On the Sample Complexity and Optimization Landscape for Quadratic Feasibility Problems

Abstract: We consider the problem of recovering a complex vector $\mathbf{x}\in \mathbb{C}<sup>n$ from $m$ quadratic measurements $${\langle A_i\mathbf{x}, \mathbf{x}\rangle}<em>{i=1}<sup>m$$. This problem, known as quadratic feasibility, encompasses the well known phase retrieval problem and has applications in a wide range of important areas including power system state estimation and x-ray crystallography. In general, not only is the the quadratic feasibility problem NP-hard to solve, but it may in fact be unidentifiable. In this paper, we establish conditions under which this problem becomes {identifiable}, and further prove isometry properties in the case when the matrices ${A_i}</em>{i=1}<sup>m$ are Hermitian matrices sampled from a complex Gaussian distribution. Moreover, we explore a nonconvex {optimization} formulation of this problem, and establish salient features of the associated optimization landscape that enables gradient algorithms with an arbitrary initialization to converge to a \emph{globally optimal} point with a high probability. Our results also reveal sample complexity requirements for successfully identifying a feasible solution in these contexts.