# Classical vs. quantum communication in XOR games

## TL;DR

This paper explores the relationship between classical and quantum communication in XOR games, introducing a new intermediate setting and providing methods to generate Bell inequality violations from communication complexity problems.

## Contribution

It introduces a framework for analyzing XOR games with limited communication and links Bell inequality violations to communication complexity, including an example with polynomial inputs.

## Key findings

- Universal method to derive Bell violations from XOR games
- Existence of XOR games with optimal quotient ratios
- Polynomial input XOR game demonstrating these properties

## Abstract

In this work we introduce an intermediate setting between quantum nonlocality and communication complexity problems. More precisely, we study the value of XOR games $G$ when Alice and Bob are allowed to use a limited amount of one-way classical communication $\omega_{o.w.-c}(G)$ (resp. one-way quantum communication $\omega_{o.w.-c}^*(G)$), where $c$ denotes the number of bits (resp. qubits). The key quantity here is the quotient $\omega_{o.w.-c}^*(G)/\omega_{o.w.-c}(G)$. We provide a universal way to obtain Bell inequality violations of general Bell functionals from XOR games for which the quotient $\omega_{o.w.-c}^*(G)/\omega_{o.w.-2c}(G)$ is larger than 1. This allows, in particular, to find (unbounded) Bell inequality violations from communication complexity problems in the same spirit as the recent work by Buhrman et al. (2016). We also provide an example of a XOR game for which the previous quotient is optimal (up to a logarithmic factor) in terms of the amount of information $c$. Interestingly, this game has only polynomially many inputs per player. For the related problem of separating the classical vs quantum communication complexity of a function, the known examples attaining exponential separation require exponentially many inputs per party.