Deutsch–Jozsa problem (D–J) has exact quantum 1-query complexity (“exact” means no error), but requires super-exponential queries for the optimal classical deterministic decision trees. D–J problem is equivalent to a symmetric partial Boolean function, and in fact, all symmetric partial Boolean functions having exact quantum 1-query complexity have been found out and these functions can be computed by D–J algorithm. A special case is that all symmetric Boolean functions with exact quantum 1-query complexity follow directly and these functions are also all total Boolean functions with exact quantum 1-query complexity obviously. Then there are pending problems concerning partial Boolean functions having exact quantum 1-query complexity and new results have been found, but some problems are still open. In this paper, we review these results regarding exact quantum 1-query complexity and in particular, we also obtain a new result that a partial Boolean function with exact quantum 1-query complexity is constructed and it cannot be computed by D–J algorithm. Further problems are pointed out for future study.
Deutsch–Jozsa problem (D–J) has exact quantum 1-query complexity (“exact” means no error), but requires super-exponential queries for the optimal classical deterministic decision trees. D–J problem is equivalent to a symmetric partial Boolean function, and in fact, all symmetric partial Boolean functions having exact quantum 1-query complexity have been found out and these functions can be computed by D–J algorithm. A special case is that all symmetric Boolean functions with exact quantum 1-query complexity follow directly and these functions are also all total Boolean functions with exact quantum 1-query complexity obviously. Then there are pending problems concerning partial Boolean functions having exact quantum 1-query complexity and new results have been found, but some problems are still open. In this paper, we review these results regarding exact quantum 1-query complexity and in particular, we also obtain a new result that a partial Boolean function with exact quantum 1-query complexity is constructed and it cannot be computed by D–J algorithm. Further problems are pointed out for future study.
Early in 1992, Deutsch-Jozsa algorithm computed a symmetric partial Boolean function with a single quantum query, and thus achieved the best separation between classical deterministic and exact quantum query complexity. Recently, it was clarified that all symmetric partial Boolean functions with a single quantum query can be computed exactly by Deutsch-Jozsa algorithm. For the general partial Boolean functions with a single quantum query, the latest characterizations is complex and not very satisfactory. Based on this, this paper proves and discovers three new results: (1) Under a new equivalence, each partial Boolean function with a single quantum query can be transformed to a simple partial Boolean function whose polynomial degree is just one; (2) For partial Boolean functions up to four bits, there are only 10 non-trivial partial Boolean functions with a single quantum query; (3) For each quantum 1-query algorithm with undefined measurement, there exists a constructive method for finding out all partial Boolean functions that can be computed exactly by the algorithm.
The query model (or black-box model) has attracted much attention from the communities of both classical and quantum computing. Usually, quantum advantages are revealed by presenting a quantum algorithm that has a better query complexity than its classical counterpart. For example, the well-known quantum algorithms including Deutsch-Jozsa algorithm, Simon algorithm and Grover algorithm all show a considerable advantage of quantum computing from the viewpoint of query complexity. Recently we have considered in (Phys. Rev. A. {\bf 101}, 02232 (2020)) the problem: what functions can be computed by an exact one-query quantum algorithm? This problem has been addressed for total Boolean functions but still open for partial Boolean functions. Thus, in this paper we continue to characterize the computational power of exact one-query quantum algorithms for partial Boolean functions by giving several necessary and sufficient conditions. By these conditions, we construct some new functions that can be computed exactly by one-query quantum algorithms but have essential difference from the already known ones. Note that before our work, the known functions that can be computed by exact one-query quantum algorithms are all symmetric functions, whereas the ones constructed in this papers are generally asymmetric.
