Bounds on approximating Max kXOR with quantum and classical local algorithms
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We consider the power of local algorithms for approximately solving Max kXOR, a generalization of two constraint satisfaction problems previously studied with classical and quantum algorithms (MaxCut and Max E3LIN2). On instances with either random signs or no overlapping clauses and D+1 clauses per variable, we calculate the average satisfying fraction of the depth-1 QAOA and compare with a generalization of the local threshold algorithm. Notably, the quantum algorithm outperforms the threshold algorithm for k > 4. On the other hand, we highlight potential difficulties for the QAOA to achieve computational quantum advantage on this problem. We first compute a tight upper bound on the maximum satisfying fraction of nearly all large random regular Max kXOR instances by numerically calculating the ground state energy density P(k) of a mean-field k-spin glass. The upper bound grows with k much faster than the performance of both one-local algorithms. We also identify a new obstruction result for low-depth quantum circuits (including the QAOA) when k=3, generalizing a result of Bravyi et al [arXiv:1910.08980] when k=2. We conjecture that a similar obstruction exists for all k.
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