Gap-preserving reductions and RE-completeness of independent set games

TL;DR

This paper introduces a framework for gap-preserving reductions in quantum complexity, establishing MIP*–RE completeness for independent set games and demonstrating undecidability in the quantum setting.

Contribution

It presents the first gap-preserving reduction framework in the quantum setting and proves MIP*–RE completeness for independent set game problems.

Findings

MIP*–RE encompasses certain quantum problems with undecidable instances.

Decidability of the independent set problem differs drastically between classical and quantum settings.

A new stability theorem for perturbing almost projective measurements to genuine projective measurements.

Abstract

In complexity theory, gap-preserving reductions play a crucial role in studying hardness of approximation and in analyzing the relative complexity of multiprover interactive proof systems. In the quantum setting, multiprover interactive proof systems with entangled provers correspond to gapped promise problems for nonlocal games, and the recent result MIP$^*$=RE \cite{ji2020mipre} shows that these are in general undecidable. However, the relative complexity of problems within MIP$^*$ is still not well-understood, as establishing gap-preserving reductions in the quantum setting presents new challenges. In this paper, we introduce a framework to study such reductions and use it to establish MIP$^*$-completeness of the gapped promise problem for the natural class of independent set games. In such a game, the goal is to determine whether a given graph contains an independent set of a specified size. We construct families of independent set games with constant question size for which the gapped promise problem is undecidable. In contrast, the same problem is decidable in polynomial time in the classical setting. To carry out our reduction, we establish a new stability theorem, which could be of independent interest, allowing us to perturb families of almost PVMs to genuine PVMs.