Shenggen Zheng

One-Way Finite Automata with Quantum and Classical States

In this paper, we introduce and explore a new model of quantum finite automata (QFA). Namely, one-way finite automata with quantum and classical states (1QCFA), a one way version of two-way finite automata with quantum and classical states (2QCFA) introduced by Ambainis and Watrous in 2002 [3]. First, we prove that coin-tossing one-way probabilistic finite automata (coin-tossing 1PFA) [23] and one-way quantum finite automata with control language (1QFACL) [6] as well as several other models of QFA, can be simulated by 1QCFA. Afterwards, we explore several closure properties for the family of languages accepted by 1QCFA. Finally, the state complexity of 1QCFA is explored and the main succinctness result is derived. Namely, for any prime m and any e1 > 0, there exists a language L m that cannot be recognized by any measure-many one-way quantum finite automata (MM-1QFA) [12] with bounded error \(\frac{7}{9}+\epsilon_1\), and any 1PFA recognizing it has at last m states, but L m can be recognized by a 1QCFA for any error bound e > 0 with O(logm) quantum states and 12 classical states.

On the state complexity of semi-quantum finite automata

Some of the most interesting and important results concerning quantum finite automata are those showing that they can recognize certain languages with (much) less resources than corresponding classical finite automata \cite{Amb98,Amb09,AmYa11,Ber05,Fre09,Mer00,Mer01,Mer02,Yak10,ZhgQiu112,Zhg12}. This paper shows three results of such a type that are stronger in some sense than other ones because (a) they deal with models of quantum automata with very little quantumness (so-called semi-quantum one- and two-way automata with one qubit memory only); (b) differences, even comparing with probabilistic classical automata, are bigger than expected; (c) a trade-off between the number of classical and quantum basis states needed is demonstrated in one case and (d) languages (or the promise problem) used to show main results are very simple and often explored ones in automata theory or in communication complexity, with seemingly little structure that could be utilized.

Potential of Quantum Finite Automata with Exact Acceptance

The potential of the exact quantum information processing is an interesting, important and intriguing issue. For examples, it has been believed that quantum tools can provide significant, that is larger than polynomial, advantages in the case of exact quantum computation only, or mainly, for problems with very special structures. We will show that this is not the case. In this paper the potential of quantum finite automata producing outcomes not only with a (high) probability, but with certainty (so called exactly) is explored in the context of their uses for solving promise problems and with respect to the size of automata. It is shown that for solving particular classes {An}n=1∞ of promise problems, even those without some very special structure, that succinctness of the exact quantum finite automata under consideration, with respect to the number of (basis) states, can be very small (and constant) though it grows proportional to n in the case deterministic finite automata (DFAs) of the same power are used. This is here demonstrated also for the case that the component languages of the promise problems solvable by DFAs are non-regular. The method used can be applied in finding more exact quantum finite automata or quantum algorithms for other promise problems.

On the State Complexity of Semi-quantum Finite Automata

Some of the most interesting and important results concerning quantum finite automata are those showing that they can recognize certain languages with much less resources than corresponding classical finite automata. This paper shows three results of such a type that are stronger in some sense than other ones because a they deal with models of quantum finite automata with very little quantumness so-called semi-quantum one- and two-way finite automata; b differences, even comparing with probabilistic classical automata, are bigger than expected; c a trade-off between the number of classical and quantum basis states needed is demonstrated in one case and d languages or the promise problem used to show main results are very simple and often explored ones in automata theory or in communication complexity, with seemingly little structure that could be utilized.

SOME LANGUAGES RECOGNIZED BY TWO-WAY FINITE AUTOMATA WITH QUANTUM AND CLASSICAL STATES

Two-way finite automata with quantum and classical states (2QCFA) were introduced by Ambainis and Watrous, and it was shown that 2QCFA have superiority over two-way probabilistic finite automata (2PFA) for recognizing some non-regular languages such as the language Leq = {anbn ∣ n ∈ N} and the palindrome language Lpal = {w ∈ {a,b}* ∣ w=wR}, where xR is x in the reverse order. It is interesting to find more languages like these that witness the superiority of 2QCFA over 2PFA. In this paper, we consider the language Lm = {xcy ∣ ∑ = {a,b,c},x,y ∈ {a,b}*, ∣x∣ = ∣y∣} that is similar to the middle language Lmiddle = {xay ∣ x,y ∈ {a,b}*, ∣x∣ = ∣y∣}. We prove that the language Lm show that Lm can be recognized by 2PFA with bounded error, but only in exponential expected time. Thus Lm is another witness of the fact that 2QCFA are more powerful than their classical counterparts.

Two-Tape Finite Automata with Quantum and Classical States

Two-way finite automata with quantum and classical states (2QCFA) were introduced by Ambainis and Watrous, and two-way two-tape deterministic finite automata (2TFA) were introduced by Rabin and Scott. In this paper we study 2TFA and propose a new computing model called two-way two-tape finite automata with quantum and classical states (2TQCFA). First, we give efficient 2TFA algorithms for identifying languages which can be recognized by 2QCFA. Second, we give efficient 2TQCFA algorithms to recognize several languages whose status vis-a-vis 2QCFA have been posed as open questions, such as

Promise problems solved by quantum and classical finite automata

L_{\mathit{square}}=\{a^{n}b^{n^{2}}\mid n\in \mathbf{N}\}

Application of distributed semi-quantum computing model in phase estimation☆

. Third, we show that

Capability of local operations and classical communication to distinguish bipartite unitary operations

\{a^{n}b^{n^{k}}\mid n\in \mathbf{N}\}

Time-Space Complexity Advantages for Quantum Computing

can be recognized by (k+1)-tape deterministic finite automata ((k+1)TFA). Finally, we introduce k-tape automata with quantum and classical states (kTQCFA) and prove that