The Deutsch-Jozsa algorithm is one of the first examples of a quantum algorithm that is exponentially faster than any possible deterministic classical algorithm. It was proposed by Deutsch and Jozsa in 1992 with improvements by Cleve, Ekert, Macchiavello, and Mosca in 1998. The Deutsch-Jozsa problem is a promise problem and we can equivalently describe it as a partial function ${\text{DJ}}_{n}^{0}:{{0,1}}^{n}\ensuremath{\rightarrow}{0,1}$ defined as DJn0(x)=1 for ∣x∣=n/2,DJn0(x)=0 for ∣x∣=0,n, and it is undefined for the rest of the cases, where n is even, and ∣x∣ is the Hamming weight of x. The optimal quantum algorithm needs only one query to compute DJn0 but the classical deterministic algorithm requires ${2}^{n\ensuremath{-}1}+1$ queries to compute it in the worse case. In this article, we generalize the Deutsch-Jozsa problem as DJnk(x)=1 for ∣x∣=n/2,DJnk(x)=0 for ∣x∣ in the set ${0,1,...,k,n\ensuremath{-}k,n\ensuremath{-}k+1,...,n}$, and it is undefined for the rest of the cases, where $0\ensuremath{\le}k<n/2$. In particular, we give and prove an optimal exact quantum query algorithm with complexity k+1 for computing the generalized Deutsch-Jozsa problem DJnk. It is clear that the case of k=0 is in accordance with the Deutsch-Jozsa problem. Also, we give a method for finding the approximate and exact degrees of symmetric partial Boolean functions.
On the exact quantum query complexity of MODmn and EXACTk,ln
The query model has generated considerable interest in both classical and quantum computing communities. Typically, quantum advantages are demonstrated by showcasing a quantum algorithm with a better query complexity compared to its classical counterpart. Exact quantum query algorithms play a pivotal role in developing quantum algorithms. For example, the Deutsch-Jozsa algorithm demonstrated exponential quantum advantages over classical deterministic algorithms. As an important complexity measure, exact quantum query complexity describes the minimum number of queries required to solve a specific problem exactly using a quantum algorithm. In this paper, we consider the exact quantum query complexity of the following two n-bit symmetric functions MODmn:{0,1}n→{0,...,m−1} and EXACTk,ln:{0,1}n→{0,1}, which are defined as MODmn(x)=∣x∣modm and $ \text{EXACT}_{k,l}^n(x) = 1$ iff ∣x∣∈{k,l}, where ∣x∣ is the number of 1's in x. Our results are as follows: i) We present an optimal quantum algorithm for computing MODmn, achieving a query complexity of ⌈n(1−m1)⌉ for 1<m≤n. This settles a conjecture proposed by Cornelissen, Mande, Ozols and de Wolf (2021). Based on this algorithm, we show the exact quantum query complexity of a broad class of symmetric functions that map {0,1}n to a finite set X is less than n. ii) When l−k≥2, we give an optimal exact quantum query algorithm to compute EXACTk,ln for the case k=0 or k=1,l=n−1. This resolves the conjecture proposed by Ambainis, Iraids and Nagaj (2017) partially.
In Exact Quantum Query model, almost all of the Boolean functions for which non-trivial query algorithms exist are symmetric in nature. The most well known techniques in this domain exploit parity decision trees, in which the parity of two bits can be obtained by a single query. Thus, exact quantum query algorithms outperforming parity decision trees are rare. In this paper we first obtain optimal exact quantum query algorithms (Qalgo(f)) for a direct sum based class of Ω(22n) non-symmetric functions. We construct these algorithms by analyzing the algebraic normal form together with a novel untangling strategy. Next we obtain the generalized parity decision tree complexity (D⊕(f)) analysing the Walsh Spectrum. Finally, we show that query complexity of Qalgo is ⌈43n⌉ whereas D⊕(f) varies between n−1 and ⌈43n⌉+1 for different classes, underlining linear separation between the two measures in many cases. To the best of our knowledge, this is the first family of algorithms beyond generalized parity (and thus parity) for a large class of non-symmetric functions. We also implement these techniques for a larger (doubly exponential in 4n) class of Maiorana-McFarland type functions, but could only obtain partial results using similar algorithmic techniques.
Query complexity is a model of computation in which functions are evaluated by making queries to the variables. In a very recent paper [Physical Review A 101, 022325 (2020)], Chen, Ye and Li provided a characterization of exact one-query quantum algorithms. We first note that this result is an immediate corollary of what was proved five years back by Montanaro, Jozsa and Mitchison [Algorithmica 71, pp. 775-796 (2015)]. Further, in this model, we concentrate on understanding the differences between classical and quantum computation in terms of number of influencing variables of a function given lowest query complexity in the classical domain. We explore the parity decision tree model to portray possible separations between classical and quantum computations in specific cases. Our results classify different classes of Boolean functions on n-variables (n being a function of k), that can be evaluated (i) with exactly k queries in both classical and exact quantum model or (ii) with k queries in classical model and (k−1) queries in exact quantum model.