Quantum soundness of the classical low individual degree test

Abstract: Low degree tests play an important role in classical complexity theory, serving as basic ingredients in foundational results such as $\mathsf{MIP} = \mathsf{NEXP}$ [BFL91] and the PCP theorem [AS98,ALM+98]. Over the last ten years, versions of these tests which are sound against quantum provers have found increasing applications to the study of nonlocal games and the complexity class~$\mathsf{MIP}^*$. The culmination of this line of work is the result $\mathsf{MIP}^* = \mathsf{RE}$ [arXiv:2001.04383]. One of the key ingredients in the first reported proof of $\mathsf{MIP}^* = \mathsf{RE}$ is a two-prover variant of the low degree test, initially shown to be sound against multiple quantum provers in [arXiv:1302.1242]. Unfortunately a mistake was recently discovered in the latter result, invalidating the main result of [arXiv:1302.1242] as well as its use in subsequent works, including [arXiv:2001.04383]. We analyze a variant of the low degree test called the low individual degree test. Our main result is that the two-player version of this test is sound against quantum provers. This soundness result is sufficient to re-derive several bounds on~$\mathsf{MIP}^*$ that relied on [arXiv:1302.1242], including $\mathsf{MIP}^* = \mathsf{RE}$.