We consider two combinatorial problems. The first we call "search with wildcards": given an unknown n-bit string x, and the ability to check whether any subset of the bits of x is equal to a provided query string, the goal is to output x. We give a nearly optimal O(√n log n) quantum query algorithm for search with wildcards, beating the classical lower bound of Ω(n) queries. Rather than using amplitude amplification or a quantum walk, our algorithm is ultimately based on the solution to a state discrimination problem. The second problem we consider is combinatorial group testing, which is the task of identifying a subset of at most k special items out of a set of n items, given the ability to make queries of the form "does the set S contain any special items?" for any subset S of the n items. We give a simple quantum algorithm which uses O(k) queries to solve this problem, as compared with the classical lower bound of Ω(k log(n/k)) queries.
['Andris Ambainis', 'Ashley Montanaro']
Quantum algorithms for search with wildcards and combinatorial group testing