Abstract
We show that for any Boolean function f on {0,1}n, the bounded-error quantum communication complexity of XOR functions $f\circ \oplus$ satisfies that $Q\epsilon(f\circ \oplus) = O(2d (\log|\hat f|{1,\epsilon} + \log \frac{n}{\epsilon}) \log(1/\epsilon))$, where d is the F2-degree of f, and $|\hat f|{1,\epsilon} = \min{g:|f-g|\infty \leq \epsilon} |\hat f|1$. This implies that the previous lower bound $Q\epsilon(f\circ \oplus) = \Omega(\log|\hat f|{1,\epsilon})$ by Lee and Shraibman \cite{LS09} is tight for f with low F2-degree. The result also confirms the quantum version of the Log-rank Conjecture for low-degree XOR functions. In addition, we show that the exact quantum communication complexity satisfies $QE(f) = O(2d \log |\hat f|0)$, where $|\hat f|0$ is the number of nonzero Fourier coefficients of f. This matches the previous lower bound $QE(f(x,y)) = \Omega(\log rank(M_f))$ by Buhrman and de Wolf \cite{BdW01} for low-degree XOR functions.
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