Characterizations of symmetrically partial Boolean functions with exact quantum query complexity

Abstract

We give and prove an optimal exact quantum query algorithm with complexity $k+1$ for computing the promise problem (i.e., symmetric and partial Boolean function) $DJ_n^k$ defined as: $DJ_n^k(x)=1$ for $|x|=n/2$, $DJ_n^k(x)=0$ for $|x|$ in the set $\{0, 1,\ldots, k, n-k, n-k+1,\ldots,n\}$, and it is undefined for the rest cases, where $n$ is even, $|x|$ is the Hamming weight of $x$. The case of $k=0$ is the well-known Deutsch-Jozsa problem. We outline all symmetric (and partial) Boolean functions with degrees 1 and 2, and prove their exact quantum query complexity. Then we prove that any symmetrical (and partial) Boolean function $f$ has exact quantum 1-query complexity if and only if $f$ can be computed by the Deutsch-Jozsa algorithm. We also discover the optimal exact quantum 2-query complexity for distinguishing between inputs of Hamming weight $\{ \lfloor n/2\rfloor, \lceil n/2\rceil \}$ and Hamming weight in the set $\{ 0, n\}$ for all odd $n$. In addition, a method is provided to determine the degree of any symmetrical (and partial) Boolean function.