In the distributed Deutsch-Jozsa promise problem, two parties are to determine whether their respective strings x, y ∈ {0, 1} n are at the Hamming distance H(x, y) = 0 or H(x, y) = n 2 . Buhrman et al. (STOC' 98) proved that the exact quantum communication complexity of this problem is O(log n) while the deterministic communication complexity is Ω(n). This was the first impressive (exponential) gap between quantum and classical communication complexity. In this paper, we generalize the above distributed Deutsch-Jozsa promise problem to determine, for any fixed n 2 ≤ k ≤ n, whether H(x, y) = 0 or H(x, y) = k, and show that an exponential gap between exact quantum and deterministic communication complexity still holds if k is an even such that 1 2 n ≤ k < (1 − λ)n, where 0 < λ < 1 2 is given. We also deal with a promise version of the well-known disjointness problem and show also that for this promise problem there exists an exponential gap between quantum (and also probabilistic) communication complexity and deterministic communication complexity of the promise version of such a disjointness problem. Finally, some applications to quantum, probabilistic and deterministic finite automata of the results obtained are demonstrated. (D. Qiu).
2We prove that quantum communication complexity of DISJ λ is not more than log 3 3λ (3 + 2 log n) while the deterministic communication complexity is Ω(n). For example, if λ = 1 4 , then the quantum communication complexity of DISJ λ is not more than 3 + 2log n while the deterministic communication complexity is more than 0.007n. We prove also that probabilistic communication complexity of DISJ λ is not more than log 3 λ log n. Therefore, there is an exponential gap between quantum (and also probabilistic) communication complexity and deterministic communication complexity of the above promise problem.Number of states is a natural complexity measure for all models of finite automata and state complexity of finite automata is one of the research fields with many applications [36]. There is a variety of methods how to prove lower bounds on the state complexity and methods as well as the results of communication complexity are among the main ones [23,25,26]. In this paper we also show how to make use of our new communication complexity results to get new state complexity bounds.The paper is structured as follows. In Section 2 basic needed concepts and notations are introduced and models involved are described in details. Communication complexities and query complexities of the promise problems EQ k and DJ k are investigated in Section 3. Communication complexity of the promise problem DISJ λ is dealt with in Section 4. Applications to finite automata are explored in Section 5. Some open problems are discussed in Section 6.
PreliminariesIn this section, we recall some basic definitions about communication complexity, query complexity and quantum finite automata. Concerning basic concepts and notations of quantum information processing, we refer the reader to [18,29].
Communication comp...