011.08.012, 2012;Timofte: J Math Anal Appl 284(1):174-190. https:// doi.org/10.1016/ S0022-247X(03)00301-9, 2003).