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Quantum algorithm for learning secret strings and its experimental demonstration

In this paper, we consider the secret-string-learning problem in the teacher-student setting: the teacher has a secret string $s\in {\{0,1\}^{n}}$, and the student wants to learn the secret $s$ by question-answer interactions with the teacher, where at each time, the student can ask the teacher with a pair $(x, q) \in {\{0,1\}^{n}}\times\{0,1,\cdots, n-1\}$ and the teacher returns a bit given by the oracle $f_{s}(x,q)$ that indicates whether the length of the longest common prefix of $s$ and $x$ is greater than $q$ or not. Our contributions are as follows. (i) We prove that any classical deterministic algorithm needs at least $n$ queries to the oracle $f_{s}$ to learn the $n$-bit secret string $s$ in both the worst case and the average case, and also present an optimal classical deterministic algorithm learning any $s$ using $n$ queries. (ii) We obtain a quantum algorithm learning the $n$-bit secret string $s$ with certainty using $\left\lceil n/2\right\rceil$ queries to the oracle $f_s$, thus proving a double speedup over classical counterparts. (iii) Experimental demonstrations of our quantum algorithm on the IBM cloud quantum computer are presented, with average success probabilities of $85.3\%$ and $82.5\%$ for all cases with $n=2$ and $n=3$ , respectively.