Distributed Grover's algorithm

Abstract

Let Boolean function $f:\{0,1\}^n\longrightarrow \{0,1\}$ where $|\{x\in\{0,1\}^n| f(x)=1\}|=a\geq 1$. To search for an $x\in\{0,1\}^n$ with $f(x)=1$, by Grover's algorithm we can get the objective with query times $\lfloor \frac{\pi}{4}\sqrt{\frac{2^n}{a}} \rfloor$. In this paper, we propose a distributed Grover's algorithm for computing $f$ with lower query times and smaller number of input bits. More exactly, for any $k$ with $n>k\geq 1$, we can decompose $f$ into $2^k$ subfunctions, each which has $n-k$ input bits, and then the objective can be found out by computing these subfunctions with query times at most $\sum_{i=1}^{r_i} \lfloor \frac{\pi}{4}\sqrt{\frac{2^{n-k}}{b_i}} \rfloor+\lceil\sqrt{2^{n-k}}\rceil+2t_a+1$ for some $1\leq b_i\leq a$ and $r_i\leq 2t_a+1$, where $t_a=\lceil 2\pi\sqrt{a}+11\rceil$. In particular, if $a=1$, then our distributed Grover's algorithm only needs $\lfloor \frac{\pi}{4}\sqrt{2^{n-k}} \rfloor$ queries, versus $\lfloor \frac{\pi}{4}\sqrt{2^{n}} \rfloor$ queries of Grover's algorithm. %When $n$ qubits belong to middle scale but still are a bit difficult to be processed in practice, $n-k$ qubits are likely feasible for appropriate $k$ in physical realizability. Finally, we propose an efficient algorithm of constructing quantum circuits for realizing the oracle corresponding to any Boolean function with conjunctive normal form (CNF).