Matching upper bounds on symmetric predicates in quantum communication complexity

TL;DR

This paper establishes tight bounds on the quantum communication complexity of symmetric functions composed with other functions, extending previous results to scenarios without shared entanglement and improving bounds for specific cases.

Contribution

It generalizes existing bounds to the no-entanglement setting and provides tight upper bounds for symmetric functions composed with AND, matching or surpassing previous lower bounds.

Findings

Quantum communication complexity bounds are tight for symmetric functions with and without shared entanglement.

The paper extends prior results to the no-entanglement model, broadening applicability.

Improves upon Razborov's bounds for specific symmetric functions in the no-entanglement setting.

Abstract

In this paper, we focus on the quantum communication complexity of functions of the form $f \circ G = f(G(X_1, Y_1), \ldots, G(X_n, Y_n))$ where $f: \{0, 1\}^n \to \{0, 1\}$ is a symmetric function, $G: \{0, 1\}^j \times \{0, 1\}^k \to \{0, 1\}$ is any function and Alice (resp. Bob) is given $(X_i)_{i \leq n}$ (resp. $(Y_i)_{i \leq n}$). Recently, Chakraborty et al. [STACS 2022] showed that the quantum communication complexity of $f \circ G$ is $O(Q(f)\mathrm{QCC}_\mathrm{E}(G))$ when the parties are allowed to use shared entanglement, where $Q(f)$ is the query complexity of $f$ and $\mathrm{QCC}_\mathrm{E}(G)$ is the exact communication complexity of $G$. In this paper, we first show that the same statement holds without shared entanglement, which generalizes their result. Based on the improved result, we next show tight upper bounds on $f \circ \mathrm{AND}_2$ for any symmetric function $f$ (where $\textrm{AND}_2 : \{0, 1\} \times \{0, 1\} \to \{0, 1\}$ denotes the 2-bit AND function) in both models: with shared entanglement and without shared entanglement. This matches the well-known lower bound by Razborov~[Izv. Math. 67(1) 145, 2003] when shared entanglement is allowed and improves Razborov's bound when shared entanglement is not allowed.