RE-completeness of entangled constraint satisfaction problems

TL;DR

This paper demonstrates that many classical NP-complete constraint satisfaction problems become RE-complete when extended to quantum entangled settings, indicating their undecidability in this broader context.

Contribution

It extends the understanding of CSP complexity to quantum entangled frameworks, showing NP-complete problems are RE-complete and undecidable in this setting.

Findings

NP-complete CSPs become RE-complete with quantum entanglement

All boolean CSPs, including 3SAT and 3-coloring, are undecidable in this setting

The paper introduces improved techniques for separating constraints and constructing gadgets

Abstract

Constraint satisfaction problems (CSPs) are a natural class of decision problems where one must decide whether there is an assignment to variables that satisfies a given formula. Schaefer's dichotomy theorem, and its extension to all alphabets due to Bulatov and Zhuk, shows that CSP languages are either efficiently decidable, or NP-complete. It is possible to extend CSP languages to quantum assignments using the formalism of nonlocal games. Due to the equality of complexity classes MIP$^\ast=$ RE, general succinctly-presented entangled CSPs are RE-complete. In this work, we show that a wide range of NP-complete CSPs become RE-complete in this setting, including all boolean CSPs, such as 3SAT, as well as $3$-colouring. This also implies that these CSP languages remain undecidable even when not succinctly presented. To show this, we work in the weighted algebra framework introduced by Mastel and Slofstra, where synchronous strategies for a nonlocal game are represented by tracial states on an algebra. Along the way, we improve the subdivision technique in order to be able to separate constraints in the CSP while preserving constant soundness, construct commutativity gadgets for all boolean CSPs, and show a variety of relations between the different ways of presenting CSPs as games.